diff --git a/.vscode/launch.json b/.vscode/launch.json
index 867396e489..53d2180992 100644
--- a/.vscode/launch.json
+++ b/.vscode/launch.json
@@ -4,6 +4,15 @@
   // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
   "version": "0.2.0",
   "configurations": [
+    {
+      "type": "node",
+      "request": "launch",
+      "name": "Launch Program",
+      "program": "${file}",
+      "skipFiles": [
+        "<node_internals>/**"
+      ]
+    },
     {
       "type": "node",
       "request": "launch",
@@ -60,12 +69,6 @@
       ],
       "internalConsoleOptions": "openOnSessionStart"
     },
-    {
-      "type": "node",
-      "request": "launch",
-      "name": "Launch Program",
-      "program": "${workspaceRoot}\\packages\\stryker-mocha-runner\""
-    },
     {
       "type": "node",
       "request": "attach",
diff --git a/packages/api/src/check/MutantStatus.ts b/packages/api/src/check/MutantStatus.ts
new file mode 100644
index 0000000000..fe94b88bb2
--- /dev/null
+++ b/packages/api/src/check/MutantStatus.ts
@@ -0,0 +1,10 @@
+export enum MutantStatus {
+  Init = 'init',
+  Ignored = 'ignored',
+  NoCoverage = 'noCoverage',
+  Killed = 'killed',
+  Survived = 'survived',
+  TimedOut = 'timedOut',
+  RuntimeError = 'runtimeError',
+  CompileError = 'compileError',
+}
diff --git a/packages/typescript-checker/.eslintrc b/packages/typescript-checker/.eslintrc
new file mode 100644
index 0000000000..7e107338ab
--- /dev/null
+++ b/packages/typescript-checker/.eslintrc
@@ -0,0 +1,3 @@
+{
+    "extends": "../../.eslintrc.js"
+}
\ No newline at end of file
diff --git a/packages/typescript-checker/.mocharc.jsonc b/packages/typescript-checker/.mocharc.jsonc
new file mode 100644
index 0000000000..a12650f47f
--- /dev/null
+++ b/packages/typescript-checker/.mocharc.jsonc
@@ -0,0 +1,12 @@
+{
+  "require": ["source-map-support/register"],
+  "file": [
+    // This file is included first
+    "dist/test/helpers/setup-tests.js"
+  ],
+  "spec": [
+    "dist/test/unit/**/*.js",
+    "dist/test/integration/**/*.js"
+  ],
+  "timeout": 10000
+}
diff --git a/packages/typescript-checker/.npmignore b/packages/typescript-checker/.npmignore
new file mode 100644
index 0000000000..026a43bb64
--- /dev/null
+++ b/packages/typescript-checker/.npmignore
@@ -0,0 +1,6 @@
+*
+
+!dist/src/**/*.js
+!dist/src/**/*.map
+!dist/src/**/*.ts
+!src/**/*
diff --git a/packages/typescript-checker/.npmrc b/packages/typescript-checker/.npmrc
new file mode 100644
index 0000000000..9cf9495031
--- /dev/null
+++ b/packages/typescript-checker/.npmrc
@@ -0,0 +1 @@
+package-lock=false
\ No newline at end of file
diff --git a/packages/typescript-checker/.vscode/launch.json b/packages/typescript-checker/.vscode/launch.json
new file mode 100644
index 0000000000..b251812147
--- /dev/null
+++ b/packages/typescript-checker/.vscode/launch.json
@@ -0,0 +1,21 @@
+{
+  "version": "0.2.0",
+  "configurations": [
+    {
+      "type": "node",
+      "request": "launch",
+      "name": "Unit / Integration tests",
+      "program": "${workspaceRoot}/../../node_modules/mocha/bin/_mocha",
+      "internalConsoleOptions": "openOnSessionStart",
+      "outFiles": [
+        "${workspaceRoot}/dist/**/*.js"
+      ],
+      "skipFiles": [
+        "<node_internals>/**"
+      ],
+      "args": [
+        "--no-timeout"
+      ]
+    }
+  ]
+}
diff --git a/packages/typescript-checker/README.md b/packages/typescript-checker/README.md
new file mode 100644
index 0000000000..2764a27194
--- /dev/null
+++ b/packages/typescript-checker/README.md
@@ -0,0 +1,84 @@
+**NOTE:** This readme describes a checker plugin set to release in Stryker v4. It is not usable yet! 
+Please see the [master branch](https://github.com/stryker-mutator/stryker/tree/master) for the current release of Stryker. 
+
+
+[![Mutation testing badge](https://img.shields.io/endpoint?style=flat&url=https%3A%2F%2Fbadge-api.stryker-mutator.io%2Fgithub.com%2Fstryker-mutator%2Fstryker%2Fmaster%3Fmodule%3Dtypescript)](https://dashboard.stryker-mutator.io/reports/github.com/stryker-mutator/stryker/master?module=typescript-checker)
+[![Build Status](https://github.com/stryker-mutator/stryker/workflows/CI/badge.svg)](https://github.com/stryker-mutator/stryker/actions?query=workflow%3ACI+branch%3Amaster)
+[![NPM](https://img.shields.io/npm/dm/@stryker-mutator/typescript-checker.svg)](https://www.npmjs.com/package/@stryker-mutator/typescript-checker)
+[![Node version](https://img.shields.io/node/v/@stryker-mutator/typescript.svg)](https://img.shields.io/node/v/@stryker-mutator/typescript-checker.svg)
+[![Slack Chat](https://img.shields.io/badge/slack-chat-brightgreen.svg?logo=slack)](https://join.slack.com/t/stryker-mutator/shared_invite/enQtOTUyMTYyNTg1NDQ0LTU4ODNmZDlmN2I3MmEyMTVhYjZlYmJkOThlNTY3NTM1M2QxYmM5YTM3ODQxYmJjY2YyYzllM2RkMmM1NjNjZjM)
+
+![Stryker](https://github.com/stryker-mutator/stryker/raw/master/stryker-80x80.png)
+
+# Typescript checker
+
+A TypeScript type checker plugin for [Stryker](https://stryker-mutator.io), the ~~JavaScript~~ *TypeScript* Mutation testing framework.
+This plugin enables type checking on mutants, so you won't have to waste time on mutants which results in a type error.
+
+## Features
+
+👽 Type check each mutant. Invalid mutants will be marked as `CompileError` in your Stryker report.  
+🧒 Easy to setup, only your `tsconfig.json` file is needed.  
+🔢 Type check is done in-memory, no side effects on disk.  
+🎁 Support for both single typescript projects as well as projects with project references (`--build` mode).  
+
+## Install
+
+First, install Stryker itself (you can follow the [quickstart on the website](https://stryker-mutator.io/quickstart.html))
+
+Next, install this package:
+
+```bash
+npm install --save-dev @stryker-mutator/typescript-checker
+```
+
+## Configuring
+
+You can configure the typescript checker in the `stryker.conf.js` (or `stryker.conf.json`) file.
+
+```js
+// stryker.conf.json
+{
+  "checkers": ["typescript"],
+  "typescriptChecker": {
+    "tsconfigFile": "tsconfig.json"
+  }
+}
+```
+
+### `typescriptChecker.tsconfigFile` [`string`]
+
+Default: `'tsconfig.json'`
+
+The path to your [tsconfig](https://www.typescriptlang.org/docs/handbook/tsconfig-json.html). Project references _are supported_, `--build` mode will be enabled automatically when references are found in your tsconfig.json file.
+
+*Note: the following compiler options are always overridden by @stryker-mutator/typescript-checker to ovoid false positives. See [issue 391](https://github.com/stryker-mutator/stryker/issues/391#issue-259829320) for more information on this*
+
+```json
+{
+  "compilerOptions": {
+    "allowUnreachableCode": true,
+    "noUnusedLocals": false,
+    "noUnusedParameters": false
+  }
+}
+```
+
+## Peer dependencies
+
+The `@stryker-mutator/typescript-checker` package  for `stryker` to enable `typescript` support. As such, you should make sure you have the correct versions of its dependencies installed:
+
+* `typescript`
+* `@stryker-mutator/core`
+
+For the current versions, see the `peerDependencies` section in the [package.json](https://github.com/stryker-mutator/stryker/blob/master/packages/typescript/package.json).
+
+
+## Load the plugin
+
+In this plugin the `@stryker-mutator/typescript-checker`' must be loaded into Stryker.
+The easiest way to achieve this, is *not have a `plugins` section* in your config file. That way, all plugins starting with `"@stryker-mutator/"` will be loaded.
+
+If you do decide to choose specific modules, don't forget to add `"@stryker-mutator/typescript-checker"` to the list of plugins to load.
+
+
diff --git a/packages/typescript-checker/package.json b/packages/typescript-checker/package.json
new file mode 100644
index 0000000000..9640a93cb1
--- /dev/null
+++ b/packages/typescript-checker/package.json
@@ -0,0 +1,45 @@
+{
+  "name": "@stryker-mutator/typescript-checker",
+  "version": "3.2.2",
+  "description": "A typescript type checker plugin to be used in Stryker, the JavaScript mutation testing framework",
+  "main": "dist/src/index.js",
+  "scripts": {
+    "test": "nyc --exclude-after-remap=false --check-coverage --reporter=html --report-dir=reports/coverage --lines 80 --functions 80 --branches 75 npm run mocha",
+    "mocha": "mocha",
+    "stryker": "node ../core/bin/stryker run"
+  },
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/stryker-mutator/stryker",
+    "directory": "packages/typescript-checker"
+  },
+  "engines": {
+    "node": ">=10"
+  },
+  "publishConfig": {
+    "access": "public"
+  },
+  "author": "Nico Jansen <jansennico@gmail.com>",
+  "contributors": [
+    "Simon de Lang <simondelang@gmail.com>",
+    "Nico Jansen <jansennico@gmail.com>"
+  ],
+  "license": "Apache-2.0",
+  "bugs": {
+    "url": "https://github.com/stryker-mutator/stryker/issues"
+  },
+  "homepage": "https://stryker-mutator.io",
+  "dependencies": {
+    "@stryker-mutator/api": "^3.2.2",
+    "@stryker-mutator/util": "^3.2.2",
+    "semver": "^7.3.2"
+  },
+  "devDependencies": {
+    "@stryker-mutator/test-helpers": "^3.2.2",
+    "@types/semver": "^7.2.0"
+  },
+  "peerDependencies": {
+    "typescript": ">=3.6",
+    "@stryker-mutator/core": "3.2.4"
+  }
+}
diff --git a/packages/typescript-checker/schema/typescript-checker-options.json b/packages/typescript-checker/schema/typescript-checker-options.json
new file mode 100644
index 0000000000..fa97f2ec61
--- /dev/null
+++ b/packages/typescript-checker/schema/typescript-checker-options.json
@@ -0,0 +1,22 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema",
+  "type": "object",
+  "title": "TypescriptCheckerOptions",
+  "additionalProperties": false,
+  "properties": {
+    "typescriptChecker": {
+      "title": "TypescriptOptions",
+      "additionalProperties": false,
+      "description": "Configuration for @stryker-mutator/typescript-checker.",
+      "default": {},
+      "type": "object",
+      "properties": {
+        "tsconfigFile": {
+          "description": "Location of your (root) tsconfig.json file. Project references with `--build` are supported by default.",
+          "type": "string",
+          "default": "tsconfig.json"
+        }
+      }
+    }
+  }
+}
diff --git a/packages/typescript-checker/src/fs/hybrid-file-system.ts b/packages/typescript-checker/src/fs/hybrid-file-system.ts
new file mode 100644
index 0000000000..64204bf70c
--- /dev/null
+++ b/packages/typescript-checker/src/fs/hybrid-file-system.ts
@@ -0,0 +1,69 @@
+import ts from 'typescript';
+import { Mutant } from '@stryker-mutator/api/core';
+
+import { ScriptFile } from './script-file';
+
+function toTSFileName(fileName: string) {
+  return fileName.replace(/\\/g, '/');
+}
+
+/**
+ * A very simple hybrid file system.
+ * * Readonly from disk
+ * * Writes in-memory
+ * * Hard caching
+ * * Ability to mutate one file
+ */
+export class HybridFileSystem {
+  private readonly files = new Map<string, ScriptFile | undefined>();
+  private mutatedFile: ScriptFile | undefined;
+
+  public writeFile(fileName: string, data: string) {
+    fileName = toTSFileName(fileName);
+    const existingFile = this.files.get(fileName);
+    if (existingFile) {
+      existingFile.write(data);
+    } else {
+      this.files.set(fileName, new ScriptFile(data, fileName));
+    }
+  }
+
+  public mutate(mutant: Pick<Mutant, 'fileName' | 'range' | 'replacement'>) {
+    const fileName = toTSFileName(mutant.fileName);
+    const file = this.files.get(fileName);
+    if (!file) {
+      throw new Error(`File "${mutant.fileName}" cannot be found.`);
+    }
+    if (this.mutatedFile && this.mutatedFile !== file) {
+      this.mutatedFile.resetMutant();
+    }
+    file.mutate(mutant);
+    this.mutatedFile = file;
+  }
+
+  public watchFile(fileName: string, watcher: ts.FileWatcherCallback) {
+    const file = this.getFile(fileName);
+    if (!file) {
+      throw new Error(`Cannot find file ${fileName} for watching`);
+    }
+    file.watcher = watcher;
+  }
+
+  public getFile(fileName: string): ScriptFile | undefined {
+    fileName = toTSFileName(fileName);
+    if (!this.files.has(fileName)) {
+      let content = ts.sys.readFile(fileName);
+      if (content) {
+        let modifiedTime = ts.sys.getModifiedTime!(fileName)!;
+        this.files.set(fileName, new ScriptFile(content, fileName, modifiedTime));
+      } else {
+        this.files.set(fileName, undefined);
+      }
+    }
+    return this.files.get(fileName);
+  }
+
+  public existsInMemory(fileName: string): boolean {
+    return !!this.files.get(toTSFileName(fileName));
+  }
+}
diff --git a/packages/typescript-checker/src/fs/index.ts b/packages/typescript-checker/src/fs/index.ts
new file mode 100644
index 0000000000..a338212607
--- /dev/null
+++ b/packages/typescript-checker/src/fs/index.ts
@@ -0,0 +1 @@
+export * from './hybrid-file-system';
diff --git a/packages/typescript-checker/src/fs/script-file.ts b/packages/typescript-checker/src/fs/script-file.ts
new file mode 100644
index 0000000000..13e71b0ce1
--- /dev/null
+++ b/packages/typescript-checker/src/fs/script-file.ts
@@ -0,0 +1,40 @@
+import ts from 'typescript';
+import { Mutant } from '@stryker-mutator/api/core';
+import { FileWatcherCallback } from 'typescript';
+
+export class ScriptFile {
+  private readonly originalContent: string;
+  constructor(public content: string, public fileName: string, public modifiedTime = new Date()) {
+    this.originalContent = content;
+  }
+
+  public write(content: string) {
+    this.content = content;
+    this.touch();
+  }
+
+  public watcher: FileWatcherCallback | undefined;
+
+  public mutate(mutant: Pick<Mutant, 'replacement' | 'range'>) {
+    this.guardMutationIsWatched();
+    this.content = `${this.originalContent.substr(0, mutant.range[0])}${mutant.replacement}${this.originalContent.substr(mutant.range[1])}`;
+    this.touch();
+  }
+
+  public resetMutant() {
+    this.guardMutationIsWatched();
+    this.content = this.originalContent;
+    this.touch();
+  }
+
+  private guardMutationIsWatched() {
+    if (!this.watcher) {
+      throw new Error(`No watcher registered for ${this.fileName}. Changes would go unnoticed`);
+    }
+  }
+
+  private touch() {
+    this.modifiedTime = new Date();
+    this.watcher?.(this.fileName, ts.FileWatcherEventKind.Changed);
+  }
+}
diff --git a/packages/typescript-checker/src/index.ts b/packages/typescript-checker/src/index.ts
new file mode 100644
index 0000000000..5afea94ba0
--- /dev/null
+++ b/packages/typescript-checker/src/index.ts
@@ -0,0 +1,9 @@
+import { PluginKind, declareClassPlugin } from '@stryker-mutator/api/plugin';
+
+import strykerValidationSchema from '../schema/typescript-checker-options.json';
+
+import { TypescriptChecker } from './typescript-checker';
+
+export const strykerPlugins = [declareClassPlugin(PluginKind.Checker, 'typescript', TypescriptChecker)];
+
+export { strykerValidationSchema, TypescriptChecker };
diff --git a/packages/typescript-checker/src/tsconfig-helpers.ts b/packages/typescript-checker/src/tsconfig-helpers.ts
new file mode 100644
index 0000000000..a9ecd4d0f6
--- /dev/null
+++ b/packages/typescript-checker/src/tsconfig-helpers.ts
@@ -0,0 +1,74 @@
+import { resolve } from 'path';
+
+import ts from 'typescript';
+import semver from 'semver';
+
+// Override some compiler options that have to do with code quality. When mutating, we're not interested in the resulting code quality
+// See https://github.com/stryker-mutator/stryker/issues/391 for more info
+const COMPILER_OPTIONS_OVERRIDES: Readonly<Partial<ts.CompilerOptions>> = Object.freeze({
+  allowUnreachableCode: true,
+  noUnusedLocals: false,
+  noUnusedParameters: false,
+});
+
+// When we're running in 'single-project' mode, we can safely disable emit
+const NO_EMIT_OPTIONS_FOR_SINGLE_PROJECT: Readonly<Partial<ts.CompilerOptions>> = Object.freeze({
+  noEmit: true,
+  incremental: false, // incremental and composite off: https://github.com/microsoft/TypeScript/issues/36917
+  composite: false,
+  declaration: false,
+});
+
+// When we're running in 'project references' mode, we need to enable declaration output
+const LOW_EMIT_OPTIONS_FOR_PROJECT_REFERENCES: Readonly<Partial<ts.CompilerOptions>> = Object.freeze({
+  emitDeclarationOnly: true,
+  noEmit: false,
+  declarationMap: false,
+});
+
+export function guardTSVersion() {
+  if (!semver.satisfies(ts.version, '>=3.6')) {
+    throw new Error(`@stryker-mutator/typescript-checker only supports typescript@3.6 our higher. Found typescript@${ts.version}`);
+  }
+}
+
+/**
+ * Determines whether or not to use `--build` mode based on "references" being there in the config file
+ * @param tsconfigFileName The tsconfig file to parse
+ */
+export function determineBuildModeEnabled(tsconfigFileName: string) {
+  const tsconfigFile = ts.sys.readFile(tsconfigFileName);
+  if (!tsconfigFile) {
+    throw new Error(`File "${tsconfigFileName}" not found!`);
+  }
+  const useProjectReferences = 'references' in ts.parseConfigFileTextToJson(tsconfigFileName, tsconfigFile).config;
+  return useProjectReferences;
+}
+
+/**
+ * Overrides some options to speed up compilation and disable some code quality checks we don't want during mutation testing
+ * @param parsedConfig The parsed config file
+ * @param useBuildMode whether or not `--build` mode is used
+ */
+export function overrideOptions(parsedConfig: { config?: any }, useBuildMode: boolean) {
+  const config = {
+    ...parsedConfig.config,
+    compilerOptions: {
+      ...parsedConfig.config?.compilerOptions,
+      ...COMPILER_OPTIONS_OVERRIDES,
+      ...(useBuildMode ? LOW_EMIT_OPTIONS_FOR_PROJECT_REFERENCES : NO_EMIT_OPTIONS_FOR_SINGLE_PROJECT),
+    },
+  };
+  return JSON.stringify(config);
+}
+
+/**
+ * Retrieves the referenced config files based on parsed configuration
+ * @param parsedConfig The parsed config file
+ */
+export function retrieveReferencedProjects(parsedConfig: { config?: any }): string[] {
+  if (Array.isArray(parsedConfig.config?.references)) {
+    return parsedConfig.config?.references.map((reference: any) => resolve(ts.resolveProjectReferencePath(reference)));
+  }
+  return [];
+}
diff --git a/packages/typescript-checker/src/tsconfig.json b/packages/typescript-checker/src/tsconfig.json
new file mode 100644
index 0000000000..72cd608aa5
--- /dev/null
+++ b/packages/typescript-checker/src/tsconfig.json
@@ -0,0 +1,22 @@
+{
+  "extends": "../../../tsconfig.settings.json",
+  "compilerOptions": {
+    "outDir": "../dist",
+    "rootDir": "..",
+    "esModuleInterop": true,
+    "resolveJsonModule": true,
+    "noImplicitThis": true,
+    "types": [
+      "node"
+    ],
+  },
+  "references": [
+    {
+      "path": "../../api/tsconfig.src.json"
+    },
+    {
+      "path": "../../util/tsconfig.src.json"
+    }
+  ],
+  "include": ["**/*.ts", "../src-generated/**/*.ts", "../schema/typescript-checker-options.json"]
+}
diff --git a/packages/typescript-checker/src/typescript-checker-with-stryker-options.ts b/packages/typescript-checker/src/typescript-checker-with-stryker-options.ts
new file mode 100644
index 0000000000..20521aeaf6
--- /dev/null
+++ b/packages/typescript-checker/src/typescript-checker-with-stryker-options.ts
@@ -0,0 +1,5 @@
+import { StrykerOptions } from '@stryker-mutator/api/core';
+
+import { TypescriptCheckerOptions } from '../src-generated/typescript-checker-options';
+
+export interface TypescriptCheckerWithStrykerOptions extends StrykerOptions, TypescriptCheckerOptions {}
diff --git a/packages/typescript-checker/src/typescript-checker.ts b/packages/typescript-checker/src/typescript-checker.ts
new file mode 100644
index 0000000000..568347122a
--- /dev/null
+++ b/packages/typescript-checker/src/typescript-checker.ts
@@ -0,0 +1,190 @@
+import { EOL } from 'os';
+import path from 'path';
+
+import ts from 'typescript';
+import { Checker, CheckResult, CheckStatus } from '@stryker-mutator/api/check';
+import { tokens, commonTokens } from '@stryker-mutator/api/plugin';
+import { Logger } from '@stryker-mutator/api/logging';
+import { Task, propertyPath } from '@stryker-mutator/util';
+import { Mutant, StrykerOptions } from '@stryker-mutator/api/core';
+
+import { TypescriptCheckerOptions } from '../src-generated/typescript-checker-options';
+
+import { HybridFileSystem } from './fs';
+import { TypescriptCheckerWithStrykerOptions } from './typescript-checker-with-stryker-options';
+import { determineBuildModeEnabled, overrideOptions, retrieveReferencedProjects, guardTSVersion } from './tsconfig-helpers';
+
+const diagnosticsHost: ts.FormatDiagnosticsHost = {
+  getCanonicalFileName: (fileName) => fileName,
+  getCurrentDirectory: process.cwd,
+  getNewLine: () => EOL,
+};
+
+const FILE_CHANGE_DETECTED_DIAGNOSTIC_CODE = 6032;
+
+/**
+ * An in-memory type checker implementation which validates type errors of mutants.
+ */
+export class TypescriptChecker implements Checker {
+  private readonly fs = new HybridFileSystem();
+  private currentTask: Task<CheckResult>;
+  private readonly currentErrors: ts.Diagnostic[] = [];
+  /**
+   * Keep track of all tsconfig files which are read during compilation (for project references)
+   */
+  private readonly allTSConfigFiles: Set<string>;
+
+  public static inject = tokens(commonTokens.logger, commonTokens.options);
+  private readonly tsconfigFile: string;
+
+  constructor(private readonly logger: Logger, options: StrykerOptions) {
+    this.tsconfigFile = (options as TypescriptCheckerWithStrykerOptions).typescriptChecker.tsconfigFile;
+    this.allTSConfigFiles = new Set([path.resolve(this.tsconfigFile)]);
+  }
+
+  /**
+   * Starts the typescript compiler and does a dry run
+   */
+  public async init(): Promise<void> {
+    guardTSVersion();
+    this.guardTSConfigFileExists();
+    this.currentTask = new Task();
+    const buildModeEnabled = determineBuildModeEnabled(this.tsconfigFile);
+    const compiler = ts.createSolutionBuilderWithWatch(
+      ts.createSolutionBuilderWithWatchHost(
+        {
+          ...ts.sys,
+          readFile: (fileName) => {
+            const content = this.fs.getFile(fileName)?.content;
+            if (content && this.allTSConfigFiles.has(path.resolve(fileName))) {
+              return this.adjustTSConfigFile(fileName, content, buildModeEnabled);
+            }
+            return content;
+          },
+          watchFile: (path: string, callback: ts.FileWatcherCallback) => {
+            this.fs.getFile(path)!.watcher = callback;
+            return {
+              close: () => {
+                delete this.fs.getFile(path)!.watcher;
+              },
+            };
+          },
+          writeFile: (path, data) => {
+            this.fs.writeFile(path, data);
+          },
+          createDirectory: () => {
+            // Idle, no need to create directories in the hybrid fs
+          },
+          clearScreen() {
+            // idle, never clear the screen
+          },
+          getModifiedTime: (fileName) => {
+            return this.fs.getFile(fileName)!.modifiedTime;
+          },
+          watchDirectory: (): ts.FileWatcher => {
+            // this is used to see if new files are added to a directory. Can safely be ignored for mutation testing.
+            return {
+              close() {},
+            };
+          },
+        },
+        undefined,
+        (error) => this.currentErrors.push(error),
+        (status) => this.logDiagnostic('status')(status),
+        (summary) => {
+          this.logDiagnostic('summary')(summary);
+          summary.code !== FILE_CHANGE_DETECTED_DIAGNOSTIC_CODE && this.resolveCheckResult();
+        }
+      ),
+      [this.tsconfigFile],
+      {}
+    );
+    compiler.build();
+    const result = await this.currentTask.promise;
+    if (result.status === CheckStatus.CompileError) {
+      throw new Error(`TypeScript error(s) found in dry run compilation: ${result.reason}`);
+    }
+  }
+
+  private guardTSConfigFileExists() {
+    if (!ts.sys.fileExists(this.tsconfigFile)) {
+      throw new Error(
+        `The tsconfig file does not exist at: "${path.resolve(
+          this.tsconfigFile
+        )}". Please configure the tsconfig file in your stryker.conf file using "${propertyPath<TypescriptCheckerOptions>(
+          'typescriptChecker',
+          'tsconfigFile'
+        )}"`
+      );
+    }
+  }
+
+  /**
+   * Checks whether or not a mutant results in a compile error.
+   * Will simply pass through if the file mutated isn't part of the typescript project
+   * @param mutant The mutant to check
+   */
+  public async check(mutant: Mutant): Promise<CheckResult> {
+    if (this.fs.existsInMemory(mutant.fileName)) {
+      this.clearCheckState();
+      this.fs.mutate(mutant);
+      return this.currentTask.promise;
+    } else {
+      // We allow people to mutate files that are not included in this ts project
+      return {
+        status: CheckStatus.Passed,
+      };
+    }
+  }
+
+  /**
+   * Post processes the content of a tsconfig file. Adjusts some options for speed and alters quality options.
+   * @param fileName The tsconfig file name
+   * @param content The tsconfig content
+   * @param buildModeEnabled Whether or not `--build` mode is used
+   */
+  private adjustTSConfigFile(fileName: string, content: string, buildModeEnabled: boolean) {
+    const parsedConfig = ts.parseConfigFileTextToJson(fileName, content);
+    if (parsedConfig.error) {
+      return content; // let the ts compiler deal with this error
+    } else {
+      for (const referencedProject of retrieveReferencedProjects(parsedConfig)) {
+        this.allTSConfigFiles.add(referencedProject);
+      }
+      return overrideOptions(parsedConfig, buildModeEnabled);
+    }
+  }
+
+  /**
+   * Resolves the task that is currently running. Will report back the check result.
+   */
+  private resolveCheckResult(): void {
+    if (this.currentErrors.length) {
+      const errorText = ts.formatDiagnostics(this.currentErrors, {
+        getCanonicalFileName: (fileName) => fileName,
+        getCurrentDirectory: process.cwd,
+        getNewLine: () => EOL,
+      });
+      this.currentTask.resolve({
+        status: CheckStatus.CompileError,
+        reason: errorText,
+      });
+    }
+    this.currentTask.resolve({ status: CheckStatus.Passed });
+  }
+
+  /**
+   * Clear state between checks
+   */
+  private clearCheckState() {
+    while (this.currentErrors.pop()) {
+      // Idle
+    }
+    this.currentTask = new Task();
+  }
+  private readonly logDiagnostic = (label: string) => {
+    return (d: ts.Diagnostic) => {
+      this.logger.trace(`${label} ${ts.formatDiagnostics([d], diagnosticsHost)}`);
+    };
+  };
+}
diff --git a/packages/typescript-checker/stryker.conf.js b/packages/typescript-checker/stryker.conf.js
new file mode 100644
index 0000000000..c84ec1366d
--- /dev/null
+++ b/packages/typescript-checker/stryker.conf.js
@@ -0,0 +1,8 @@
+const path = require('path');
+const settings = require('../../stryker.parent.conf');
+const moduleName = __dirname.split(path.sep).pop();
+settings.dashboard.module = moduleName;
+delete settings.mochaOptions.spec;
+delete settings.files;
+settings.coverageAnalysis = 'perTest';
+module.exports = settings;
diff --git a/packages/typescript-checker/test/helpers/factories.ts b/packages/typescript-checker/test/helpers/factories.ts
new file mode 100644
index 0000000000..de13830698
--- /dev/null
+++ b/packages/typescript-checker/test/helpers/factories.ts
@@ -0,0 +1,8 @@
+import { TypescriptOptions } from '../../src-generated/typescript-checker-options';
+
+export function createTypescriptOptions(overrides?: Partial<TypescriptOptions>): TypescriptOptions {
+  return {
+    tsconfigFile: 'tsconfig.json',
+    ...overrides,
+  };
+}
diff --git a/packages/typescript-checker/test/helpers/setup-tests.ts b/packages/typescript-checker/test/helpers/setup-tests.ts
new file mode 100644
index 0000000000..f900fedb46
--- /dev/null
+++ b/packages/typescript-checker/test/helpers/setup-tests.ts
@@ -0,0 +1,20 @@
+import chai from 'chai';
+import chaiAsPromised from 'chai-as-promised';
+import sinonChai from 'sinon-chai';
+import sinon from 'sinon';
+import { testInjector } from '@stryker-mutator/test-helpers';
+
+chai.use(sinonChai);
+chai.use(chaiAsPromised);
+
+let originalCwd: string;
+
+beforeEach(() => {
+  originalCwd = process.cwd();
+});
+
+afterEach(() => {
+  sinon.restore();
+  testInjector.reset();
+  process.chdir(originalCwd);
+});
diff --git a/packages/typescript-checker/test/integration/project-references.it.spec.ts b/packages/typescript-checker/test/integration/project-references.it.spec.ts
new file mode 100644
index 0000000000..51a933833d
--- /dev/null
+++ b/packages/typescript-checker/test/integration/project-references.it.spec.ts
@@ -0,0 +1,72 @@
+import path from 'path';
+import fs from 'fs';
+
+import { expect } from 'chai';
+import { Mutant, Range } from '@stryker-mutator/api/core';
+import { CheckResult, CheckStatus } from '@stryker-mutator/api/check';
+import { testInjector, factory } from '@stryker-mutator/test-helpers';
+
+import { TypescriptChecker } from '../../src';
+import { createTypescriptOptions } from '../helpers/factories';
+
+const resolveTestResource = (path.resolve.bind(
+  path,
+  __dirname,
+  '..' /* integration */,
+  '..' /* test */,
+  '..' /* dist */,
+  'testResources',
+  'project-references'
+) as unknown) as typeof path.resolve;
+
+describe('Typescript checker on a project with project references', () => {
+  let sut: TypescriptChecker;
+
+  beforeEach(() => {
+    process.chdir(resolveTestResource());
+    testInjector.options.typescriptChecker = createTypescriptOptions({ tsconfigFile: 'tsconfig.root.json' });
+    sut = testInjector.injector.injectClass(TypescriptChecker);
+    return sut.init();
+  });
+
+  it('should not write output to disk', () => {
+    expect(fs.existsSync(resolveTestResource('dist')), 'Output was written to disk!').false;
+  });
+
+  it('should be able to validate a mutant', async () => {
+    const mutant = createMutant('src/todo.ts', 'TodoList.allTodos.push(newItem)', 'newItem ? 42 : 43');
+    const expectedResult: CheckResult = {
+      status: CheckStatus.Passed,
+    };
+    const actualResult = await sut.check(mutant);
+    expect(actualResult).deep.eq(expectedResult);
+  });
+
+  it('should allow unused local variables (override options)', async () => {
+    const mutant = createMutant('src/todo.ts', 'TodoList.allTodos.push(newItem)', '42');
+    const expectedResult: CheckResult = {
+      status: CheckStatus.Passed,
+    };
+    const actual = await sut.check(mutant);
+    expect(actual).deep.eq(expectedResult);
+  });
+});
+
+const fileContents = Object.freeze({
+  ['src/todo.ts']: fs.readFileSync(resolveTestResource('src', 'todo.ts'), 'utf8'),
+  ['test/todo.spec.ts']: fs.readFileSync(resolveTestResource('test', 'todo.spec.ts'), 'utf8'),
+});
+
+function createMutant(fileName: 'src/todo.ts' | 'test/todo.spec.ts', findText: string, replacement: string, offset: number = 0): Mutant {
+  const originalOffset: number = fileContents[fileName].indexOf(findText);
+  if (originalOffset === -1) {
+    throw new Error(`Cannot find ${findText} in ${fileName}`);
+  }
+  const range: Range = [originalOffset + offset, originalOffset + findText.length];
+  return factory.mutant({
+    fileName: resolveTestResource(fileName),
+    mutatorName: 'foo-mutator',
+    range,
+    replacement,
+  });
+}
diff --git a/packages/typescript-checker/test/integration/single-project.it.spec.ts b/packages/typescript-checker/test/integration/single-project.it.spec.ts
new file mode 100644
index 0000000000..d02581972c
--- /dev/null
+++ b/packages/typescript-checker/test/integration/single-project.it.spec.ts
@@ -0,0 +1,130 @@
+import path from 'path';
+import fs from 'fs';
+
+import { testInjector, factory } from '@stryker-mutator/test-helpers';
+import { expect } from 'chai';
+import { Mutant, Range } from '@stryker-mutator/api/core';
+import { CheckResult, CheckStatus } from '@stryker-mutator/api/check';
+
+import { TypescriptChecker } from '../../src';
+import { createTypescriptOptions } from '../helpers/factories';
+
+const resolveTestResource = (path.resolve.bind(
+  path,
+  __dirname,
+  '..' /* integration */,
+  '..' /* test */,
+  '..' /* dist */,
+  'testResources',
+  'single-project'
+) as unknown) as typeof path.resolve;
+
+describe('Typescript checker on a single project', () => {
+  let sut: TypescriptChecker;
+
+  beforeEach(() => {
+    process.chdir(resolveTestResource());
+    testInjector.options.typescriptChecker = createTypescriptOptions();
+    sut = testInjector.injector.injectClass(TypescriptChecker);
+    return sut.init();
+  });
+
+  it('should not write output to disk', () => {
+    expect(fs.existsSync(resolveTestResource('dist')), 'Output was written to disk!').false;
+  });
+
+  it('should be able to validate a mutant that does not result in an error', async () => {
+    const mutant = createMutant('todo.ts', 'TodoList.allTodos.push(newItem)', 'newItem? 42: 43');
+    const expectedResult: CheckResult = {
+      status: CheckStatus.Passed,
+    };
+    const actual = await sut.check(mutant);
+    expect(actual).deep.eq(expectedResult);
+  });
+
+  it('should be able invalidate a mutant that does result in a compile error', async () => {
+    const mutant = createMutant('todo.ts', 'TodoList.allTodos.push(newItem)', '"This should not be a string 🙄"');
+    const actual = await sut.check(mutant);
+    expect(actual.status).deep.eq(CheckStatus.CompileError);
+    expect(actual.reason).has.string('todo.ts(15,9): error TS2322');
+  });
+
+  it('should be able validate a mutant that does not result in a compile error after a compile error', async () => {
+    // Arrange
+    const mutantCompileError = createMutant('todo.ts', 'TodoList.allTodos.push(newItem)', '"This should not be a string 🙄"');
+    const mutantWithoutError = createMutant('todo.ts', 'return TodoList.allTodos', '[]', 7);
+    const expectedResult: CheckResult = {
+      status: CheckStatus.Passed,
+    };
+
+    // Act
+    await sut.check(mutantCompileError);
+    const actual = await sut.check(mutantWithoutError);
+
+    // Assert
+    expect(actual).deep.eq(expectedResult);
+  });
+
+  it('should be able to invalidate a mutant that results in an error in a different file', async () => {
+    const result = await sut.check(createMutant('todo.ts', 'return totalCount;', ''));
+    expect(result.status).eq(CheckStatus.CompileError);
+    expect(result.reason).has.string('todo.spec.ts(4,7): error TS2322');
+  });
+
+  it('should be able to validate a mutant after a mutant in a different file resulted in a transpile error', async () => {
+    // Act
+    await sut.check(createMutant('todo.ts', 'return totalCount;', ''));
+    const result = await sut.check(createMutant('todo.spec.ts', "'Mow lawn'", "'this is valid, right?'"));
+
+    // Assert
+    const expectedResult: CheckResult = {
+      status: CheckStatus.Passed,
+    };
+    expect(result).deep.eq(expectedResult);
+  });
+
+  it('should be allow mutations in unrelated files', async () => {
+    // Act
+    const result = await sut.check(createMutant('not-type-checked.js', 'bar', 'baz'));
+
+    // Assert
+    const expectedResult: CheckResult = {
+      status: CheckStatus.Passed,
+    };
+    expect(result).deep.eq(expectedResult);
+  });
+
+  it('should allow unused local variables (override options)', async () => {
+    const mutant = createMutant('todo.ts', 'TodoList.allTodos.push(newItem)', '42');
+    const expectedResult: CheckResult = {
+      status: CheckStatus.Passed,
+    };
+    const actual = await sut.check(mutant);
+    expect(actual).deep.eq(expectedResult);
+  });
+});
+
+const fileContents = Object.freeze({
+  ['todo.ts']: fs.readFileSync(resolveTestResource('src', 'todo.ts'), 'utf8'),
+  ['todo.spec.ts']: fs.readFileSync(resolveTestResource('src', 'todo.spec.ts'), 'utf8'),
+  ['not-type-checked.js']: fs.readFileSync(resolveTestResource('src', 'not-type-checked.js'), 'utf8'),
+});
+
+function createMutant(
+  fileName: 'todo.ts' | 'todo.spec.ts' | 'not-type-checked.js',
+  findText: string,
+  replacement: string,
+  offset: number = 0
+): Mutant {
+  const originalOffset: number = fileContents[fileName].indexOf(findText);
+  if (originalOffset === -1) {
+    throw new Error(`Cannot find ${findText} in ${fileName}`);
+  }
+  const range: Range = [originalOffset + offset, originalOffset + findText.length];
+  return factory.mutant({
+    fileName: resolveTestResource('src', fileName),
+    mutatorName: 'foo-mutator',
+    range,
+    replacement,
+  });
+}
diff --git a/packages/typescript-checker/test/integration/typescript-checkers-errors.it.spec.ts b/packages/typescript-checker/test/integration/typescript-checkers-errors.it.spec.ts
new file mode 100644
index 0000000000..18e2a236a7
--- /dev/null
+++ b/packages/typescript-checker/test/integration/typescript-checkers-errors.it.spec.ts
@@ -0,0 +1,46 @@
+import path from 'path';
+
+import { testInjector } from '@stryker-mutator/test-helpers';
+import { expect } from 'chai';
+
+import { createTypescriptOptions } from '../helpers/factories';
+import { TypescriptChecker } from '../../src';
+
+const resolveTestResource = (path.resolve.bind(
+  path,
+  __dirname,
+  '..' /* integration */,
+  '..' /* test */,
+  '..' /* dist */,
+  'testResources',
+  'errors'
+) as unknown) as typeof path.resolve;
+
+describe('Typescript checker errors', () => {
+  let sut: TypescriptChecker;
+
+  beforeEach(() => {
+    testInjector.options.typescriptChecker = createTypescriptOptions();
+    sut = testInjector.injector.injectClass(TypescriptChecker);
+  });
+
+  it('should reject initialization if initial compilation failed', async () => {
+    process.chdir(resolveTestResource('compile-error'));
+    await expect(sut.init()).rejectedWith('TypeScript error(s) found in dry run compilation: add.ts(2,3): error TS2322:');
+  });
+
+  it('should reject initialization if tsconfig was invalid', async () => {
+    process.chdir(resolveTestResource('invalid-tsconfig'));
+    await expect(sut.init()).rejectedWith('TypeScript error(s) found in dry run compilation: tsconfig.json(1,1): error TS1005:');
+  });
+
+  it("should reject when tsconfig file doesn't exist", async () => {
+    process.chdir(resolveTestResource('empty-dir'));
+    await expect(sut.init()).rejectedWith(
+      `The tsconfig file does not exist at: "${resolveTestResource(
+        'empty-dir',
+        'tsconfig.json'
+      )}". Please configure the tsconfig file in your stryker.conf file using "typescriptChecker.tsconfigFile"`
+    );
+  });
+});
diff --git a/packages/typescript-checker/test/tsconfig.json b/packages/typescript-checker/test/tsconfig.json
new file mode 100644
index 0000000000..c3f30c9d6a
--- /dev/null
+++ b/packages/typescript-checker/test/tsconfig.json
@@ -0,0 +1,20 @@
+{
+  "extends": "../../../tsconfig.settings.json",
+  "compilerOptions": {
+    "outDir": "../dist/test",
+    "rootDir": ".",
+    "esModuleInterop": true,
+    "types": [
+      "mocha",
+      "node"
+    ]
+  },
+  "references": [
+    {
+      "path": "../src"
+    },
+    {
+      "path": "../../test-helpers/tsconfig.src.json"
+    }
+  ]
+}
diff --git a/packages/typescript-checker/test/unit/fs/hybrid-file-system.spec.ts b/packages/typescript-checker/test/unit/fs/hybrid-file-system.spec.ts
new file mode 100644
index 0000000000..462865e4a0
--- /dev/null
+++ b/packages/typescript-checker/test/unit/fs/hybrid-file-system.spec.ts
@@ -0,0 +1,180 @@
+import sinon from 'sinon';
+import ts from 'typescript';
+import { expect } from 'chai';
+
+import { HybridFileSystem } from '../../../src/fs';
+
+describe('fs', () => {
+  describe(HybridFileSystem.name, () => {
+    class Helper {
+      public readFileStub = sinon.stub(ts.sys, 'readFile');
+      public getModifiedTimeStub = sinon.stub(ts.sys, 'getModifiedTime');
+    }
+    let sut: HybridFileSystem;
+    let helper: Helper;
+    beforeEach(() => {
+      helper = new Helper();
+      sut = new HybridFileSystem();
+    });
+
+    describe(HybridFileSystem.prototype.writeFile.name, () => {
+      it('should create a new in-memory file', () => {
+        sut.writeFile('add.js', 'a + b');
+        const file = sut.getFile('add.js');
+        expect(file).ok;
+        expect(file!.content).eq('a + b');
+        expect(file!.fileName).eq('add.js');
+      });
+
+      it('should override an existing file', () => {
+        sut.writeFile('add.js', 'a + b');
+        sut.writeFile('add.js', 'a - b');
+        const file = sut.getFile('add.js');
+        expect(file!.content).eq('a - b');
+      });
+
+      it('should convert path separator to forward slashes', () => {
+        sut.writeFile('test\\foo\\a.js', 'a');
+        const actual = sut.getFile('test/foo/a.js');
+        expect(actual).ok;
+        expect(actual!.content).eq('a');
+      });
+    });
+
+    describe(HybridFileSystem.prototype.getFile.name, () => {
+      it("should read the file from disk if it wasn't loaded yet", () => {
+        // Arrange
+        const modifiedTime = new Date(2010, 1, 1);
+        helper.readFileStub.returns('content from disk');
+        helper.getModifiedTimeStub.returns(modifiedTime);
+
+        // Act
+        const actual = sut.getFile('foo.js');
+
+        // Assert
+        expect(helper.readFileStub).calledWith('foo.js');
+        expect(helper.getModifiedTimeStub).calledWith('foo.js');
+        expect(actual).ok;
+        expect(actual!.fileName).eq('foo.js');
+        expect(actual!.content).eq('content from disk');
+        expect(actual!.modifiedTime).eq(modifiedTime);
+      });
+
+      it('should convert path separator to forward slashes', () => {
+        helper.readFileStub.returns('content from disk');
+        helper.getModifiedTimeStub.returns(new Date(2010, 1, 1));
+        sut.getFile('test\\foo\\a.js');
+        expect(helper.readFileStub).calledWith('test/foo/a.js');
+        expect(helper.getModifiedTimeStub).calledWith('test/foo/a.js');
+      });
+
+      it("should cache a file that doesn't exists", () => {
+        sut.getFile('not-exists.js');
+        sut.getFile('not-exists.js');
+        expect(helper.readFileStub).calledOnce;
+      });
+    });
+
+    describe(HybridFileSystem.prototype.watchFile.name, () => {
+      it('should register a watcher', () => {
+        // Arrange
+        helper.readFileStub.returns('foobar');
+        const watcherCallback = sinon.stub();
+
+        // Act
+        sut.watchFile('foo.js', watcherCallback);
+        sut.writeFile('foo.js', 'some-content');
+
+        // Asset
+        expect(watcherCallback).calledWith('foo.js', ts.FileWatcherEventKind.Changed);
+      });
+
+      it('should convert path separator to forward slashes', () => {
+        helper.readFileStub.returns('content from disk');
+        helper.getModifiedTimeStub.returns(new Date(2010, 1, 1));
+        sut.watchFile('test\\foo\\a.js', sinon.stub());
+        expect(helper.readFileStub).calledWith('test/foo/a.js');
+      });
+
+      it("should throw if file doesn't exist", () => {
+        expect(() => sut.watchFile('not-exists.js', sinon.stub())).throws('Cannot find file not-exists.js for watching');
+      });
+    });
+
+    describe(HybridFileSystem.prototype.mutate.name, () => {
+      it('should mutate the file in-memory', () => {
+        // Arrange
+        helper.readFileStub.returns('a + b');
+        sut.watchFile('a.js', sinon.stub());
+
+        // Act
+        sut.mutate({ fileName: 'a.js', range: [2, 3], replacement: '-' });
+
+        // Assert
+        expect(sut.getFile('a.js')!.content).eq('a - b');
+      });
+
+      it('should convert path separator to forward slashes', () => {
+        helper.readFileStub.returns('a + b');
+        sut.watchFile('test/foo/a.js', sinon.stub());
+        sut.mutate({ fileName: 'test\\foo\\a.js', range: [2, 3], replacement: '-' });
+        expect(sut.getFile('test/foo/a.js')!.content).eq('a - b');
+      });
+
+      it('should notify the watcher', () => {
+        // Arrange
+        const watcher = sinon.stub();
+        helper.readFileStub.returns('a + b');
+        sut.watchFile('a.js', watcher);
+
+        // Act
+        sut.mutate({ fileName: 'a.js', range: [2, 3], replacement: '-' });
+
+        // Assert
+        expect(watcher).calledWith('a.js', ts.FileWatcherEventKind.Changed);
+      });
+
+      it('should reset previously mutated file', () => {
+        // Arrange
+        helper.readFileStub.withArgs('a.js').returns('a + b').withArgs('b.js').returns('"foo" + "bar"');
+        sut.watchFile('a.js', sinon.stub());
+        sut.watchFile('b.js', sinon.stub());
+
+        // Act
+        sut.mutate({ fileName: 'a.js', range: [2, 3], replacement: '-' });
+        sut.mutate({ fileName: 'b.js', range: [6, 7], replacement: '-' });
+
+        // Assert
+        expect(sut.getFile('a.js')!.content).eq('a + b');
+        expect(sut.getFile('b.js')!.content).eq('"foo" - "bar"');
+      });
+
+      it("should throw if file doesn't exist", () => {
+        expect(() => sut.mutate({ fileName: 'a.js', range: [2, 3], replacement: '-' })).throws('File "a.js" cannot be found.');
+      });
+    });
+
+    describe(HybridFileSystem.prototype.existsInMemory.name, () => {
+      it('should return true if it exists', () => {
+        sut.writeFile('a.js', 'a + b');
+        expect(sut.existsInMemory('a.js')).true;
+      });
+
+      it('should return false if it does not exist', () => {
+        sut.writeFile('b.js', 'a + b');
+        expect(sut.existsInMemory('a.js')).false;
+      });
+
+      it('should return false it is cached to not exist', () => {
+        helper.readFileStub.returns(undefined);
+        sut.getFile('a.js'); // caches that it doesn't exists
+        expect(sut.existsInMemory('a.js')).false;
+      });
+
+      it('should convert path separator to forward slashes', () => {
+        sut.writeFile('test/foo/a.js', 'foobar');
+        expect(sut.existsInMemory('test\\foo\\a.js')).true;
+      });
+    });
+  });
+});
diff --git a/packages/typescript-checker/test/unit/fs/script-file.spec.ts b/packages/typescript-checker/test/unit/fs/script-file.spec.ts
new file mode 100644
index 0000000000..220eb6bc51
--- /dev/null
+++ b/packages/typescript-checker/test/unit/fs/script-file.spec.ts
@@ -0,0 +1,139 @@
+import { expect } from 'chai';
+import sinon from 'sinon';
+import ts from 'typescript';
+
+import { ScriptFile } from '../../../src/fs/script-file';
+
+describe('fs', () => {
+  describe(ScriptFile.name, () => {
+    beforeEach(() => {
+      sinon.useFakeTimers();
+    });
+    describe('constructor', () => {
+      it('should reflect content, name and modified date', () => {
+        const modifiedTime = new Date(2010, 1, 1, 2, 3, 4, 4);
+        const sut = new ScriptFile('foo()', 'foo.js', modifiedTime);
+        expect(sut.fileName).eq('foo.js');
+        expect(sut.content).eq('foo()');
+        expect(sut.modifiedTime).eq(modifiedTime);
+      });
+
+      it('should default modifiedDate to now', () => {
+        const now = new Date(2010, 1, 2, 3, 4, 5, 6);
+        sinon.clock.setSystemTime(now);
+        const sut = new ScriptFile('', '');
+        expect(sut.modifiedTime.valueOf()).eq(new Date(2010, 1, 2, 3, 4, 5, 6).valueOf());
+      });
+    });
+
+    describe(ScriptFile.prototype.mutate.name, () => {
+      let sut: ScriptFile;
+      beforeEach(() => {
+        sut = new ScriptFile('add(a, b) { return a + b };', 'add.js', new Date(2010, 1, 1));
+        sut.watcher = sinon.stub();
+      });
+
+      it('should throw when no watcher is registered', () => {
+        sut.watcher = undefined;
+        expect(() => sut.mutate({ range: [21, 22], replacement: '-' })).throws('No watcher registered for add.js. Changes would go unnoticed');
+      });
+
+      it('should mutate the current content', () => {
+        sut.mutate({ range: [21, 22], replacement: '-' });
+        expect(sut.content).eq('add(a, b) { return a - b };');
+      });
+
+      it('should update the modified date', () => {
+        // Arrange
+        const now = new Date(2015, 1, 2, 3, 4, 5, 6);
+        sinon.clock.setSystemTime(now);
+
+        // Act
+        sut.mutate({ range: [1, 2], replacement: '' });
+
+        // Assert
+        expect(sut.modifiedTime).deep.eq(now);
+      });
+
+      it('should notify the file system watcher', () => {
+        sut.mutate({ range: [1, 2], replacement: '' });
+        expect(sut.watcher).calledWith('add.js', ts.FileWatcherEventKind.Changed);
+      });
+    });
+
+    describe(ScriptFile.prototype.resetMutant.name, () => {
+      let sut: ScriptFile;
+
+      beforeEach(() => {
+        sut = new ScriptFile('add(a, b) { return a + b };', 'add.js', new Date(2010, 1, 1));
+        sut.watcher = sinon.stub();
+      });
+
+      it('should reset the content after a mutation', () => {
+        sut.mutate({ replacement: 'replaces', range: [0, sut.content.length] });
+        sut.resetMutant();
+        expect(sut.content).eq('add(a, b) { return a + b };');
+      });
+
+      it('should throw when no watcher is registered', () => {
+        sut.watcher = undefined;
+        expect(() => sut.resetMutant()).throws('No watcher registered for add.js. Changes would go unnoticed');
+      });
+
+      it('should reset the content after two mutations', () => {
+        sut.mutate({ replacement: 'replaces', range: [0, sut.content.length] });
+        sut.mutate({ replacement: 'replaced a second time', range: [0, sut.content.length] });
+        sut.resetMutant();
+        expect(sut.content).eq('add(a, b) { return a + b };');
+      });
+
+      it('should notify the file system watcher', () => {
+        sut.resetMutant();
+        expect(sut.watcher).calledWith('add.js', ts.FileWatcherEventKind.Changed);
+      });
+
+      it('should update the modified date', () => {
+        // Arrange
+        const now = new Date(2015, 1, 2, 3, 4, 5, 6);
+        sinon.clock.setSystemTime(now);
+
+        // Act
+        sut.resetMutant();
+
+        // Assert
+        expect(sut.modifiedTime).deep.eq(now);
+      });
+    });
+
+    describe(ScriptFile.prototype.write.name, () => {
+      let sut: ScriptFile;
+
+      beforeEach(() => {
+        sut = new ScriptFile('add(a, b) { return a + b };', 'add.js', new Date(2010, 1, 1));
+      });
+
+      it('should write to the content', () => {
+        sut.write('overridden');
+        expect(sut.content).eq('overridden');
+      });
+
+      it('should inform the fs watcher', () => {
+        sut.watcher = sinon.stub();
+        sut.write('overridden');
+        expect(sut.watcher).calledWith('add.js', ts.FileWatcherEventKind.Changed);
+      });
+
+      it('should update the modified date', () => {
+        // Arrange
+        const now = new Date(2015, 1, 2, 3, 4, 5, 6);
+        sinon.clock.setSystemTime(now);
+
+        // Act
+        sut.write('overridden');
+
+        // Assert
+        expect(sut.modifiedTime).deep.eq(now);
+      });
+    });
+  });
+});
diff --git a/packages/typescript-checker/test/unit/typescript-helpers.spec.ts b/packages/typescript-checker/test/unit/typescript-helpers.spec.ts
new file mode 100644
index 0000000000..34db146d0e
--- /dev/null
+++ b/packages/typescript-checker/test/unit/typescript-helpers.spec.ts
@@ -0,0 +1,135 @@
+import path from 'path';
+
+import sinon from 'sinon';
+import ts from 'typescript';
+import { expect } from 'chai';
+
+import { determineBuildModeEnabled, overrideOptions, retrieveReferencedProjects, guardTSVersion } from '../../src/tsconfig-helpers';
+
+describe('typescript-helpers', () => {
+  describe(determineBuildModeEnabled.name, () => {
+    let readFileStub: sinon.SinonStub;
+
+    beforeEach(() => {
+      readFileStub = sinon.stub(ts.sys, 'readFile');
+    });
+
+    it('should throw an error if the tsconfig file could not be found', () => {
+      expect(() => determineBuildModeEnabled('tsconfig.json')).throws('File "tsconfig.json" not found');
+    });
+
+    it('should return true if the tsconfig file has references', () => {
+      readFileStub.returns('{ "references": [] }');
+      expect(determineBuildModeEnabled('foo.json')).true;
+      expect(readFileStub).calledWith('foo.json');
+    });
+
+    it('should return true if the tsconfig file has no references', () => {
+      readFileStub.returns('{ "compilerOptions": {} }');
+      expect(determineBuildModeEnabled('foo.json')).false;
+    });
+  });
+
+  describe(overrideOptions.name, () => {
+    it('should allow unreachable and unused code', () => {
+      expect(JSON.parse(overrideOptions({ config: {} }, false)).compilerOptions).deep.include({
+        allowUnreachableCode: true,
+        noUnusedLocals: false,
+        noUnusedParameters: false,
+      });
+      expect(
+        JSON.parse(
+          overrideOptions({ config: { compilerOptions: { allowUnreachableCode: false, noUnusedLocals: true, noUnusedParameters: true } } }, false)
+        ).compilerOptions
+      ).deep.include({
+        allowUnreachableCode: true,
+        noUnusedLocals: false,
+        noUnusedParameters: false,
+      });
+    });
+
+    it('should set --noEmit options when `--build` mode is off', () => {
+      expect(JSON.parse(overrideOptions({ config: {} }, false)).compilerOptions).deep.include({
+        noEmit: true,
+        incremental: false,
+        composite: false,
+        declaration: false,
+      });
+      expect(
+        JSON.parse(
+          overrideOptions(
+            {
+              config: {
+                compilerOptions: {
+                  noEmit: false,
+                  incremental: true,
+                  composite: true,
+                  declaration: true,
+                },
+              },
+            },
+            false
+          )
+        ).compilerOptions
+      ).deep.include({
+        noEmit: true,
+        incremental: false,
+        composite: false,
+        declaration: false,
+      });
+    });
+
+    it('should set --emitDeclarationOnly options when `--build` mode is on', () => {
+      expect(JSON.parse(overrideOptions({ config: {} }, true)).compilerOptions).deep.include({
+        emitDeclarationOnly: true,
+        noEmit: false,
+        declarationMap: false,
+      });
+      expect(
+        JSON.parse(
+          overrideOptions(
+            {
+              config: {
+                compilerOptions: {
+                  emitDeclarationOnly: false,
+                  noEmit: true,
+                  declarationMap: true,
+                },
+              },
+            },
+            true
+          )
+        ).compilerOptions
+      ).deep.include({
+        emitDeclarationOnly: true,
+        noEmit: false,
+        declarationMap: false,
+      });
+    });
+  });
+
+  describe(retrieveReferencedProjects.name, () => {
+    it('should result in an empty array when references are missing', () => {
+      expect(retrieveReferencedProjects({ config: { compilerOptions: {} } })).deep.eq([]);
+    });
+
+    it('should retrieve referenced projects', () => {
+      expect(retrieveReferencedProjects({ config: { references: [{ path: 'some.json' }] } })).deep.eq([path.resolve('some.json')]);
+    });
+  });
+
+  describe(guardTSVersion.name, () => {
+    it('should throw if typescript@2.5.0', () => {
+      sinon.stub(ts, 'version').value('3.5.0');
+      expect(guardTSVersion).throws('@stryker-mutator/typescript-checker only supports typescript@3.6 our higher. Found typescript@3.5.0');
+    });
+    it('should not throw if typescript@3.6.0', () => {
+      sinon.stub(ts, 'version').value('3.6.0');
+      expect(guardTSVersion).not.throws();
+    });
+    it('should not throw if typescript@4.0.0', () => {
+      sinon.stub(ts, 'version').value('4.0.0');
+      expect(guardTSVersion).not.throws();
+    });
+  });
+});
diff --git a/packages/typescript-checker/testResources/errors/compile-error/add.ts b/packages/typescript-checker/testResources/errors/compile-error/add.ts
new file mode 100644
index 0000000000..5c40274cca
--- /dev/null
+++ b/packages/typescript-checker/testResources/errors/compile-error/add.ts
@@ -0,0 +1,3 @@
+function add(a: number, b: number): string {
+  return a + b;
+}
diff --git a/packages/typescript-checker/testResources/errors/compile-error/tsconfig.json b/packages/typescript-checker/testResources/errors/compile-error/tsconfig.json
new file mode 100644
index 0000000000..b8a31e14ea
--- /dev/null
+++ b/packages/typescript-checker/testResources/errors/compile-error/tsconfig.json
@@ -0,0 +1,6 @@
+{
+  "compilerOptions": {
+    "target": "ES5",
+    "types": []
+  }
+}
diff --git a/packages/typescript-checker/testResources/errors/empty-dir/.gitkeep b/packages/typescript-checker/testResources/errors/empty-dir/.gitkeep
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/packages/typescript-checker/testResources/errors/invalid-tsconfig/tsconfig.json b/packages/typescript-checker/testResources/errors/invalid-tsconfig/tsconfig.json
new file mode 100644
index 0000000000..072ca9248e
--- /dev/null
+++ b/packages/typescript-checker/testResources/errors/invalid-tsconfig/tsconfig.json
@@ -0,0 +1 @@
+invalid tsconfig file
diff --git a/packages/typescript-checker/testResources/project-references/src/todo.ts b/packages/typescript-checker/testResources/project-references/src/todo.ts
new file mode 100644
index 0000000000..7ac1dd6631
--- /dev/null
+++ b/packages/typescript-checker/testResources/project-references/src/todo.ts
@@ -0,0 +1,23 @@
+export interface ITodo {
+  name: string;
+  description: string;
+  completed: boolean;
+}
+
+class Todo implements ITodo {
+  constructor(public name: string, public description: string, public completed: boolean) { }
+}
+
+export class TodoList {
+  public static allTodos: Todo[] = [];
+  createTodoItem(name: string, description: string) {
+    let newItem = new Todo(name, description, false);
+    let totalCount: number = TodoList.allTodos.push(newItem);
+    return totalCount;
+  }
+
+  allTodoItems(): ITodo[] {
+    return TodoList.allTodos;
+  }
+}
+
diff --git a/packages/typescript-checker/testResources/project-references/src/tsconfig.json b/packages/typescript-checker/testResources/project-references/src/tsconfig.json
new file mode 100644
index 0000000000..af64ba5a29
--- /dev/null
+++ b/packages/typescript-checker/testResources/project-references/src/tsconfig.json
@@ -0,0 +1,6 @@
+{
+  "extends": "../tsconfig.settings",
+  "compilerOptions": {
+    "outDir": "../dist/src"
+  }
+}
diff --git a/packages/typescript-checker/testResources/project-references/test/todo.spec.ts b/packages/typescript-checker/testResources/project-references/test/todo.spec.ts
new file mode 100644
index 0000000000..4d0aa4f7a2
--- /dev/null
+++ b/packages/typescript-checker/testResources/project-references/test/todo.spec.ts
@@ -0,0 +1,11 @@
+import { TodoList } from '../src/todo';
+
+const list = new TodoList();
+const n: number = list.createTodoItem('Mow lawn', 'Mow moving forward.')
+console.log(n);
+
+function addTodo(name = 'test', description = 'test') {
+  list.createTodoItem(name, description);
+}
+
+addTodo();
diff --git a/packages/typescript-checker/testResources/project-references/test/tsconfig.json b/packages/typescript-checker/testResources/project-references/test/tsconfig.json
new file mode 100644
index 0000000000..22f321ef72
--- /dev/null
+++ b/packages/typescript-checker/testResources/project-references/test/tsconfig.json
@@ -0,0 +1,9 @@
+{
+  "extends": "../tsconfig.settings",
+  "compilerOptions": {
+    "outDir": "../dist/test"
+  },
+  "references": [
+    { "path": "../src" }
+  ]
+}
diff --git a/packages/typescript-checker/testResources/project-references/tsconfig.root.json b/packages/typescript-checker/testResources/project-references/tsconfig.root.json
new file mode 100644
index 0000000000..d54e2806ef
--- /dev/null
+++ b/packages/typescript-checker/testResources/project-references/tsconfig.root.json
@@ -0,0 +1,7 @@
+{
+  "files": [],
+  "references": [
+    { "path": "./src" },
+    { "path": "./test" }
+  ]
+}
diff --git a/packages/typescript-checker/testResources/project-references/tsconfig.settings.json b/packages/typescript-checker/testResources/project-references/tsconfig.settings.json
new file mode 100644
index 0000000000..447ab8f8ac
--- /dev/null
+++ b/packages/typescript-checker/testResources/project-references/tsconfig.settings.json
@@ -0,0 +1,18 @@
+{
+  "compilerOptions": {
+    "strict": true,
+    "target": "es5",
+    "esModuleInterop": true,
+    "moduleResolution": "node",
+    "module": "commonjs",
+    "composite": true,
+    "declaration": true,
+    "declarationMap": true,
+
+    // These settings should be overridden by the typescript checker
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+
+    "types": []
+  }
+}
diff --git a/packages/typescript-checker/testResources/single-project/src/not-type-checked.js b/packages/typescript-checker/testResources/single-project/src/not-type-checked.js
new file mode 100644
index 0000000000..74aa322268
--- /dev/null
+++ b/packages/typescript-checker/testResources/single-project/src/not-type-checked.js
@@ -0,0 +1 @@
+const foo = 'bar';
diff --git a/packages/typescript-checker/testResources/single-project/src/todo.spec.ts b/packages/typescript-checker/testResources/single-project/src/todo.spec.ts
new file mode 100644
index 0000000000..7a3ec6a0c0
--- /dev/null
+++ b/packages/typescript-checker/testResources/single-project/src/todo.spec.ts
@@ -0,0 +1,12 @@
+import { TodoList } from './todo';
+
+const list = new TodoList();
+const n: number = list.createTodoItem('Mow lawn', 'Mow moving forward.')
+console.log(n);
+
+function addTodo(name = 'test', description = 'test') {
+  list.createTodoItem(name, description);
+}
+
+
+addTodo();
diff --git a/packages/typescript-checker/testResources/single-project/src/todo.ts b/packages/typescript-checker/testResources/single-project/src/todo.ts
new file mode 100644
index 0000000000..7ac1dd6631
--- /dev/null
+++ b/packages/typescript-checker/testResources/single-project/src/todo.ts
@@ -0,0 +1,23 @@
+export interface ITodo {
+  name: string;
+  description: string;
+  completed: boolean;
+}
+
+class Todo implements ITodo {
+  constructor(public name: string, public description: string, public completed: boolean) { }
+}
+
+export class TodoList {
+  public static allTodos: Todo[] = [];
+  createTodoItem(name: string, description: string) {
+    let newItem = new Todo(name, description, false);
+    let totalCount: number = TodoList.allTodos.push(newItem);
+    return totalCount;
+  }
+
+  allTodoItems(): ITodo[] {
+    return TodoList.allTodos;
+  }
+}
+
diff --git a/packages/typescript-checker/testResources/single-project/tsconfig.json b/packages/typescript-checker/testResources/single-project/tsconfig.json
new file mode 100644
index 0000000000..92e62065bc
--- /dev/null
+++ b/packages/typescript-checker/testResources/single-project/tsconfig.json
@@ -0,0 +1,16 @@
+{
+  "compilerOptions": {
+    "strict": true,
+    "target": "es5",
+    "esModuleInterop": true,
+    "moduleResolution": "node",
+    "module": "commonjs",
+    "outDir": "dist",
+
+    // These settings should be overridden by the typescript checker
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    
+    "types": []
+  }
+}
diff --git a/packages/typescript-checker/tsconfig.json b/packages/typescript-checker/tsconfig.json
new file mode 100644
index 0000000000..d5c40e5337
--- /dev/null
+++ b/packages/typescript-checker/tsconfig.json
@@ -0,0 +1,11 @@
+{
+  "files": [],
+  "references": [
+    {
+      "path": "./src"
+    },
+    {
+      "path": "./test"
+    }
+  ]
+}
diff --git a/packages/typescript-checker/tsconfig.stryker.json b/packages/typescript-checker/tsconfig.stryker.json
new file mode 100644
index 0000000000..3338399303
--- /dev/null
+++ b/packages/typescript-checker/tsconfig.stryker.json
@@ -0,0 +1,11 @@
+{
+  "extends": "./test/tsconfig.json",
+  "compilerOptions": {
+    "rootDir": ".",
+    "outDir": "dist"
+  },
+  "include": [
+    "src",
+    "test"
+  ],
+}
diff --git a/tsconfig.json b/tsconfig.json
index 77b67225ca..6b03636b24 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -15,6 +15,7 @@
     { "path": "packages/mocha-runner" },
     { "path": "packages/mutator-specification" },
     { "path": "packages/typescript" },
+    { "path": "packages/typescript-checker" },
     { "path": "packages/vue-mutator" },
     { "path": "packages/wct-runner" },
     { "path": "packages/webpack-transpiler" },
diff --git a/workspace.code-workspace b/workspace.code-workspace
index 1391450dbd..f25ee2c2d7 100644
--- a/workspace.code-workspace
+++ b/workspace.code-workspace
@@ -56,6 +56,10 @@
       "name": "typescript",
       "path": "packages/typescript"
     },
+    {
+      "name": "typescript-checker",
+      "path": "packages/typescript-checker"
+    },
     {
       "name": "vue-mutator",
       "path": "packages/vue-mutator"