diff --git a/lib/tsserver.js b/lib/tsserver.js
index 77348ce6e8aaf..ed62dd90686ef 100644
--- a/lib/tsserver.js
+++ b/lib/tsserver.js
@@ -42743,9 +42743,10 @@ var ts;
                 this.host = host;
                 this.project = project;
                 this.ls = null;
-                this.filenameToScript = {};
                 this.roots = [];
-                this.resolvedModuleNames = ts.createFileMap(ts.createGetCanonicalFileName(host.useCaseSensitiveFileNames));
+                var getCanonicalFileName = ts.createGetCanonicalFileName(host.useCaseSensitiveFileNames);
+                this.resolvedModuleNames = ts.createFileMap(getCanonicalFileName);
+                this.filenameToScript = ts.createFileMap(getCanonicalFileName);
                 this.moduleResolutionHost = {
                     fileExists: function (fileName) { return _this.fileExists(fileName); },
                     readFile: function (fileName) { return _this.host.readFile(fileName); }
@@ -42824,33 +42825,29 @@ var ts;
             };
             LSHost.prototype.removeReferencedFile = function (info) {
                 if (!info.isOpen) {
-                    this.filenameToScript[info.fileName] = undefined;
+                    this.filenameToScript.remove(info.fileName);
                     this.resolvedModuleNames.remove(info.fileName);
                 }
             };
             LSHost.prototype.getScriptInfo = function (filename) {
-                var scriptInfo = ts.lookUp(this.filenameToScript, filename);
+                var scriptInfo = this.filenameToScript.get(filename);
                 if (!scriptInfo) {
                     scriptInfo = this.project.openReferencedFile(filename);
                     if (scriptInfo) {
-                        this.filenameToScript[scriptInfo.fileName] = scriptInfo;
+                        this.filenameToScript.set(scriptInfo.fileName, scriptInfo);
                     }
                 }
-                else {
-                }
                 return scriptInfo;
             };
             LSHost.prototype.addRoot = function (info) {
-                var scriptInfo = ts.lookUp(this.filenameToScript, info.fileName);
-                if (!scriptInfo) {
-                    this.filenameToScript[info.fileName] = info;
+                if (!this.filenameToScript.contains(info.fileName)) {
+                    this.filenameToScript.set(info.fileName, info);
                     this.roots.push(info);
                 }
             };
             LSHost.prototype.removeRoot = function (info) {
-                var scriptInfo = ts.lookUp(this.filenameToScript, info.fileName);
-                if (scriptInfo) {
-                    this.filenameToScript[info.fileName] = undefined;
+                if (!this.filenameToScript.contains(info.fileName)) {
+                    this.filenameToScript.remove(info.fileName);
                     this.roots = copyListRemovingItem(info, this.roots);
                     this.resolvedModuleNames.remove(info.fileName);
                 }
@@ -42890,7 +42887,7 @@ var ts;
                 return this.host.directoryExists(path);
             };
             LSHost.prototype.lineToTextSpan = function (filename, line) {
-                var script = this.filenameToScript[filename];
+                var script = this.filenameToScript.get(filename);
                 var index = script.snap().index;
                 var lineInfo = index.lineNumberToInfo(line + 1);
                 var len;
@@ -42904,13 +42901,13 @@ var ts;
                 return ts.createTextSpan(lineInfo.offset, len);
             };
             LSHost.prototype.lineOffsetToPosition = function (filename, line, offset) {
-                var script = this.filenameToScript[filename];
+                var script = this.filenameToScript.get(filename);
                 var index = script.snap().index;
                 var lineInfo = index.lineNumberToInfo(line);
                 return (lineInfo.offset + offset - 1);
             };
             LSHost.prototype.positionToLineOffset = function (filename, position) {
-                var script = this.filenameToScript[filename];
+                var script = this.filenameToScript.get(filename);
                 var index = script.snap().index;
                 var lineOffset = index.charOffsetToLineNumberAndPos(position);
                 return { line: lineOffset.line, offset: lineOffset.offset + 1 };