[New] add types

index.d.ts

@@ -0,0 +1,3 @@
+declare function isSymbol(value: unknown): value is (symbol | Symbol);
+
+export = isSymbol;

index.js

@@ -8,13 +8,16 @@ var safeRegexTest = require('safe-regex-test');
 if (hasSymbols) {
 	var $symToStr = callBound('Symbol.prototype.toString');
 	var isSymString = safeRegexTest(/^Symbol\(.*\)$/);
+
+	/** @type {(value: object) => value is Symbol} */
 	var isSymbolObject = function isRealSymbolObject(value) {
 		if (typeof value.valueOf() !== 'symbol') {
 			return false;
 		}
 		return isSymString($symToStr(value));
 	};
 
+	/** @type {import('.')} */
 	module.exports = function isSymbol(value) {
 		if (typeof value === 'symbol') {
 			return true;
@@ -29,7 +32,7 @@ if (hasSymbols) {
 		}
 	};
 } else {
-
+	/** @type {import('.')} */
 	module.exports = function isSymbol(value) {
 		// this environment does not support Symbols.
 		return false && value;

package.json

@@ -12,6 +12,7 @@
 		"test": "npm run tests-only",
 		"posttest": "npx npm@'>=10.2' audit --production",
 		"lint": "eslint --ext=js,mjs .",
+		"postlint": "tsc -p . && attw -P",
 		"version": "auto-changelog && git add CHANGELOG.md",
 		"postversion": "auto-changelog && git add CHANGELOG.md && git commit --no-edit --amend && git tag -f \"v$(node -e \"console.log(require('./package.json').version)\")\""
 	},
@@ -39,7 +40,15 @@
 		"safe-regex-test": "^1.0.3"
 	},
 	"devDependencies": {
+		"@arethetypeswrong/cli": "^0.17.0",
 		"@ljharb/eslint-config": "^21.1.1",
+		"@ljharb/tsconfig": "^0.2.0",
+		"@types/call-bind": "^1.0.5",
+		"@types/for-each": "^0.3.3",
+		"@types/has-symbols": "^1.0.2",
+		"@types/object-inspect": "^1.13.0",
+		"@types/safe-regex-test": "^1.0.2",
+		"@types/tape": "^5.6.5",
 		"auto-changelog": "^2.5.0",
 		"encoding": "^0.1.13",
 		"es-value-fixtures": "^1.5.0",
@@ -51,7 +60,8 @@
 		"nyc": "^10.3.2",
 		"object-inspect": "^1.13.3",
 		"safe-publish-latest": "^2.0.0",
-		"tape": "^5.9.0"
+		"tape": "^5.9.0",
+		"typescript": "next"
 	},
 	"testling": {
 		"files": "test/index.js",

test/index.js

@@ -14,6 +14,7 @@ test('non-symbol values', function (t) {
 	var nonSymbols = v.nonSymbolPrimitives.concat(
 		Object(true),
 		Object(false),
+		// @ts-expect-error TS sucks with concat
 		{},
 		[],
 		/a/g,
@@ -36,9 +37,12 @@ test('faked symbol values', function (t) {
 	});
 
 	t.test('faked @@toStringTag', { skip: !hasToStringTag }, function (st) {
+		/** @type {{ valueOf(): unknown; [Symbol.toStringTag]?: unknown }} */
 		var fakeSymbol = { valueOf: function () { return Symbol('foo'); } };
 		fakeSymbol[Symbol.toStringTag] = 'Symbol';
 		st.equal(isSymbol(fakeSymbol), false, 'object with fake Symbol @@toStringTag and valueOf returning a symbol is not a symbol');
+
+		/** @type {{ valueOf(): unknown; [Symbol.toStringTag]?: unknown }} */
 		var notSoFakeSymbol = { valueOf: function () { return 42; } };
 		notSoFakeSymbol[Symbol.toStringTag] = 'Symbol';
 		st.equal(isSymbol(notSoFakeSymbol), false, 'object with fake Symbol @@toStringTag and valueOf not returning a symbol is not a symbol');
@@ -53,12 +57,14 @@ test('faked symbol values', function (t) {
 
 test('Symbol support', { skip: !hasSymbols }, function (t) {
 	t.test('well-known Symbols', function (st) {
+		/** @type {(name: string) => name is Exclude<keyof SymbolConstructor, 'for' | 'keyFor'>} */
 		var isWellKnown = function filterer(name) {
 			return name !== 'for' && name !== 'keyFor' && !(name in filterer);
 		};
 		var wellKnownSymbols = Object.getOwnPropertyNames(Symbol).filter(isWellKnown);
 		wellKnownSymbols.forEach(function (name) {
-			var sym = Symbol[name];
+			// eslint-disable-next-line no-extra-parens
+			var sym = Symbol[/** @type {keyof SymbolConstructor} */ (name)];
 			st.equal(isSymbol(sym), true, inspect(sym) + ' is a symbol');
 		});
 		st.end();

tsconfig.json

@@ -0,0 +1,9 @@
+{
+	"extends": "@ljharb/tsconfig",
+	"compilerOptions": {
+		"target": "ES2021",
+	},
+	"exclude": [
+		"coverage"
+	]
+}