Add TypeScript definition (#11)

index.d.ts

@@ -0,0 +1,29 @@
+export interface Limit {
+	/**
+	 * @param fn - Promise-returning/async function.
+	 * @param arguments - Any arguments to pass through to `fn`. Support for passing arguments on to the `fn` is provided in order to be able to avoid creating unnecessary closures. You probably don't need this optimization unless you're pushing a lot of functions.
+	 * @returns The promise returned by calling `fn(...arguments)`.
+	 */
+	<Arguments extends unknown[], ReturnType>(
+		fn: (...arguments: Arguments) => PromiseLike<ReturnType> | ReturnType,
+		...arguments: Arguments
+	): Promise<ReturnType>;
+
+	/**
+	 * The number of promises that are currently running.
+	 */
+	readonly activeCount: number;
+
+	/**
+	 * The number of promises that are waiting to run (i.e. their internal `fn` was not called yet).
+	 */
+	readonly pendingCount: number;
+}
+
+/**
+ * Run multiple promise-returning & async functions with limited concurrency.
+ *
+ * @param concurrency - Concurrency limit. Minimum: `1`.
+ * @returns A `limit` function.
+ */
+export default function pLimit(concurrency: number): Limit;

index.js

@@ -1,7 +1,7 @@
 'use strict';
 const pTry = require('p-try');
 
-module.exports = concurrency => {
+const pLimit = concurrency => {
 	if (concurrency < 1) {
 		throw new TypeError('Expected `concurrency` to be a number from 1 and up');
 	}
@@ -47,3 +47,6 @@ module.exports = concurrency => {
 
 	return generator;
 };
+
+module.exports = pLimit;
+module.exports.default = pLimit;

index.test-d.ts

@@ -0,0 +1,18 @@
+import {expectType} from 'tsd-check';
+import pLimit from '.';
+
+const limit = pLimit(1);
+
+const input = [
+	limit(() => Promise.resolve('foo')),
+	limit(() => Promise.resolve('bar')),
+	limit(() => Promise.resolve(undefined)),
+];
+
+expectType<Promise<Array<string | undefined>>>(Promise.all(input));
+
+expectType<Promise<string>>(limit((a: string) => '', 'test'));
+expectType<Promise<string>>(limit((a: string, b: number) => Promise.resolve(''), 'test', 1));
+
+expectType<number>(limit.activeCount);
+expectType<number>(limit.pendingCount);

package.json

@@ -13,10 +13,11 @@
 		"node": ">=6"
 	},
 	"scripts": {
-		"test": "xo && ava"
+		"test": "xo && ava && tsd-check"
 	},
 	"files": [
-		"index.js"
+		"index.js",
+		"index.d.ts"
 	],
 	"keywords": [
 		"promise",
@@ -44,6 +45,7 @@
 		"in-range": "^1.0.0",
 		"random-int": "^1.0.0",
 		"time-span": "^2.0.0",
+		"tsd-check": "^0.3.0",
 		"xo": "^0.23.0"
 	}
 }