async.race module

README.md

@@ -221,6 +221,7 @@ Some functions are also available in the following forms:
 * [`retry`](#retry)
 * [`iterator`](#iterator)
 * [`times`](#times), `timesSeries`, `timesLimit`
+* [`race`](#race)
 
 ### Utils
 
@@ -1655,6 +1656,46 @@ __Related__
 
 ---------------------------------------
 
+<a name="race" />
+### race(tasks, [callback])
+
+Runs the `tasks` array of functions in parallel, without waiting until the
+previous function has completed. Once any the `tasks` completed or pass an
+error to its callback, the main `callback` is immediately called. It's
+equivalent to `Promise.race()`.
+
+__Arguments__
+
+* `tasks` - An array containing functions to run. Each function is passed
+  a `callback(err, result)` which it must call on completion with an error `err`
+  (which can be `null`) and an optional `result` value.
+* `callback(err, result)` - A callback to run once any of the
+  functions have completed. This function gets an error or result from the
+  first function that completed.
+
+__Example__
+
+```js
+async.race([
+    function(callback){
+        setTimeout(function(){
+            callback(null, 'one');
+        }, 200);
+    },
+    function(callback){
+        setTimeout(function(){
+            callback(null, 'two');
+        }, 100);
+    }
+],
+// main callback
+function(err, result){
+    // the result will be equal to 'two' as it finishes earlier
+});
+```
+
+---------------------------------------
+
 <a name="memoize"></a>
 ### memoize(fn, [hasher])
 

lib/index.js

@@ -42,6 +42,7 @@ import parallel from './parallel';
 import parallelLimit from './parallelLimit';
 import priorityQueue from './priorityQueue';
 import queue from './queue';
+import race from './race';
 import reduce from './reduce';
 import reduceRight from './reduceRight';
 import reject from './reject';
@@ -106,6 +107,7 @@ export default {
     parallelLimit: parallelLimit,
     priorityQueue: priorityQueue,
     queue: queue,
+    race: race,
     reduce: reduce,
     reduceRight: reduceRight,
     reject: reject,
@@ -188,6 +190,7 @@ export {
     parallelLimit as parallelLimit,
     priorityQueue as priorityQueue,
     queue as queue,
+    race as race,
     reduce as reduce,
     reduceRight as reduceRight,
     reject as reject,

lib/race.js

@@ -0,0 +1,14 @@
+'use strict';
+
+import isArray from 'lodash/isArray';
+import noop from 'lodash/noop';
+import once from 'lodash/once';
+
+export default function race(tasks, cb) {
+    cb = once(cb || noop);
+    if (!isArray(tasks)) return cb(new TypeError('First argument to race must be an array of functions'));
+    if (!tasks.length) return cb();
+    for (let i = 0; i < tasks.length; i++) {
+        tasks[i](cb);
+    }
+}

mocha_test/race.js

@@ -0,0 +1,70 @@
+var async = require('../lib');
+var assert = require('assert');
+
+describe('race', function () {
+    it('should call each function in parallel and callback with first result', function raceTest10(done) {
+        var finished = 0;
+        var tasks = [];
+        function eachTest(i) {
+            var index = i;
+            return function (next) {
+                finished++;
+                next(null, index);
+            };
+        }
+        for (var i = 0; i < 10; i++) {
+            tasks[i] = eachTest(i);
+        }
+        async.race(tasks, function (err, result) {
+            assert.ifError(err);
+            //0 finished first
+            assert.strictEqual(result, 0);
+            assert.strictEqual(finished, 1);
+            async.setImmediate(function () {
+                assert.strictEqual(finished, 10);
+                done();
+            });
+        });
+    });
+    it('should callback with the first error', function raceTest20(done) {
+        var tasks = [];
+        function eachTest(i) {
+            var index = i;
+            return function (next) {
+                setTimeout(function () {
+                    next(new Error('ERR' + index));
+                }, 50 - index * 2);
+            };
+        }
+        for (var i = 0; i <= 5; i++) {
+            tasks[i] = eachTest(i);
+        }
+        async.race(tasks, function (err, result) {
+            assert.ok(err);
+            assert.ok(err instanceof Error);
+            assert.strictEqual(typeof result, 'undefined');
+            assert.strictEqual(err.message, 'ERR5');
+            done();
+        });
+    });
+    it('should callback when task is empty', function raceTest30(done) {
+        async.race([], function (err, result) {
+            assert.ifError(err);
+            assert.strictEqual(typeof result, 'undefined');
+            done();
+        });
+    });
+    it('should callback in error the task arg is not an Array', function raceTest40() {
+        var errors = [];
+        async.race(null, function (err) {
+            errors.push(err);
+        });
+        async.race({}, function (err) {
+            errors.push(err);
+        });
+        assert.strictEqual(errors.length, 2);
+        assert.ok(errors[0] instanceof TypeError);
+        assert.ok(errors[1] instanceof TypeError);
+    });
+});
+