Promises based solution. Here we convert all callback based functions to promise based, and then the logic to check for errored tasks and incomplete tasks goes into the .then() function.
<pre class="shiki github-dark shaku"><code>/*type Callback = (error: Error, data: any) =&gt; voidtype AsyncFunc = ( callback: Callback, data: any) =&gt; void*//** * @param {AsyncFunc[]} funcs * @return {(callback: Callback) =&gt; void} */function parallel(funcs){ funcs = funcs.map(promisify) let out = []; let errored = false; return function(callback, value) { for(let [index, func] of funcs.entries()) { func().then( (value) =&gt; { if (errored) return; out[index] = value; if (out.length === funcs.length) { callback(undefined, out); } }, (error) =&gt; { if (!errored) { errored = true; callback(error, undefined); return; } } ) } }}function promisify(func) { return function(...args) { return new Promise((resolve, reject) =&gt; { func((err, val) =&gt; { if (err) { reject(err); } else { resolve(val); } }, ...args) }) }}</code></pre>

PROMISES BASED SOLUTION

This problem is related to 29. implement async helper - sequence(). You are asked to implement an async function helper, parallel() which works like P…