Implement an async function helper parallel() that executes multiple asynchronous functions concurrently and collects their results, similar to Promise.all(). Unlike sequence(), all functions run simultaneously rather than waiting for each other.
Requirements
- Concurrent execution – Execute all async functions simultaneously, not sequentially
- Result collection – Collect all successful results into an array in the same order
- Error handling – Return the first error encountered, ignoring subsequent errors or results
- Callback interface – Each async function follows the Node.js callback pattern
(error, data) => void
- Return async function –
parallel() should return a new async function that can be called
Key Behaviors
- Parallel execution – All functions start simultaneously and run concurrently
- Result ordering – Results are collected in the same order as the input functions
- Early error termination – On first error, ignore remaining results and pass error to callback
- Completion tracking – Wait for all functions to complete before calling final callback
- Flexible input – Accept any number of async functions in the array
Example
const async1 = (callback) => {
setTimeout(() => callback(null, 1), 100);
};
const async2 = (callback) => {
setTimeout(() => callback(null, 2), 50);
};
const async3 = (callback) => {
setTimeout(() => callback(null, 3), 150);
};
const all = parallel([async1, async2, async3]);
all((error, data) => {
console.log(data); // [1, 2, 3]
}, 'initial');
// Error handling
const asyncFail = (callback) => {
setTimeout(() => callback(new Error('Something failed')), 75);
};
const parallelWithError = parallel([async1, asyncFail, async3]);
parallelWithError((error, data) => {
console.log(error.message); // "Something failed"
console.log(data); // undefined
});
Key Challenge
The function must coordinate multiple concurrent async operations, track completion status, handle the first error encountered, and collect results in the correct order while managing the callback-based async flow.