How Can I Execute Multiple Async JavaScript Functions Concurrently?

Executing Async Functions Concurrently in JavaScript

In JavaScript, awaiting multiple async functions may give the impression that they execute sequentially rather than concurrently. This raises the question: how can we achieve parallel execution of these functions?

Using Promise.all() for Parallelism

To execute async functions in parallel, you can employ the Promise.all() function. It takes an array of promises as an argument and returns a single promise that resolves when all the promises in the array resolve.

await Promise.all([someCall(), anotherCall()]);

In this code, both someCall() and anotherCall() are called simultaneously, and the script proceeds only when both functions have finished executing.

Storing the Results

If you want to access the results of the parallel async calls, you can destructure the array returned by Promise.all():

let [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);

Caveat: Fail-Fast Behavior

It's important to note that Promise.all() follows a "fail-fast" approach. If any of the promises in the array rejects, the entire Promise.all() promise will reject with the error from the first failed promise:

// If any of the promises fails, it will reject
Promise.all([happy('happy', 100), sad('sad', 50)]).then(console.log).catch(console.log);

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