Does `Promise.all()` Guarantee the Order of Resolved Values?

Promise.all: Order of Resolved Values

The Promise.all() method enables concurrent execution and resolution of multiple promises, offering developers a powerful tool for asynchronous programming. An intriguing question arises regarding the order of resolved values within the result array of Promise.all().

According to the Mozilla Developer Network (MDN), the results obtained from the .then() callback of Promise.all() appear to be ordered in the same sequence as the input promises. For instance:

var somePromises = [1, 2, 3, 4, 5].map(Promise.resolve);
return Promise.all(somePromises).then(function(results) {
  console.log(results) //  is [1, 2, 3, 4, 5] the guaranteed result?
});

Is this behavior guaranteed by the Promises specification? Let's delve into the technical details to find out.

Preserving Order in Promise.all()

In the Promise.all(iterable) method, iterable represents a collection of promises. The PerformPromiseAll(iterator, constructor, resultCapability) function is invoked internally, iterating through the iterable using IteratorStep(iterator).

When a promise settles with a resolved value, Promise.all() Resolve is invoked. Each resolved promise carries an internal [[Index]] slot, signifying its position within the original iterable.

Consequently, the output from Promise.all() maintains a strict order that corresponds to the order of the promises in the input iterable. This holds true as long as the provided iterable is strictly ordered, such as an array.

Demonstration of Order Preservation

Here's a practical example that illustrates the order preservation in Promise.all():

<code class="javascript">// Used to display results
const write = msg => {
  document.body.appendChild(document.createElement('div')).innerHTML = msg;
};

// Different speed async operations
const slow = new Promise(resolve => {
  setTimeout(resolve, 200, 'slow');
});
const instant = 'instant';
const quick = new Promise(resolve => {
  setTimeout(resolve, 50, 'quick');
});

// The order is preserved regardless of what resolved first
Promise.all([slow, instant, quick]).then(responses => {
  responses.map(response => write(response));
});</code>

In this scenario, the 'instant' value resolves immediately, followed by 'quick' and 'slow'. However, as per the order preservation in Promise.all(), the results will be displayed in the sequence: 'slow', 'instant', 'quick', aligning with the order of the promises within the input array.

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