Preface
And are two important methods of the Promise API in JavaScript, which exhibit different behaviors when processing multiple Promise objects. Here are the main differences between them:
1. Functions and behaviors
：
Function: Receive an array (or class array object) containing multiple promises as a parameter and return a new promise.
Behavior: Only when all Promise objects in the array complete successfully (i.e., the state becomes fulfilled), the returned Promise object will complete successfully, and all the results of the Promise are returned as an array. If any of the Promise objects in the array fails (i.e., the state becomes rejected), the returned Promise object will fail immediately and return the cause of the first failed Promise as the cause of the failure.
Usage scenario: It is suitable for scenarios where you need to wait for all asynchronous operations to complete successfully before performing the next operation, such as loading multiple resources at the same time and displaying the page after all resources are loaded.
：
Function: Also receives an array (or class array object) containing multiple promises as a parameter and returns a new promise.
Behavior: This returned Promise will be resolved or rejected when any Promise state in the array becomes fulfilled or rejected, and will be returned as the result (or reason) of the first resolved Promise. If all the promises in the array are rejected, the returned promise will be rejected with the reason for the first rejected Promise.
Usage scenario: It is suitable for scenarios where the timeout mechanism is set or only care about the first asynchronous operation completed. For example, if a request is initiated and a timeout time is set, the request result is returned if the request is completed within the timeout time; if the timeout, the timeout error is returned.
2. Return value and status
: Returns a new Promise whose status depends on the status of all incoming Promise objects. If all Promises are completed successfully, the new Promise is completed successfully, and returns an array of all results; if any of the Promises fails, the new Promise also fails, and returns the first reason for the failure.
: Also returns a new Promise, but its status is determined only by the first resolved Promise. Regardless of whether the first resolution Promise succeeds or fails, the status of the new Promise will be consistent with it and will return the corresponding result or cause.
3. Example
Example:
const promise1 = (3);
const promise2 = new Promise((resolve, reject) => setTimeout(resolve, 100, 'foo'));
const promise3 = new Promise((resolve, reject) => setTimeout(resolve, 50, 'bar'));
([promise1, promise2, promise3]).then((values) => {
(values); // [3, 'foo', 'bar']
});
Example：
javascript
const promise1 = new Promise((resolve, reject) => setTimeout(resolve, 500, 'one'));
const promise2 = new Promise((resolve, reject) => setTimeout(resolve, 100, 'two'));
([promise1, promise2]).then((value) => {
(value); // "two"
});Summarize
Different mechanisms are provided when processing multiple Promise objects. It is suitable for scenarios where you need to wait for all asynchronous operations to complete successfully, and for scenarios where you only care about the first completed asynchronous operations. Understanding the difference is crucial to writing efficient and reliable asynchronous code in JavaScript.