JavaScript asynchronous programming: pending states of async/await and promises
async/await is an elegant way to handle asynchronous operations in JavaScript, but its behavior is worth exploring in depth when the Promise is in a long-term pending state. This article will analyze the performance of await when the Promise is not resolve or reject .
Scenario Analysis
Consider the following code:
async function yyy() {
console.log('yyy calls xxx to start');
let res = await xxx();
console.log('yyy call xxx ends', res);
}
async function xxx() {
return new Promise(function(resolve, reject) {
// No resolve or reject call});
}
yyy();
The xxx function returns a promise that is always in the pending state. What happens with await xxx() in the yyy function?
Environment Differences: Node.js vs. Browser
The behavior of await is affected by the operating environment:
Node.js: If Promise remains pending and there are no other active asynchronous operations (such as timers, network requests), the Node.js event loop will consider the program to be completed and exited. It won't wait for the pending Promise indefinitely.
Browser: In a browser environment, await will usually wait for the Promise to complete. However, if other asynchronous tasks are lacking to drive the event loop, the browser engine may think that the script has been executed and will not block page rendering or interaction. But if the script is executed synchronously (for example, directly
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