5 Awesome JavaScript Promise Tricks
Promises are a big invention of javascript. Previously we usesetTimeouts and settling for synchronous operations to doing everything possible to leverage this new async API. Let's check out a handful of awesome Promises tricks!
Cancel a fetch Request
One problem we instantly complained about with promises was not being able to cancel them. A simple promiseInstance.cancel() would have been excellent but never came. Instead, we were given an API that was way more complicated:
const controller = new AbortController();
const { signal } = controller;fetch("http://localhost:8000", { signal }).then(response => {
console.log(`Request 1 is complete!`);
}).catch(e => {
console.warn(`Fetch 1 error: ${e.message}`);
});// Abort request
controller.abort();
The magic here is providing the signal with each fetch request. In the JavaScript world, we inherit difficult APIs and do wonders to abstract them, and thus we'll find a way to better abstract this API.
waitForTime & waitForever
Waiting for a duration is useful in loads of production and testing situations — it’s never ideal but always helpful. I’ve used two awesome functions to make my life better:
/* Wait for milliseconds */
function waitForTime(ms) {
return new Promise(r => setTimeout(r, ms));
}/* Usage */
await waitForTime(200);/* Wait Forever */
function waitForever() {
return new Promise(r => {});
}// Usage:
await waitForever();
Don’t wait for perfect situations, wait for the time you need.
Async Array Functions
Array functions like forEach, map, and other functions are used frequently without the need for them to be synchronous. We don't think about it there's a fair amount of times we can go async with our operations.
const promises = [1, 2, 3].map(async (num) => {
console.log(num);
});await promises;
The difference in caring between async and sync is Promise.allSettled. Go async when you can!
then on Objects
Did you know that you can arbitrarily add a then method on objects to have them treated as a Promise?
j = { then: resolve => fetch("/").then(resolve) }j.then(res => console.log(res));
// Response {type: "basic", url: "https://davidwalsh.name/", redirected: false, status: 200, ok: true, …}// ... or an await...
const response = await j;
// Response {type: "basic", url: "https://davidwalsh.name/", redirected: false, status: 200, ok: true, …}
Now you know! An excellent trick most don’t know about!
Detect an Async Function
Not something you would need to do often but this post is about tricks, right? If you want to detect an asynchronous function, you always can:
async function myFunction() {}const isAsync = myFunction.constructor.name === "AsyncFunction";
JavaScript Promises are something we every day but a broader look at them allows us to innovate! Have any Promise tricks of your own? Share it with me.
Source- https://davidwalsh.name/javascript-promise-tricks
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Harsh Patel | Full-Stack Developer | http://imharshpatel.com
