js create a promise
/*
A Promise is a proxy for a value not necessarily known when the promise is created.
It allows you to associate handlers with an asynchronous action's eventual success
value or failure reason.
*/
let promise = new Promise((resolve , reject) => {
fetch("https://myAPI")
.then((res) => {
// successfully got data
resolve(res);
})
.catch((err) => {
// an error occured
reject(err);
});
});what is promise in javascript
Promises make async javascript easier as they are easy to use and write than callbacks. Basically , promise is just an object , that gives us either success of async opertion or failue of async operationspromise resolve reject
function testFunction() {
return new Promise(function (resolve, reject) {
setTimeout(function () {
resolve('hello!');
}, 1000);
})
}
testFunction()
.then(function (value) {
console.log(value);
})
.catch(function (error) {
console.log(error);
})making promises in js
getData()
.then(data => console.log(data))
.catch(error => console.log(error));Promise.resolve
var original = Promise.resolve(33);
original.then( val => console.log('origianl', val);
var cast = Promise.resolve(original);
cast.then(function(value) {
console.log('value: ' + value);
});
console.log('original === cast ? ' + (original === cast));
// logs, in order:
// original === cast ? true
// value: 33resolve vs return promise js
In simple terms, inside a then handler function:
A) When x is a value (number, string, etc):
- return x is equivalent to return Promise.resolve(x)
- throw x is equivalent to return Promise.reject(x)
B) When x is a Promise that is already settled (not
pending anymore):
- return x is equivalent to return Promise.resolve(x),
if the Promise was already resolved.
- return x is equivalent to return Promise.reject(x),
if the Promise was already rejected.
C) When x is a Promise that is pending:
- return x will return a pending Promise, and it will
be evaluated on the subsequent then.
Read more on this topic on the Promise.prototype.then() docs.Copyright © 2021 Codeinu
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