I would like to share some of my experience with javascript Promise. I will add on to this list when I have more promise examples to share.
Promise
Taken from MDN:
The
Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.
A Promise has 3 states:
i) pending: initial state when a promise has not been fulfilled nor rejected yet.
ii) fulfilled: the operation within a promise is executed successfully.
iii) rejected: the operation within a promise failed to complete.
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MDN Promise handling and chaining |
Promise related examples
1) Wrapping functions that return a callback with a promise
// a function that takes a callback function
function foo(type, callback) {
callback(type);
}
function fooPromise(param) {
return new Promise(function(resolve, reject) {
foo(param, (err, res) => {
if (err !== null)
reject(err);
resolve(res);
});
});
}
2) Returning a promise within a promise:
var innerPromise = function() {
return new Promise(function(resolve, reject) {
setTimeout(() => resolve("innerPromise fulfilled"), 1000);
});
}
var outerPromise = function() {
// return a promise that contains another promise
return new Promise(function(resolve, reject) {
// calling innerPromise
innerPromise().then(function(res) {
// innerPromise is done
console.log(res);
// resolve with some value / object
setTimeout(() => resolve("outerPromise fulfilled"), 1000);
})
});
}
outerPromise()
.then(res => console.log(res))
.catch(err => console.log(err));
// on console
// innerPromise fulfilled
// outerPromise fulfilled
I will keep updating this list. Thanks for reading!
Jun
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