Promises in ECMAScript

Demo 1

Create new Javascript file named demo1.js. This file contains code demo as below:

var p = new Promise((resolve, reject) => resolve(5));
p.then((val) => console.log('Value: ' + val));

Run Demo 1

Use node demo1.js statement run code demo

Value: 5

Demo 2

Create new Javascript file named demo2.js. This file contains code demo as below:

const isSmallThenTen = (num) => {
    return new Promise((resolve, reject) => {
        if (num < 10) {
            resolve(true)
        } else {
            reject(false)
        }
    })
};

isSmallThenTen(91)
    .then(res => console.log('The number is smaller then 10'))
    .catch(err => console.log('The number is greater than 10'));

isSmallThenTen(5)
    .then(res => console.log('The number is smaller then 10'))
    .catch(err => console.log('The number is greater than 10'));

Run Demo 2

Use node demo2.js statement run code demo

The number is smaller then 10
The number is greater than 10

Demo 3

Create new Javascript file named demo3.js. This file contains code demo as below:

const timeoutIn = (time) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(time), time)
    })
}

timeoutIn(2000).then(res => console.log(`Resloved in ${res/1000} seconds`))

Run Demo 3

Use node demo3.js statement run code demo

Resloved in 2 seconds

Demo 4

Create new Javascript file named demo4.js. This file contains code demo as below:

var p1 = new Promise((resolve, reject) => {
    if (true)
        throw new Error("rejected!");
    else
        resolve(4);
});
p1.then((val) => val + 2)
    .then((val) => console.log("got ", val))
    .catch((err) => console.log("error: ", err.message));

var p2 = new Promise((resolve, reject) => {
    resolve(4);
});
p2.then((val) => val + 2)
    .then((val) => { throw new Error("step 2 failed!") })
    .then((val) => console.log("got ", val))
    .catch((err) => console.log("error: ", err.message));

Run Demo 4

Use node demo4.js statement run code demo

error:  rejected!
error:  step 2 failed!