ES6 Promise Example

promises.js

/*
$ node promises.js 
1: Hello
2: Hello, world
3: Hello, world. I am a Promise.
4: Error: REJECTION!
*/

const assert = require('assert')


new Promise((resolve, reject) => {

    // Some asynchronous object that will eventually resolve
    setTimeout(() => {
        resolve('Hello')
    }, 300);

}).then(result => {

    // We will get 'Hello'
    assert.strictEqual(result, 'Hello')
    console.log(`1: ${result}`)

    // Now we can chain a result, if we want
    return result + ', world'

}).then(result => {

    // We will get 'Hello, world'
    assert.strictEqual(result, 'Hello, world')
    console.log(`2: ${result}`)

    // We can continue this chain, and it can either be a random value, or a
    // Promise, so let's return a Promise.
    return new Promise((resolve, reject) => {
        resolve(`${result}. I am a Promise.`)
    })

}).then(result => {

    // Note how the result is not a Promise,
    // but is 'Hello, world. I am a Promise.'
    // This is because the Promise is resolved before this callback is called,
    // and this callback is called with the value that was resolved.
    assert.strictEqual(result, 'Hello, world. I am a Promise.')
    console.log(`3: ${result}`)

    // Now, we could also reject. whe
    return new Promise((resolve, reject) => {
        reject(new Error("REJECTION!"))
    })

}).then(result => {
    assert.fail("I should never be called")

}).then(result => {
    assert.fail("Neither will I")

}).catch(error => {

    // The previous two are skipped because of the reject.
    console.log(`4: ${error}`)
    assert.strictEqual(error.message, "REJECTION!")
})