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Intro to Promises

Why JS only has one thread

JS was designed to be a simple scripting language for the Netscape browser to make websites interactive.

Not the powerhouse that can do all the things like today.

Only having one thread makes programs simpler because you don't have to deal with locks and concurrency issues.

We can still run code on multiple threads

How the event loop works


Why callbacks?

  • execute code after something else finishes


function printContents(err, data) {
    if (err) console.error(err);

fs.readFile("package.json", "utf8", printContents);

Problem with callbacks: Callback hell

const fs = require("fs");

fs.readFile("package.json", "utf8", function (err, data) {
    if (err) console.error(err);
    fs.readFile("file2", "utf8", function (err, data) {
        if (err) console.error(err);
        fs.readFile("file3", "utf8", function (err, data) {
            if (err) console.error(err);
            fs.readFile("file4", "utf8", function (err, data) {
                if (err) console.error(err);
                fs.readFile("file5", "utf8", function (err, data) {
                    if (err) console.error(err);
                    fs.readFile("file6", "utf8", function (err, data) {
                        if (err) console.error(err);
                        fs.readFile("file7", "utf8", function (err, data) {
                            if (err) console.error(err);
                            fs.readFile("file8", "utf8", function (err, data) {
                                if (err) console.error(err);

Problem of Inversion of Control


UI Dot Dev Async JS

Busy Restaurant

  • callbacks: give the restaurant (3rd party library) your phone number (callback)
    • we trust that they'll use the callback the way we expect
    • call it once
    • call it at the right time
    • pass in the right args
    • reality
      • they can run the callback multiple times
      • phone: can send you ads
      • can't take back your phone number


  • fulfill a contract
    • can't resolved 3 times in a row
      • (state machine: once it's resolved, it can't go to another state)
  • promises: restaurant gives you a beeper that lets you know when you have a table
  • you decide what to do when the .then() is called or the .catch() is called


.then() example

let file1;
file8Promise = fs
    .readFile("file1", "utf8")
    .then(function (_file1) {
        // NOTE: you NEED to return in order for the data to get passed into the function
        file1 = _file1;
        return fs.promises.readFile("file2", "utf8");
    .then(function (file2) {
        console.log(_file1); // errors out
        return fs.promises.readFile("file3", "utf8");
    .then(function (file3) {
        return fs.promises.readFile("file4", "utf8");
    .then(function (file4) {
        return fs.promises.readFile("file5", "utf8");
    .then(function (file5) {
        return fs.promises.readFile("file6", "utf8");
    .then(function (file6) {
        return fs.promises.readFile("file7", "utf8");
    .then(function (file7) {
        return fs.promises.readFile("file8", "utf8");
    .catch(function () {
        // if any of the 8 error out


async function readFiles() {
    try {
        const file1 = await fs.promises.readFile("file1", "utf8");
        const file2 = await fs.promises.readFile("file2", "utf8");
        const file3 = await fs.promises.readFile("file3", "utf8");
        const file4 = await fs.promises.readFile("file4", "utf8");
        const file5 = await fs.promises.readFile("file5", "utf8");
        const file6 = await fs.promises.readFile("file6", "utf8");
        const file7 = await fs.promises.readFile("file7", "utf8");
        const file8 = await fs.promises.readFile("file8", "utf8");
    } catch (error) {

Using useEffect with async/await

  • useEffect can only return a cleanup function
  • need a wrapper
useEffect(() => {
    const make_api_call = async () => {
        const response = await fetch();
        const data = await response.json();

What's a promise

My definition

  • magic box
  • call .then() or await
    • it'll return the value
    • or throw an error

One of 3 states

  • pending
  • fulfilled
  • rejected

What can promises do that callbacks can't?

Run Promises simultaneously

Promise.all([api(), api2(), api3()]).then(function(result) {
    //do work. result is an array contains the values of the three fulfilled promises.
}).catch(function(error) {

Non-intro notes to promises

Last update: 2023-04-24