Javascript Promises are not difficult. However, lots of people find it a little bit hard to understand at the beginning. Therefore, I would like to write down the way I understand promises, in a dummy way.
Understanding Promises
A Promise in short:
“Imagine you are a kid. Your mom promises you that she’ll get you a new phone next week.”
You don’t know if you will get that phone until next week. Your mom can either really buy you a brand new phone, or she doesn’t, because she is not happy :(.
That is a promise. A promise has 3 states. They are:
- Pending: You don’t know if you will get that phone
- Fulfilled: Mom is happy, she buys you a brand new phone
- Rejected: Mom is unhappy, she doesn’t buy you a phone
Creating a Promise
Let’s convert this to JavaScript.
// ES5: Part 1
var isMomHappy = false;
// Promise
var willIGetNewPhone = new Promise(
function (resolve, reject) {
if (isMomHappy) {
var phone = {
brand: 'Samsung',
color: 'black'
};
resolve(phone); // fulfilled
} else {
var reason = new Error('mom is not happy');
reject(reason); // reject
}
}
);
The code is quite expressive in itself.
Below is how a promise syntax look like normally:
// promise syntax look like this
new Promise(function (resolve, reject) { ... } );
Consuming Promises
Now that we have the promise, let’s consume it.
// ES5: Part 2
var willIGetNewPhone = ... // continue from part 1
// call our promise
var askMom = function () {
willIGetNewPhone
.then(function (fulfilled) {
// yay, you got a new phone
console.log(fulfilled);
// output: { brand: 'Samsung', color: 'black' }
})
.catch(function (error) {
// oops, mom don't buy it
console.log(error.message);
// output: 'mom is not happy'
});
};
askMom();
Let’s run the example and see the result!
Demo: https://jsbin.com/nifocu/1/edit?js,console
Chaining Promises
Promises are chainable.
Let’s say, you, the kid, promises your friend that you will show them the new phone when your mom buy you one.
That is another promise. Let’s write it!
// ES5
// 2nd promise
var showOff = function (phone) {
return new Promise(
function (resolve, reject) {
var message="Hey friend, I have a new " +
phone.color + ' ' + phone.brand + ' phone';
resolve(message);
}
);
};
Notes: We can shorthen the above code by written as below:
// shorten it
// 2nd promise
var showOff = function (phone) {
var message="Hey friend, I have a new " +
phone.color + ' ' + phone.brand + ' phone';
return Promise.resolve(message);
};
Let’s chain the promises. You, the kid can only start the showOff promise after the willIGetNewPhone promise.
// call our promise
var askMom = function () {
willIGetNewPhone
.then(showOff) // chain it here
.then(function (fulfilled) {
console.log(fulfilled);
// output: 'Hey friend, I have a new black Samsung phone.'
})
.catch(function (error) {
// oops, mom don't buy it
console.log(error.message);
// output: 'mom is not happy'
});
};
That’s how easy to chain the promise.
Promises are Asynchronous
Promises are asynchronous. Let’s log a message before and after we call the promise.
// call our promise
var askMom = function () {
console.log('before asking Mom'); // log before
willIGetNewPhone
.then(showOff)
.then(function (fulfilled) {
console.log(fulfilled);
})
.catch(function (error) {
console.log(error.message);
});
console.log('after asking mom'); // log after
}
What is the sequence of expected output? Probably you expect:
1. before asking Mom
2. Hey friend, I have a new black Samsung phone.
3. after asking mom
However, the actual output sequence is:
1. before asking Mom
2. after asking mom
3. Hey friend, I have a new black Samsung phone.
Don’t believe me? Go try it yourself: https://jsbin.com/vepateq/edit?js,console !
Why? Because life (or JS) waits for no man.
You, the kid, wouldn’t stop playing while waiting for your mom promise (the new phone). Don’t you? That’s something we call asynchronous, the code will run without blocking or waiting for the result. Anything that need to wait for promise to proceed, you put that in .then.
Here is the full example in ES5.
// ES5: Full example
var isMomHappy = true;
// Promise
var willIGetNewPhone = new Promise(
function (resolve, reject) {
if (isMomHappy) {
var phone = {
brand: 'Samsung',
color: 'black'
};
resolve(phone); // fulfilled
} else {
var reason = new Error('mom is not happy');
reject(reason); // reject
}
}
);
// 2nd promise
var showOff = function (phone) {
var message="Hey friend, I have a new " +
phone.color + ' ' + phone.brand + ' phone';
return Promise.resolve(message);
};
// call our promise
var askMom = function () {
willIGetNewPhone
.then(showOff) // chain it here
.then(function (fulfilled) {
console.log(fulfilled);
// output: 'Hey friend, I have a new black Samsung phone.'
})
.catch(function (error) {
// oops, mom don't buy it
console.log(error.message);
// output: 'mom is not happy'
});
};
askMom();
Promises in ES5, ES6/2015, ES7/Next
ES5 – Majority browsers
The demo code is workable in ES5 environments (all major browsers + NodeJs) if you include Bluebird promise library. It’s because ES5 doesn’t support promises out of the box. Another famous promise library is Q by Kris Kowal.
ES6 / ES2015 – Modern browsers, NodeJs v6
The demo code works out of the box because ES6 supports promises natively. In addition, with ES6 functions, we can further simplify the code with fat arrow => and use const and let.
Here is the full example in ES6 code:
//_ ES6: Full example_
const isMomHappy = true;
// Promise
const willIGetNewPhone = new Promise(
(resolve, reject) => { // fat arrow
if (isMomHappy) {
const phone = {
brand: 'Samsung',
color: 'black'
};
resolve(phone);
} else {
const reason = new Error('mom is not happy');
reject(reason);
}
}
);
// 2nd promise
const showOff = function (phone) {
const message="Hey friend, I have a new " +
phone.color + ' ' + phone.brand + ' phone';
return Promise.resolve(message);
};
// call our promise
const askMom = function () {
willIGetNewPhone
.then(showOff)
.then(fulfilled => console.log(fulfilled)) // fat arrow
.catch(error => console.log(error.message)); // fat arrow
};
askMom();
Notes that all the var are replaced with const. All the function(resolve, reject) has been simplified to (resolve, reject) =>. There are a few benefits come with these changes. Read more on:-
ES7 – Async Await make the syntax look prettier
ES7 introduce async and await syntax. It makes the asynchronous syntax look prettier and easier to understand, without the .then and .catch.
Rewrite our example with ES7 syntax.
// ES7: Full example
const isMomHappy = true;
// Promise
const willIGetNewPhone = new Promise(
(resolve, reject) => {
if (isMomHappy) {
const phone = {
brand: 'Samsung',
color: 'black'
};
resolve(phone);
} else {
const reason = new Error('mom is not happy');
reject(reason);
}
}
);
// 2nd promise
async function showOff(phone) {
return new Promise(
(resolve, reject) => {
var message="Hey friend, I have a new " +
phone.color + ' ' + phone.brand + ' phone';
resolve(message);
}
);
};
// call our promise in ES7 async await style
async function askMom() {
try {
console.log('before asking Mom');
let phone = await willIGetNewPhone;
let message = await showOff(phone);
console.log(message);
console.log('after asking mom');
}
catch (error) {
console.log(error.message);
}
}
// async await it here too
(async () => {
await askMom();
})();
Why Promises and When to Use Them?
Why do we need promises? How’s the world look like before promise? Before answering these questions, let’s go back to the fundamental.
Normal Function vs Async Function
Let’s take a look at these two example, both example perform addition of two number, one add using normal function, the other add remotely.
Normal Function to Add Two Numbers
// add two numbers normally
function add (num1, num2) {
return num1 + num2;
}
const result = add(1, 2); // you get result = 3 immediately
Async Function to Add Two numbers
// add two numbers remotely
// get the result by calling an API
const result = getAddResultFromServer('http://www.example.com?num1=1&num2=2');
// you get result = "undefined"
If you add the numbers with normal function, you get the result immediately. However when you issue a remote call to get result, you need to wait, you can’t get the result immediately.
Or put it this way, you don’t know if you will get the result because the server might be down, slow in response, etc. You don’t want your entire process to be blocked while waiting for the result.
Calling APIs, downloading files, reading files are among some of the usual async operations that you’ll perform.
World Before Promises: Callback
Must we use promise for asynchronous call? Nope. Prior to Promise, we use callback. Callback is just a function you call when you get the return result. Let’s modify the previous example to accept a callback.
// add two numbers remotely
// get the result by calling an API
function addAsync (num1, num2, callback) {
// use the famous jQuery getJSON callback API
return $.getJSON('http://www.example.com', {
num1: num1,
num2: num2
}, callback);
}
addAsync(1, 2, success => {
// callback
const result = success; // you get result = 3 here
});
The syntax looks ok, why do we need promises then?
What if You Want to Perform Subsequent Async Action?
Let’s say, instead of just add the numbers one time, we want to add 3 times. In a normal function, we do this:-
// add two numbers normally
let resultA, resultB, resultC;
function add (num1, num2) {
return num1 + num2;
}
resultA = add(1, 2); // you get resultA = 3 immediately
resultB = add(resultA, 3); // you get resultB = 6 immediately
resultC = add(resultB, 4); // you get resultC = 10 immediately
console.log('total' + resultC);
console.log(resultA, resultB, resultC);
How it looks like with callbacks?
// add two numbers remotely
// get the result by calling an API
let resultA, resultB, resultC;
function addAsync (num1, num2, callback) {
// use the famous jQuery getJSON callback API
// https://api.jquery.com/jQuery.getJSON/
return $.getJSON('http://www.example.com', {
num1: num1,
num2: num2
}, callback);
}
addAsync(1, 2, success => {
// callback 1
resultA = success; // you get result = 3 here
addAsync(resultA, 3, success => {
// callback 2
resultB = success; // you get result = 6 here
addAsync(resultB, 4, success => {
// callback 3
resultC = success; // you get result = 10 here
console.log('total' + resultC);
console.log(resultA, resultB, resultC);
});
});
});
Demo: https://jsbin.com/barimo/edit?html,js,console
The syntax is less user friendly. In a nicer term, It looks like a pyramid, but people usually refer this as “callback hell”, because the callback nested into another callback. Imagine you have 10 callbacks, your code will nested 10 times!
Escape From Callback Hell
Promises come in to rescue. Let’s look at the promise version of the same example.
// add two numbers remotely using observable
let resultA, resultB, resultC;
function addAsync(num1, num2) {
// use ES6 fetch API, which return a promise
// What is .json()? https://developer.mozilla.org/en-US/docs/Web/API/Body/json
return fetch(`http://www.example.com?num1=${num1}&num2=${num2}`)
.then(x => x.json());
}
addAsync(1, 2)
.then(success => {
resultA = success;
return resultA;
})
.then(success => addAsync(success, 3))
.then(success => {
resultB = success;
return resultB;
})
.then(success => addAsync(success, 4))
.then(success => {
resultC = success;
return resultC;
})
.then(success => {
console.log('total: ' + success)
console.log(resultA, resultB, resultC)
});
Demo: https://jsbin.com/qafane/edit?js,console
With promises, we flatten the callback with .then. In a way, it looks cleaner because of no callback nesting. Of course, with ES7 async syntax, we can even further enhance this example, but I leave that to you. 🙂
New Kid On the Block: Observables
Before you settle down with promises, there is something that has come about to make it even easier to deal with async data called Observables.
Observables are lazy event streams which can emit zero or more events, and may or may not finish.source
Some key differences between promises and observable are:
Fear not, let look at the same demo written with Observables. In this example, I am using RxJS for the observables.
let Observable = Rx.Observable;
let resultA, resultB, resultC;
function addAsync(num1, num2) {
// use ES6 fetch API, which return a promise
const promise = fetch(`http://www.example.com?num1=${num1}&num2=${num2}`)
.then(x => x.json());
return Observable.fromPromise(promise);
}
addAsync(1,2)
.do(x => resultA = x)
.flatMap(x => addAsync(x, 3))
.do(x => resultB = x)
.flatMap(x => addAsync(x, 4))
.do(x => resultC = x)
.subscribe(x => {
console.log('total: ' + x)
console.log(resultA, resultB, resultC)
});
Demo: https://jsbin.com/dosaviwalu/edit?js,console
Notes:
Observables can do more funky stuff easily. For example, delay add function by 3 seconds with just one line of code or retry so you can retry a call a certain number of times.
...
addAsync(1,2)
.delay(3000) // delay 3 seconds
.do(x => resultA = x)
...
Well, let’s talk about Observables in future post. (You may read one of my RxJs post here too!)
Summary
Get yourself familiar with callbacks and promises. Understand them and use them. Don’t worry about Observables, just yet. All three can factor into your development depending on the situation.
That’s it. Hopefully this article smoothen your path to tame the JavaScript promises. Happy coding!
