Understanding the For…of Loop In JavaScript

Understanding the For…of Loop In JavaScript

<strong>In this article, we will look into the for...of statement to see how it works and where it can be used to write better code in our JS applications.</strong>

In this article, we will look into the for...of statement to see how it works and where it can be used to write better code in our JS applications.

In JavaScript, we have so many looping statements:

  • while statement
  • do...while statement
  • for statement
  • for...in statement
  • for...of statement

All these have one basic function: they repeat until a certain condition is met.

In this article, we will look into the for...of statement to see how it works and where it can be used to write better code in our JS applications.

Tip: Build JS apps faster with Bit (open-source). It lets you quickly discover, share and install components/modules across your apps. Give it a try.

for…of

for...of is a type of for statement to cycles through iterables(iterable objects) until it reaches the end of the line.

Let’s look at a basic example:

let arr = [2,4,6,8,10]
for(let a of arr) {
    log(a)
}
// It logs:
// 2
// 4
// 6
// 8
// 10

With much less code than the for statement, we looped through the arr array.

let myname = "Nnamdi Chidume"
for (let a of myname) {
    log(a)
}
// It logs:
// N
// n
// a
// m
// d
// i
//
// C
// h
// i
// d
// u
// m
// e

You know if we used for loop, we will have to employ some mathematics and logic to know when we reached the end of myname and quit. But you see with for…of loop we save ourselves some headache :).

As we can see for...of has the following general definition:

for ( variable of iterable) {    //...}

variable - holds the value of each property of the iterable on each iteration. iterable - is the object to be iterated upon.

Iterables and Iterator

At the definition of for…of loop, we said it “cycles through iterables(iterable objects)”. So with this, it means to tell us that for...of loop could not be used unless the item it is going to try to loop over is an iterable.

Then, what are iterables?

Simply put, Iterables are objects that iteration could be performed on. In ECMAScript 2015 a coupla additions were made. These additions were new protocols. And among the protocols were the Iterator protocol and Iterable protocol.

According to Mozilla Developer, “The iterable protocol allows JavaScript objects to define or customize their iteration behavior, such as what values are looped over in a for…of construct.” and “In order to be iterable, an object must implement the @@iterator method, meaning that the object (or one of the objects up its prototype chain) must have a property with a @@iterator key which is available via constant Symbol.iterator."

What this actually means is that, for your objects to be able to be looped through by for...of it must be iterable in other words it must have the weird @@iterator as property. That’s conforming to the iterable protocol.

So when the object with the @@iterator property is to be iterated by for...of, the @@iterator method is called by the same for...of. The @@iterator must return an iterator.

Now, the Iterator protocol defines a way by which a stream of values could be returned from an object. An iterator must implement the next method. The next method has a set of rules to follow:

  • while statement
  • do...while statement
  • for statement
  • for...in statement
  • for...of statement

Example:

const createIterator = function () {
    var array = ['Nnamdi','Chidume']
    return  {
        next: function() {
            if(this.index == 0) {
                this.index++
                return { value: array[this.index], done: false }
            }
            if(this.index == 1) {
                return { value: array[this.index], done: true }
            }
        },
        index: 0 
    }
}
const iterator = createIterator()
log(iterator.next()) // Nnamdi
log(iterator.next()) // Chidume

Basically, the @@iterator method returns an iterator which the for...of uses to cycle through the implementing object to get the values. So, if an object doesn’t have the @@iterator method and/or returns an iterator, the for...of statement on it won’t work.

const nonIterable = //...
 for( let a of nonIterable) {
     // ...
 }
for( let a of nonIterable) {
               ^
TypeError: nonIterable is not iterable

Examples of Iterables are:

  • while statement
  • do...while statement
  • for statement
  • for...in statement
  • for...of statement

Notice that Object is missing. Object is not an iterable. If we try to use loop through an object’s properties using the for…of loop:

let obj {
    firstname: "Nnamdi",
    surname: "Chidume"
}
for(const a of obj) {
    log(a)
}

It will throw an error:

for(const a of obj) {               ^
TypeError: obj is not iterable

We can check if an object is iterable by doing this:

const str = new String('Chidume');
log(typeof str[Symbol.iterator]);
function

See, it logs a function, that shows @@iterator property is present in String. If we try Object:

const obj = {
    surname: "Chidume"
}
log(typeof obj[Symbol.iterator]);
undefined

Woo!! undefined means not present.

for…of: Array

An Array is an iterable.

log(typeof new Array("Nnamdi", "Chidume")[Symbol.iterator]);
// function

That’s why we can perform for...of on it.

const arr = ["Chidume", "Nnamdi", "loves", "JS"]
for(const a of arr) {
    log(a)
}
// It logs:
// Chidume
// Nnamdi
// loves
// JS
const arr = new Array("Chidume", "Nnamdi", "loves", "JS")
for(const a of arr) {
    log(a)
}
// It logs:
// Chidume
// Nnamdi
// loves
// JS

for…of: String

String is also iterable.

const myname = "Chidume Nnamdi"
for(const a of myname) {
    log(a)
}
// It logs:
// C
// h
// i
// d
// u
// m
// e
// 
// N
// n
// a
// m
// d
// i
const str = new String("The Young")
for(const a of str) {
    log(a)
}
// It logs:
// T
// h
// e
// 
// Y
// o
// u
// n
// g

for…of: Map

const map = new Map([["surname", "Chidume"],["firstname","Nnamdi"]])
for(const a of map) {
    log(a)
}
// It logs:
// ["surname", "Chidume"]
// ["firstname","Nnamdi"]
for(const [key, value] of map) {
    log(`key: ${key}, value: ${value}`)
}
// It logs:
// key: surname, value: Chidume
// key: firstname, value: Nnamdi

for…of: Set

const set = new Set(["Chidume","Nnamdi"])
for(const a of set) {
    log(a)
}
// It logs:
// Chidume
// Nnamdi

for…of: TypedArray

const typedarray = new Uint8Array([0xe8, 0xb4, 0xf8, 0xaa]);
for (const a of typedarray) {
  log(a);
}
// It logs:
// 232
// 180
// 248
// 170

for…of: arguments

arguments is iterable? Well, let’s check it out:

// testFunc.js
function testFunc(arg) {
    log(typeof arguments[Symbol.iterator])
}
testFunc()
$ node testFunc
function

Well, that settles it. If we investigate further, arguments is actually of type IArguments and the class implementing the IArguments interface has the @@iterator property which makes arguments iterable.

// testFunc.js
function testFunc(arg) {
    log(typeof arguments[Symbol.iterator])
    for(const a of arguments) {
        log(a)
    }
}
testFunc("Chidume")
// It:
// Chidume

for…of: Custom Iterables

Like we demonstrated in the previous sections we can create a custom iterable that can be iterated by for..of.

var obj = {}
obj[Symbol.iterator] = function() {
    var array = ["Chidume", "Nnamdi"]
    return {
        next: function() {
            let value = null
            if (this.index == 0) {
                value = array[this.index]
                this.index++
                    return { value, done: false }
            }
            if (this.index == 1) {
                value = array[this.index]
                this.index++
                    return { value, done: false }
            }
            if (this.index == 2) {
                return { done: true }
            }
        },
        index: 0
    }
};

I created an object obj and to make it iterable, I assigned a @@iterator property to it using the [Symbol.iterator]. Then, I made the function to return an iterator.

//...
return {
    next: function() {...}
}
//...

Remember, an iterator must have a next() function.

Inside the next function, I implemented the values will be returning to for…of during iteration. Looking at it above, you will see that what I did is quite clear.

Let’s test this our obj against a for…of to see what will happen:

// customIterableTest.js
//...
for (let a of obj) {
    log(a)
}
$ node customIterableTest
Chidume
Nnamdi

Yea!!! You see it worked!

Making Object and plain objects iterable

Plain objects are not iterable and also objects from Object are not iterable.

We can by-pass this by adding @@iterator to the Object.prototype with a custom iterator.

Object.prototype[Symbol.iterator] = function() {
    let properties = Object.keys(this)
    let count = 0
    let isdone = false
    let next = () => {
        let value = this[properties[count]]
        if (count == properties.length) {
            isdone = true
        }
        count++
        return { done: isdone, value }
    }
    return { next }
}

The properties variable holds the properties of the object gotten using the Object.keys() call. In the next function, we simply return each value from the properties variable and update the count so as to get the next value from the properties variable using the count variable as the index. When the count equals the length of the properties we set done to true, so the iteration stops.

Testing using Object:

let o = new Object()
o.s = "SK"
o.me = 'SKODA'
for (let a of o) {
    log(a)
}
SK
SKODA

It works!!!

With plain objects:

let dd = {
    shit: 900,
    opp: 800
}
for (let a of dd) {
    log(a)
}
900
800

Tada!! :)

So we can add this as a polyfill so we can use for…of on objects where ever we want in our app.

Using for…of on ES6 classes

We can use for…of to iterate through a list of data in an instance of a class.

class Profiles {
    constructor(profiles) {
        this.profiles = profiles
    }
}
const profiles = new Profiles([
    {
        firstname: "Nnamdi",
        surname: "Chidume"
    },
    {
        firstname: "Philip",
        surname: "David"
    }
])

The class Profiles has a property profile that holds an array of users. We may need to display this data in our app using for…of. If we do this:

//...
for(const a of profiles) {
    log(a)
}

Obviously, it won’t work

for(const a of profiles) {
               ^
TypeError: profiles is not iterable

To make profiles iterable remember the rules:

  • while statement
  • do...while statement
  • for statement
  • for...in statement
  • for...of statement

We define the @@iterator property using the familiar constant [Symbol.iterator].

class Profiles {
    constructor(profiles) {
            this.profiles = profiles
        }
        [Symbol.iterator]() {
            let props = this.profiles
            let propsLen = this.profiles.length
            let count = 0
            return {
                next: function() {
                    if (count < propsLen) {
                        return { value: props[count++], done: false }
                    }
                    if (count == propsLen) {
                        return { done: true }
                    }
                }
            }
        }
}

Then, if we run:

//...
for(const a of profiles) {
    log(a)
}
$ node profile.js
{ firstname: 'Nnamdi', surname: 'Chidume' }
{ firstname: 'Philip', surname: 'David' }

We have our profiles property displayed.

Async Iterator

A new construct was introduced to ECMAScript 2018 to be able to loop through an array of Promises, this new construct is for-await-of and a new Symbol Symbol.asyncIterator.

The Symbol.asyncIterator function in an iterable returns an iterator that returns a Promise.

const f = {
    [Symbol.asyncIterator]() {
        return new Promise(...)
    }
}

The difference between [Symbol.iterator] and [Symbol.asyncIterator] is that the former returns { value, done } while the latter returns a Promise that resolves to { value, done }.

Our f above will look like this:

const f = {
    [Symbol.asyncIterator]() {
        return {
            next: function() {
                if (this.index == 0) {
                    this.index++
                        return new Promise(res => res({ value: 900, done: false }))
                }
                return new Promise(res => res({ value: 1900, done: true }))
            },
            index: 0
        }
    }
}

The f is an async iterable. You see it always returns a Promise, the Promise has a resolve function that returns a value at each iteration.

To iterate through f, we will not use for..of rather we will use the new for-await-of like this:

// ...
async function fAsyncLoop(){
    for await (const _f of f) {
        log(_f)
    }
}
fAsyncLoop()
$ node fAsyncLoop.js
900

We can also use this for-await-of to loop through an array of Promises:

const arrayOfPromises = [
    new Promise(res => res("Nnamdi")),
    new Promise(res => res("Chidume"))
]
async function arrayOfPromisesLoop(){
    for await (const p of arrayOfPromises) {
        log(p)
    }
}
arrayOfPromisesLoop()
$ node arrayOfPromisesLoop.js
Nnamdi
Chidume

Conclusion

In this post we dug deep into for...ofloop. We started by defining what for…of is, and went on to see what makes what iterable. Then, we looked at the complete list of iterables in JS and went through each of them to see how to work with for...of loop on them.

Like I said in the beginning, for…of saves us a lot of complexities and logic and helps make our code looks cleaner and readable. If you haven’t tried this awesome for- loop mutation, I think now will be the right time to do so.

If you have any question regarding this or anything I should add, correct or remove, feel free to comment below, and anything or DM me. Thanks for reading! :)

Originally published by Chidume Nnamdi at *[https://blog.bitsrc.io](https://blog.bitsrc.io "https://blog.bitsrc.io*")

Learn More

☞ The Complete JavaScript Course 2019: Build Real Projects!

☞ Become a JavaScript developer - Learn (React, Node,Angular)

☞ JavaScript: Understanding the Weird Parts

☞ Vue JS 2 - The Complete Guide (incl. Vue Router & Vuex)

☞ The Full JavaScript & ES6 Tutorial - (including ES7 & React)

☞ JavaScript - Step By Step Guide For Beginners

☞ The Web Developer Bootcamp

☞ MERN Stack Front To Back: Full Stack React, Redux & Node.js

javascript

Bootstrap 5 Complete Course with Examples

Bootstrap 5 Tutorial - Bootstrap 5 Crash Course for Beginners

Nest.JS Tutorial for Beginners

Hello Vue 3: A First Look at Vue 3 and the Composition API

Building a simple Applications with Vue 3

Deno Crash Course: Explore Deno and Create a full REST API with Deno

How to Build a Real-time Chat App with Deno and WebSockets

Convert HTML to Markdown Online

HTML entity encoder decoder Online

The essential JavaScript concepts that you should understand

The essential JavaScript concepts that you should understand - For successful developing and to pass a work interview

Data Types In JavaScript

JavaScript data types are kept easy. While JavaScript data types are mostly similar to other programming languages; some of its data types can be unique. Here, we’ll outline the data types of JavaScript.

JavaScript Memory Management System

The main goal of this article is help to readers to understand that how memory management system performs in JavaScript. I will use a shorthand such as GC which means Garbage Collection. When the browsers use Javascript, they need any memory location to store objects, functions, and all other things. Let’s deep in dive that how things going to work in GC.

Create a Line Through Effect with JavaScript

In this post we are going to create an amazing line through effect, with help of CSS and lots of JavaScript. So, head over to your terminal and create a folder LineThroughEffect. Create three files -index.html, main.js and styles.css inside it.

Grokking Call(), Apply() and Bind() Methods in JavaScript

In this article, we will have a look at the call(), apply() and bind() methods of JavaScript. Basically these 3 methods are used to control the invocation of the function.