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.

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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

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 Tutorial: if-else Statement in JavaScript

JavaScript Tutorial: if-else Statement in JavaScript

This JavaScript tutorial is a step by step guide on JavaScript If Else Statements. Learn how to use If Else in javascript and also JavaScript If Else Statements. if-else Statement in JavaScript. JavaScript's conditional statements: if; if-else; nested-if; if-else-if. These statements allow you to control the flow of your program's execution based upon conditions known only during run time.

Decision Making in programming is similar to decision making in real life. In programming also we face some situations where we want a certain block of code to be executed when some condition is fulfilled.
A programming language uses control statements to control the flow of execution of the program based on certain conditions. These are used to cause the flow of execution to advance and branch based on changes to the state of a program.

JavaScript’s conditional statements:

  • if
  • if-else
  • nested-if
  • if-else-if

These statements allow you to control the flow of your program’s execution based upon conditions known only during run time.

  • if: if statement is the most simple decision making statement. It is used to decide whether a certain statement or block of statements will be executed or not i.e if a certain condition is true then a block of statement is executed otherwise not.
    Syntax:
if(condition) 
{
   // Statements to execute if
   // condition is true
}

Here, condition after evaluation will be either true or false. if statement accepts boolean values – if the value is true then it will execute the block of statements under it.
If we do not provide the curly braces ‘{‘ and ‘}’ after if( condition ) then by default if statement will consider the immediate one statement to be inside its block. For example,

if(condition)
   statement1;
   statement2;

// Here if the condition is true, if block 
// will consider only statement1 to be inside 
// its block.

Flow chart:

Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate If statement 

var i = 10; 

if (i > 15) 
document.write("10 is less than 15"); 

// This statement will be executed 
// as if considers one statement by default 
document.write("I am Not in if"); 

< /script> 

Output:

I am Not in if
  • if-else: The if statement alone tells us that if a condition is true it will execute a block of statements and if the condition is false it won’t. But what if we want to do something else if the condition is false. Here comes the else statement. We can use the else statement with if statement to execute a block of code when the condition is false.
    Syntax:
if (condition)
{
    // Executes this block if
    // condition is true
}
else
{
    // Executes this block if
    // condition is false
}


Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate If-else statement 

var i = 10; 

if (i < 15) 
document.write("10 is less than 15"); 
else
document.write("I am Not in if"); 

< /script> 

Output:

i is smaller than 15
  • nested-if A nested if is an if statement that is the target of another if or else. Nested if statements means an if statement inside an if statement. Yes, JavaScript allows us to nest if statements within if statements. i.e, we can place an if statement inside another if statement.
    Syntax:
if (condition1) 
{
   // Executes when condition1 is true
   if (condition2) 
   {
      // Executes when condition2 is true
   }
}

Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate nested-if statement 

var i = 10; 

if (i == 10) { 

// First if statement 
if (i < 15) 
	document.write("i is smaller than 15"); 

// Nested - if statement 
// Will only be executed if statement above 
// it is true 
if (i < 12) 
	document.write("i is smaller than 12 too"); 
else
	document.write("i is greater than 15"); 
} 
< /script> 

Output:

i is smaller than 15
i is smaller than 12 too
  • if-else-if ladder Here, a user can decide among multiple options.The if statements are executed from the top down. As soon as one of the conditions controlling the if is true, the statement associated with that if is executed, and the rest of the ladder is bypassed. If none of the conditions is true, then the final else statement will be executed.
if (condition)
    statement;
else if (condition)
    statement;
.
.
else
    statement;


Example:

<script type = "text/javaScript"> 
// JavaScript program to illustrate nested-if statement 

var i = 20; 

if (i == 10) 
document.wrte("i is 10"); 
else if (i == 15) 
document.wrte("i is 15"); 
else if (i == 20) 
document.wrte("i is 20"); 
else
document.wrte("i is not present"); 
< /script> 

Output:

i is 20

How to Retrieve full Profile of LinkedIn User using Javascript

How to Retrieve full Profile of LinkedIn User using Javascript

I am trying to retrieve the full profile (especially job history and educational qualifications) of a linkedin user via the Javascript (Fetch LinkedIn Data Using JavaScript)

Here we are fetching LinkedIn data like Username, Email and other fields using JavaScript SDK.

Here we have 2 workarounds.

  1. Configuration of linkedIn developer api
  2. Javascript Code to fetch records

Configuration of linkedIn developer api

In order to fetch records, first we need to create developer api in linkedin which will act as token/identity while fetching data from other linkedin accounts.

So to create api, navigate to https://linkedin.com/developer/apps and click on 'Create Application'.

After navigating, fill in details like name, description and other required fields and then submit.

As we submit, it will create Client ID and Client Secret shown below, which we will be using in our code while communicating to fetch records from other LinkedIn account.

Note: We need to provide localhost Url here under Oauth 2.0. I am using my localhost, but you can probably use other production URLs under Oauth 2.0 where your app is configured. It will make your api  consider the Url as trusted which fetching records.

Javascript Code to fetch records

For getting user details like first name, last name,User image can be written as,

<script type="text/javascript" src="https://platform.linkedin.com/in.js">  
    api_key: XXXXXXX //Client ID  
    onLoad: OnLinkedInFrameworkLoad //Method that will be called on page load  
    authorize: true  
</script>  
<script type="text/javascript">  
    function OnLinkedInFrameworkLoad() {  
        IN.Event.on(IN, "auth", OnLinkedInAuth);  
    }  
  
    function OnLinkedInAuth() {  
        IN.API.Profile("me").result(ShowProfileData);  
    }  
  
    function ShowProfileData(profiles) {  
        var member = profiles.values[0];  
        var id = member.id;  
        var firstName = member.firstName;  
        var lastName = member.lastName;  
        var photo = member.pictureUrl;  
        var headline = member.headline;  
        //use information captured above  
        var stringToBind = "<p>First Name: " + firstName + " <p/><p> Last Name: " + lastName + "<p/><p>User ID: " + id + " and Head Line Provided: " + headline + "<p/>"  
        document.getElementById('profiles').innerHTML = stringToBind;  
    }  
</script>    

Kindly note we need to include 'https://platform.linkedin.com/in.js' as src under script type as it will act on this Javascript SDK provided by Linkedin.

In the same way we can also fetch records of any organization with the companyid as keyword.

<head>  
    <script type="text/javascript" src="https://platform.linkedin.com/in.js">  
        api_key: XXXXXXX ////Client ID  
        onLoad: onLinkedInLoad  
        authorize: true  
    </script>  
</head>  
  
<body>  
    <div id="displayUpdates"></div>  
    <script type="text/javascript">  
        function onLinkedInLoad() {  
            IN.Event.on(IN, "auth", onLinkedInAuth);  
            console.log("On auth");  
        }  
  
        function onLinkedInAuth() {  
            var cpnyID = XXXXX; //the Company ID for which we want updates  
            IN.API.Raw("/companies/" + cpnyID + "/updates?event-type=status-update&start=0&count=10&format=json").result(displayCompanyUpdates);  
            console.log("After auth");  
        }  
  
        function displayCompanyUpdates(result) {  
            var div = document.getElementById("displayUpdates");  
            var el = "<ul>";  
            var resValues = result.values;  
            for (var i in resValues) {  
                var share = resValues[i].updateContent.companyStatusUpdate.share;  
                var isContent = share.content;  
                var isTitled = isContent,  
                    isLinked = isContent,  
                    isDescription = isContent,  
                    isThumbnail = isContent,  
                    isComment = isContent;  
                if (isTitled) {  
                    var title = isContent.title;  
                } else {  
                    var title = "News headline";  
                }  
                var comment = share.comment;  
                if (isLinked) {  
                    var link = isContent.shortenedUrl;  
                } else {  
                    var link = "#";  
                }  
                if (isDescription) {  
                    var description = isContent.description;  
                } else {  
                    var description = "No description";  
                }  
                /* 
                if (isThumbnailz) { 
                var thumbnailUrl = isContent.thumbnailUrl; 
                } else { 
                var thumbnailUrl = "http://placehold.it/60x60"; 
                } 
                */  
                if (share) {  
                    var content = "<a target='_blank' href=" + link + ">" + comment + "</a><br>";  
                    //el += "<li><img src='" + thumbnailUrl + "' alt=''>" + content + "</li>";  
                    el += "<li><div>" + content + "</div></li>";  
                }  
                console.log(share);  
            }  
            el += "</ul>";  
            document.getElementById("displayUpdates").innerHTML = el;  
        }  
    </script>  
</body>  

We can get multiple metadata while fetching records for any any organization. We can get company updates as shown below.

Conclusion

We can also fetch any company specific data like company job updates/post, total likes, comments, and number of views along with a lot of metadata we can fetch which I have shown below.

Thank you for reading !

7 Best Javascript Iframe Libraries

7 Best Javascript Iframe Libraries

Iframes let you build user experiences into embeddable ‘cross-domain components’, which let users interact with other sites without being redirected. I have compiled 7 best Javascript iframe libraries.

Iframes let you build user experiences into embeddable ‘cross-domain components’, which let users interact with other sites without being redirected. I have compiled 7 best Javascript iframe libraries.

1. Zoid

A cross-domain component toolkit, supporting:

  • Render an iframe or popup on a different domain, and pass down props, including objects and functions
  • Call callbacks natively from the child window without worrying about post-messaging or cross-domain restrictions
  • Create and expose components to share functionality from your site to others!
  • Render your component directly as a React, Vue or Angular component!
    It's 'data-down, actions up' style components, but 100% cross-domain using iframes and popups!

Download


2. Postmate

Postmate is a promise-based API built on postMessage. It allows a parent page to speak with a child iFrame across origins with minimal effort.

Download


3. Iframe Resizer

Keep same and cross domain iFrames sized to their content with support for window/content resizing, in page links, nesting and multiple iFrames

Demo

Download


4. Iframely

Embed proxy. Supports over 1800 domains via custom parsers, oEmbed, Twitter Cards and Open Graph

Demo

Download


5. React Frame component

This component allows you to encapsulate your entire React application or per component in an iFrame.

Demo

Download


6. Seamless.js

A seamless iframe makes it so that visitors are unable to distinguish between content within the iframe and content beside the iframe. Seamless.js is a JavaScript library (with no dependencies) that makes working with iframes easy by doing all the seamless stuff for you automatically.

Demo

Download


7. Porthole

A proxy to safely communicate to cross-domain iframes in javascript

Demo

Download


Thank for read!