Revising ECMAScript2015-ES6 to ECMAScript2019-ES10
ECMAScript6 -ECMAScript 2015

Its the protocol on which Javascript works. So we can say its javascript version.

Features introduced in EC6

let keyword

The let statement allows us to declare a variable with block scope and we cannot redeclare the same object in the same scope

var x = 10;

{

let x = 2;

let x = 4; // ERROR (cant redeclare)

x = 5; // works fine

// In this block x is equal to 5

}

// here x is 10

const keyword

The const statement is the same as let, the only difference is its value cant be changed. They also have block scope.

var x = 10;

{

const x = 2;

// In this block x is equal to 2

x = 4; // ERROR

}

// here x is 10

Arrow Functions

Arrow functions changed the way we write a function

//In Javascript 5

var x = function add(num1,num2) {

return num1 + num2;

} // In Javascript6 or ES6

const y = (num1,num2) => num1+ num2;

or if we have multiple lines of code to evaluate then we can wrap it in curly brackets and can return the result.

const z = (num1,num2) => {

console.log(num1 + " " + num2);

return num1*num2;

}

Some properties of Arrow functions They don’t have there own this . Arrow functions are not hoisted. They must be defined before use. Always use const to define them because their definition is not going to change.

Classes in Javascript

ES6 introduced classes.

A class is a type of function but instead of using a function keyword, we use class keyword to declare them, and properties are assigned inside the constructor.

The constructor method is called always when a class object is initialized

class Person {

let personName;

constructor(personName) {

this.personName = personName;

} let getName = () => personName;

} let person1 = new Person("Alice");

person1.getName(); // Alice

Default parameter values

In ES6 we can allow function parameters to have default parameter value.

const mult = (x,y = 10) => x*y; mult(5); // 50

We can define the default value in the function parameter. So if we don’t pass any parameter value we can use the default value example: Here we have 2 functions 1 with a default value and other without a default value.

const sum = (a=0,b=0,c = 0) => {

return a+b+c;

} const add = (a,b,c) => {

return a+b+c;

} sum(1,2); // 3 add(1,2); // NaN

In the first function, we have a default value of c, So we got the result. In the second function, we have to provide all the parameter value otherwise it will return NaN

Array.find()

This method is used to find the value from an array as per the condition passed as a parameter to it. It will return the first value which satisfies the condition.

let numbers = [1,2,3,4,5];

numbers.find(x => x >2); // It will return 3

Array.findIndex()

It’s the same as the Array.find(), the only difference is it will return the index of the value which satisfies function condition.

let numbers = [1,2,3,4,5];

numbers.findIndex(x => x >2) // It will return 2 (index)

Added new Number properties

Number.EPSILON //2.220446049250313e-16

Number.MIN_SAFE_INTEGER //-9007199254740991

Number.MAX_SAFE_INTEGER //9007199254740991

Number.isInteger()

It is used to check value is integer or not

Number.isInteger(10) // true

Number.isInteger(10.5) // false

Number.isSafeInteger()

A safe integer is an integer that can be exactly represented as a double-precision number.

Number.isSafeInteger(10); // returns true

Number.isSafeInteger(12345678901234567890); // returns false

isFinite()

It is used to check if a number is finite or not It will return false if the value is Infinite or NaN otherwise true.

isFinite(10/1) // finite

isFinite(10.0) // false

isFinite("alpha") // true

isNaN()

It will return true when the value is not a number else return false

isNaN() // true

isNaN("") // false

isNaN("alpha") // true

isNaN(1) // false

isNaN(1.1) // false

isNaN(true) //false

Exponential Operator

The exponentiation operator (**) raises the first operand to the power of the second operand. previously we have to use Math.pow() function like

Math.pow(5,2) //25

Now let x = 5**2; // 25

Object.is()

This method is used to compare 2 values are equal or not. This method is slightly different from === in that id we compare -0 with 0,

-0 === 0 // true

Object.is(-0,0) // false

Similarly in the case of NaN

console.log(NaN === NaN); // false

console.log(Object.is(NaN,NaN)) // true

String Interpolation

String interpolation is used to add dynamic string value in a constant string. Till ES5 we used to write code like :

const name ={firstName: 'Harbajan', lastName: 'singh'} let heading = "Indian best spinner name is " + name.firstName + " " + name.lastName;

Now in ES6

`Indian best spinner name is ${name.firstName} ${name.lastName}`

String Repeating

If we want to create a string by repeating some specific string then we can use this method.

('ABC').repeat(4); // "ABCABCABCABC"

Spread operator

Spread operator allows an iterable to expand in places where 0+ arguments are expected. It is mostly used in the variable array where there is more than 1 values are expected

const arr = [1,2,3,4,5];

console.log(...arr); // 1 2 3 4 5 console.log(arr); // (5) [1, 2, 3, 4, 5]

Here we can see the difference between printing array and printing values. Here we are printing array values using the spread operator.

Merging array using the spread operator

const arr1 = [1,2,3];

const arr2 = [4,5];

const arr = [...arr1,...arr2]; // [1,2,3,4,5]

converting char array from string

const name = "Yuvraj";

const chars = [...name]; //(6) ["Y","u","v","r","a","j"];

Destructuring Assignment

This makes it possible to unpack values from arrays, or properties from objects, into distinct variables.

let arr = [1,2,3,4,5];; const [a,b,c,d,e] = arr;

console.log(a); //1

console.log(b); //2

console.log(c); //3

console.log(d); //4

console.log(e); //5

It’s not important to give variables equal to the array size. We can give lesser variables also like :

let arr = [1,2,3,4,5]; const [a,b] = arr;

console.log(a); //1

console.log(b); //2

Similarly, we have it in Object

let obj = {firstName: 'Kapil', lastName: 'Dev'}; let {firstName:name} = obj; console.log(name) // Kapil

ECMAScript7 -ECMAScript 2016

ES7 is a much smaller release than ES6. It has only 2 new changes

Array.prototype.includes

Till ES6 to check the presence of an element in an array, we have to do some boilerplate code using the indexOf method. indexOf returns the position of an element present in an array. But we wanted a boolean value. So, we have to do some extra checks to convert it into boolean.

let fruits = ['apple','mango','pineapple'];

// wrong implenentation

if(fruits.indexOf('aaple')) {

console.log('Available');

}

First, we see why the indexOf is not a perfect implementation. In the above example, we can see we are trying to find an element in an array, which is present in the 0th position. So, indexOf will return 0 and in if condition 0 turns into false. So even if the value is present we have to add some extra checks.

So we have to do something like:

let fruits = ['apple','mango','pineapple'];

// till ES6

if(fruits.indexOf('aaple') !==1)

console.log('Available');

}

Now we have a very simple method in ES7. where we need not add these extra checks

let fruits = ['apple','mango','pineapple'];

// ES7

if(fruits.includes('apple'))

console.log('Available');

}

includes method returns boolean. So no need to do extra checks.

Exponentiation Operator

This operator is mostly used in doing Maths and some calculations. Till ES6 we use to use Math.pow() function or sometimes write our boilerplate code to get a simple exponential functionality.

Math.pow(2,3); //8## ECMAScript7 -ECMAScript 2016

In ES7 they gave a very simple and shorter syntax to achieve the same thing.

2**3 //8

ECMAScript8 -ECMAScript 2017

String padding

It will add some extra characters to a string. It can be in front of the string or at the end. There are 2 methods in String padding

padStart() padEnd()

'script'.padStart(6); // 'script'

'script'.padStart(7); // ' script'

'script'.padStart(10,'java') // 'javascript' 'java'.padEnd(4); // 'java'

'java'.padEnd(5); // 'java '

'java'.padEnd(10,'script'); // 'javascript'

Object.values()

This method returns an array of all the values present in an object

let name = {firstName: 'sachin', lastName: 'Tendulkar'}

Object.values(name); // (2) ["sachin", "Tendulkar"]

Object.entries()

This method returns an array of key-value pairs of an object.

let name = {firstName: 'sachin', lastName: 'Tendulkar'}

Object.entries(name); // Output

[

[

"firstName",

"sachin"

],

[

"lastName",

"Tendulkar"

]

]

getOwnPropertyDescriptors()

This method will return the description of all the properties of an object.

properties like value, writable, enumerable, configurable, get and set

let name = {firstName: 'sachin', lastName: 'Tendulkar'}

Object.getOwnPropertyDescriptors(name) // Output

{

"firstName": {

"value": "sachin",

"writable": true,

"enumerable": true,

"configurable": true

},

"lastName": {

"value": "Tendulkar",

"writable": true,

"enumerable": true,

"configurable": true

}

}

Async functions

When Promises were introduced in ES6, they were meant to solve a problem with asynchronous code, and they did, but over the 2 years that separated ES6 and ES2017, it was clear that promises could not be the final solution. Promises were introduced to solve the famous callback hell problem, but they introduced complexity on their own, and syntax complexity. They were good primitives around which a better syntax could be exposed to the developers: enter async functions.

function getResult() {

return new Promise((resolve) => {

setTimeout(() => resolve('Results'), 5000)

})

} async function execute() {

console.log(await getResult())

} console.log('Before')

execute()

console.log('After') //Output

Before

After

Result //after 5s

ECMAScript9 -ECMAScript 2018

Asynchronous iterators

Asynchronous iterators are just like synchronous iterators except the next method returns a promise. To consume asynchronous iterators, we can now use the await keyword with for … of loops.

async function example() {

// Regular iterator

for (const item of NumberIterator) {

// …

}

// Async iterator

for await (const item of AsyncNumberIterator) {

// …

}

}

Regex changes

There are lots of regex changes happen in ES9.

Rest/Spread Properties

Rest/Spread operator first introduced in ES6. now we have some new updates on the operator. we can use it to copy all the remaining properties in a new object.

const values = {a: 1, b: 2, c: 3, d: 4};

const {a, ...n} = values;

console.log(a); //1

console.log(n); // prints {b: 2, c: 3, d: 4}

Promise finally method

Finally-method is introduced in the promise which will always get executed even its a success response or an error response.

fetch('/files')

.then(data => data.json())

.catch(err => console.error(err))

.finally(() => console.log('processed!'))

ECMAScript10 -ECMAScript 2019

Array.flat()

This method is used for flattening 1 level in an array.

let fruits = [['apple','mango'],['banana','pineapple']];

fruits.flat();

// ["apple", "mango", "banana", "pineapple"]

Array.flatMap()

This method is a combination of flat and map methods of an array. let we have an array of fruits, we

let sentences = [

"JavaScript Array flatMap()",

" ",

"is",

" ",

"Awesome"

]; let words = sentences.flatMap(s => s.split(' '));

console.log(words); // Output

[ 'JavaScript', 'Array', 'flatMap()', '', '', 'is', '', '', 'Awesome' ]

String trim

It has 2 methods- trimStart() and trimEnd() which trims the blank space from start and end respectively.

const str = ' untrimmedString ';

str.trimStart(); // 'untrimmedString '; str.trimEnd(); // ' untrimmedString';

Object.fromEntries()

It is used to create a new Object from a given object. It adds all the key-value pairs of a given object to a new object.

const fruits = [['mango','1'],['pineapple','2']];

const fruitsFromArr = Object.fromEntries(fruits);

// {mango: 1,pineapple:2} const fruits = {mango: 1,pineapple:2};

const fruitsFromObj = Object.fromEntries(fruits);

// {mango: 1,pineapple:2}

Optional Catch Binding

Previously till ES9, we have to provide a parameter in the catch block of the try-catch statement. Now it’s optional.

try {

// do something

}

catch {

// catch block without parameter

}

Function’s toString() method updated

Previously when we use to do toString on a function, It removes all the white space, new lines and comments but now from ES10 everything will be retained

const sum = (a,b) => {

return a +b;

} sum.toString(); // output const sum = (a,b) => {

return a +b;

}

Symbol Description Accessor

When you create a symbol you can provide a string as a description:

const symbol = Symbol("Some description");

Previously, we need to call the toString method to get the description.

symbol.toString();

Now, we have a getter to access the description
['Rohan Aggarwal']
['Ecmascript 2017', 'Ecmascript', 'Ecmascript2019', 'JavaScript', 'Ecmascript2018']