Javascript const

const is one of the many features introduced in ES6 such as let, arrow functions etc. This article will explain the usage of const with examples.
Syntax
const keyword is followed by a user defined variable name to declare it as a constant. Example,

// declaring a constant integer
const num = 12;
// declaring a constant string
const name = 'codippa';

Name of variable should be as per javascript variable naming rules.
As a naming convention in most programming languages, constants are declared in capital letters. This is a convention, not a rule, you can define it in lower case as well.
Variables declared with const should be initialized at the time of declaration or else there will be an error. Example,
Below statement

will result in

So, you need to provide a value to a const variable at the time of declaring it. Above statement can be corrected as

const primitives
Primitive types are those which hold a value such as integers, decimal values, string, boolean etc. Primitives declared with const can not be modified again. Example,

// valid
const x = 12;
// error !!
x = 25;

will result in

const Objects
A javascript object declared with const can not be assigned again but you can modify its properties or add new properties to it.
That is, following is not allowed.

// define const object
const obj = {'name': 'codippa'};

// reassign to new object
obj = {'language': 'javascript'};

and will result in

But, this is legal.

// initialize object
const obj = {'device': 'laptop'};
// add properties
obj['brand'] = 'Lenovo';
obj['frequency'] = 2.4;

// update properties
obj['brand'] = 'Sony';

const Arrays
Similar to objects, a javascript array declared with const can not be re-initialized but you can add or push new elements to it.
Thus,

// define const array
const arr = [2, 4];

// reassign to new array
arr = [];

is invalid while, this is allowed

// initialize array
const arr = ['xyz', 'swfd'];
// add elements
arr.push('abcd');

Block scope
A const declared in a block is visible only inside that block and you can not modify its value in that block after initialization.
But, outside the block, the variable is not visible and can be re-initialized. Example,

// declare a constant
const c = 23;
{
 // declare a constant with the same name
 const c = 46;
 console.log(c)  // prints 46 
}
// no problem
console.log(c)  // prints 23

Above code works perfectly fine. This is because the variable declared inside the curly braces is visible only in that block.
const function
In javascript, a function can be assigned to a variable as shown below.

f = function() {
  console.log('Called');
}

Same variable can later be assigned a different function.

But, if the variable is declared using const, then it can not be reassigned a function after it is initialized the first time.
Thus, below code

// assign function to const
const f = function() {
   console.log('Called');
}

// reassign function to variable
f = function() {
   console.log('Called Again');
}

will result in

const arrow function
Arrow function is a new concept added in javascript ES6 where there is no need to use function keyword while defining functions and there is an arrow(=>) between function parameters and body.
More on arrow functions here.

An arrow function can also be assigned to a variable as

f = () => {
  console.log('Called');
}

Similar to normal functions, if arrow function is assigned to a const variable, you can not reassign it again as shown below.

// assign function to const
const f = () => {
   console.log('Called');
}

// reassign function to variable
f = () =>{
   console.log('Called Again');
}

This will again be a TypeError.

const Hoisting
Javascript hoisting means you can use a variable declared with var before declaring them. Example,

b = 20;
let b;
console.log(b);   // prints 20

But with const, this is not the case.
That is, you can not use a variable declared with const before declaring. Same code that uses const in place of var as below

b = 20;
const b = 10;
console.log(b);

gives error

const browser support
Minimum browser versions that support const are

Browser	Version
Firefox	36
Chrome	21
Edge	12
IE	11
Safari	5.1
Opera	9

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