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ECMAScript 2015
ES2015 introduced two important new JavaScript keywords: let and const.
Variables defined with const behave like let variables, except they cannot be reassigned:
Example
PI = 3.14; // This will give an error
PI = PI + 10; // This will also give an error
Block Scope
Declaring a variable with const is similar to let when it comes to Block Scope.
The x declared in the block, in this example, is not the same as the x declared outside the block:
Example
// Here x is 10
{
const x = 2;
// Here x is 2
}
// Here x is 10
You can learn more about Block Scope in the previous chapter: JavaScript Let.
Assigned when Declared
JavaScript const variables must be assigned a value when they are declared:
Incorrect
Correct
Not Real Constants
The keyword const is a little misleading.
It does NOT define a constant value. It defines a constant reference to a value.
Because of this, we cannot change constant primitive values, but we can change the properties of constant objects.
Primitive Values
If we assign a primitive value to a constant, we cannot change the primitive value:
Example
PI = 3.14; // This will give an error
PI = PI + 10; // This will also give an error
Constant Objects can Change
You can change the properties of a constant object:
Example
const car = {type:'Fiat', model:'500', color:'white'};
// You can change a property:
car.color = 'red';
// You can add a property:
car.owner = 'Johnson';
But you can NOT reassign a constant object:
Example
car = {type:'Volvo', model:'EX60', color:'red'}; // ERROR
Constant Arrays can Change
You can change the elements of a constant array:
Example
const cars = ['Saab', 'Volvo', 'BMW'];
// You can change an element:
cars[0] = 'Toyota';
// You can add an element:
cars.push('Audi');
But you can NOT reassign a constant array:
Example
cars = ['Toyota', 'Volvo', 'Audi']; // ERROR
Browser Support
The const keyword is not supported in Internet Explorer 10 or earlier.
The following table defines the first browser versions with full support for the const keyword:
| Chrome 49 | IE / Edge 11 | Firefox 36 | Safari 10 | Opera 36 |
| Mar, 2016 | Oct, 2013 | Feb, 2015 | Sep, 2016 | Mar, 2016 |
Redeclaring
Redeclaring a JavaScript var variable is allowed anywhere in a program:
Example
var x = 3; // Allowed
x = 4; // Allowed
Redeclaring or reassigning an existing var or let variable to const, in the same scope, or in the same block, is not allowed:
Example
const x = 2; // Not allowed
{
let x = 2; // Allowed
const x = 2; // Not allowed
}
Redeclaring or reassigning an existing const variable, in the same scope, or in the same block, is not allowed:
Example
const x = 3; // Not allowed
x = 3; // Not allowed
var x = 3; // Not allowed
let x = 3; // Not allowed
{
const x = 2; // Allowed
const x = 3; // Not allowed
x = 3; // Not allowed
var x = 3; // Not allowed
let x = 3; // Not allowed
}
Redeclaring a variable with const, in another scope, or in another block, is allowed:
Example
{
const x = 3; // Allowed
}
{
const x = 4; // Allowed
}
Hoisting
Variables defined with var are hoisted to the top and can be initialized at any time (if you don't know what Hoisting is, read our Hoisting Chapter).
Meaning: You can use the variable before it is declared:
Example
This is OK:
Try it Yourself »Variables defined with const are hoisted to the top of the block, but not initialized.
Meaning: The block of code is aware of the variable, but it cannot be used until it has been declared.
The variable is in a 'temporal dead zone' from the start of the block until it is declared.
Using a const variable before it is declared, is a syntax errror, so the code will simply not run.
Example
This code will not run:
Try it Yourself »Plugs - Friction Fit
Plugs - Threaded
Javascript Compressor And Obfuscator
Javascript Condenser Online
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