Author: MDBootstrap
ECMAScript 2015
ES2015 introduced two important new JavaScript keywords: let and const.
These two keywords provide Block Scope variables (and constants) in JavaScript.
Before ES2015, JavaScript had only two types of scope: Global Scope and Function Scope.
Global Scope
Variables declared Globally (outside any function) have Global Scope.
var carName = "Volvo";
// code here can use carName
function myFunction() {
// code here can also use carName
}
Global variables can be accessed from anywhere in a JavaScript program.
Function Scope
Variables declared Locally (inside a function) have Function Scope.
// code here can NOT use carName
function myFunction() {
var carName = "Volvo";
// code here CAN use carName
}
// code here can NOT use carName
Local variables can only be accessed from inside the function where they are declared.
JavaScript Block Scope
Variables declared with the var keyword can not have Block
Scope.
Variables declared inside a block {} can be accessed from outside the block.
{
var x = 2;
}
// x CAN be used here
Before ES2015 JavaScript did not have Block Scope.
Variables declared with the let keyword can have Block Scope.
Variables declared inside a block {} can not be accessed from outside the block:
{
let x = 2;
}
// x can NOT be used here
Redeclaring Variables
Redeclaring a variable using the var keyword can impose problems.
Redeclaring a variable inside a block will also redeclare the variable outside the block:
var x = 10;
// Here x is 10
{
var x = 2;
// Here x is 2
}
// Here x is 2
Redeclaring a variable using the let keyword can solve this problem.
Redeclaring a variable inside a block will not redeclare the variable outside the block:
var x = 10;
// Here x is 10
{
let x = 2;
// Here x is 2
}
// Here x is 10
Loop Scope
Using var in a loop:
var i = 5;
for (var i = 0; i < 10; i++) {
// some statements
}
// Here i is 10
Using let in a loop:
let i = 5;
for (let i = 0; i < 10; i++) {
// some statements
}
// Here i is 5
In the first example, using var, the variable declared in
the loop redeclares the variable outside the loop.
In the second example, using let, the variable declared in
the loop does not redeclare the variable outside the loop.
When let is used to declare the i variable in a loop, the i
variable will only be visible within the loop.
Function Scope
Variables declared with var and let are
quite similar when declared inside a function.
They will both have Function Scope:
function myFunction() {
var carName = "Volvo"; // Function Scope
}
function myFunction() {
let carName = "Volvo"; // Function Scope
}
Global Scope
Variables declared with var and let are
quite similar when declared outside a block.
They will both have Global Scope:
var x = 2; // Global scope
let x = 2; // Global scope
Global Variables in HTML
With JavaScript, the global scope is the JavaScript environment.
In HTML, the global scope is the window object.
Global variables defined with the var keyword belong to the window
object:
var carName = "Volvo";
// code here can use window.carName
Global variables defined with the let keyword do not belong to the window object:
let carName = "Volvo";
// code here can not use window.carName
Redeclaring
Redeclaring a JavaScript variable with var is allowed
anywhere in a program:
var x = 2;
// Now x is 2
var x = 3;
// Now x is 3
Redeclaring a var variable with let, in the same scope, or in
the same block, is not allowed:
var x = 2; // Allowed
let x = 3; // Not allowed
{
var x = 4; // Allowed
let x = 5 // Not allowed
}
Redeclaring a let variable with let, in the same scope, or in
the same block, is not allowed:
let x = 2; // Allowed
let x = 3; // Not allowed
{
let x = 4; // Allowed
let x = 5; // Not allowed
}
Redeclaring a let variable with var, in the same scope, or in
the same block, is not allowed:
let x = 2; // Allowed
var x = 3; // Not allowed
{
let x = 4; // Allowed
var x = 5; // Not allowed
}
Redeclaring a variable with let, in another scope, or in
another block, is allowed:
let x = 2; // Allowed
{
let x = 3; // Allowed
}
{
let x = 4; // Allowed
}
Hoisting
Variables defined with var are hoisted to the top (if you don't know what
Hoisting is, read our
Hoisting Lesson).
You can use a variable before it is declared:
// you CAN use carName here
var carName;
Variables defined with let are not hoisted to the top.
Using a let variable before it is declared will result in a
ReferenceError.
The variable is in a "temporal dead zone" from the start of the block until it is declared:
// you can NOT use carName here
let carName;
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