Variable scope var, let, const in the JavaScript

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Variable scope var, let, const in the JavaScript | standard ES5

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обновлено: 1 Augusta 2020

In the JavaScript variables are declared in different ways:

Example

What does that mean?

var

var a = 10;

1. If the variable is declared at the beginning of the file, it is visible everywhere (inside all functions) that is, it is a global variable. Example...

2. If a variable is declared inside a function, it is visible only within that function. Example...

3. If the variable is declared at the beginning of the file and the same variable is declared inside the function, then the variable inside the function DOES NOT affect the variable at the beginning of the file (these are different variables). Example...

4. If a variable is declared anywhere and the same variable is declared just below inside block then variable inside block AFFECTS the variable above (it's the same variable). Example...

let

let b = 20;

1. same behavior as var. Example...

2. same behavior as var. Example...

3. same behavior as var. Example...

4. If a variable is declared let anywhere and same variable let announced just below inside block then variable inside block DOES NOT AFFECT THE VARIABLE ABOVE (these are different variables). Example...

const

const c = 30;

const the variable is the same as let only cannot change its meaning. Example...

d = 40;

If d the variable exists then replace the value
if d the variable does not exist then the variable will be declared as var
Example...

var

JavaScript Example 1. var variable declared at the beginning of the file is a global variable

// declare a global variable
var a = 10;

// a the variable is visible inside the loop
for (var i=1; i<=1; i++)
{
// a = 10
}

// a the variable is visible inside the function
function ShowValue()
{
// a = 10
}

JavaScript Example 2. var variable declared Inside function is a local variable

function SetValue()
{
// declare a local variable
var a = 15;
}

// call the function SetValue()
SetValue();

// after the function is executed SetValue
// a variable not defined

JavaScript Example 3. var the variable is declared at the beginning of the file and the same variable is declared inside the function

// declare a global variable
var a = 10;
var t = 30;

// declare the same variable inside the function
function SetValue()
{
var a = 100;
t = 300;
}

// call the function SetValue()
SetValue();

// after the function is executed SetValue
// variable a = 10
// variable t = 300

JavaScript Example 4. var variable and the same variable declared just below inside block

// declare a global variable
var a = 10;

// declare a variable inside block if
if (777==777)
{
var a = 56;
}

// after release from the block if
// a = 56

let

JavaScript Example 1. let variable declared at the beginning of the file is a global variable

// declare a global variable
let b = 20;

// b the variable is visible inside the loop
for (var i=1; i<=1; i++)
{
// b = 20
}

// b the variable is visible inside the function
function ShowValue()
{
// b = 20
}

JavaScript Example 2. let variable declared Inside function is a local variable

function SetValue()
{
// declare a local variable
let b = 15;
}

// call the function SetValue()
SetValue();

// after the function is executed SetValue
// b variable not defined

JavaScript Example 3. let variable declared at the beginning of the file and the same variable is declared inside a function

// declare a global variable
let b = 20;
let t = 30;

// declare the same variable inside the function
function SetValue()
{
let b = 200;
t = 300;
}

// call the function SetValue()
SetValue();

// after the function is executed SetValue
// b = 20
// t = 300

JavaScript Example 4. let variable and the same variable declared just below inside block

// declare a global variable
let b = 10;

// declare a variable inside the block if
if (777==777)
{
let b = 56;
}

// after release from block if
// b = 10

const

JavaScript Example

// declare a variable const
const c = 30;

c = 120; // error! the second time you cannot set the value

empty variable declaration

JavaScript Example

// declare a global variable
var e = 10;

function SetValue()
{
// e variable already exists therefore, let's replace the value
e = 40;
}

// call the function SetValue()
SetValue();

// after the function is executed
// e = 40

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