What is the difference between let, var, and const?
In this article, I’m explaining the importance of
these three keywords in JavaScript and TypeScript. I also provided various examples to deeply
understand and use them and what happened when?
Let
see about: Var vs. Let vs. Const
var:
-
1. var
is function-scoped
2. var
return undefined when accessing a variable before it's declared
let:
-
1. let
is block-scoped
2. let
throw ReferenceError when accessing a variable before it's declared
const:-
1. Const
is block-scoped
2. Const
throw ReferenceError when accessing a variable before it's declared
3. Const
cannot be reassigned
Let
see the Examples to understand what happens when -
What will happen when the following code is executed?
var
x = 5;
console.log(x);
if (true)
{
var
x = 6;
console.log(x);
}
console.log(x);
//The Output will
be -
5
6
6
What will happen when the following code is executed?
var
x = 5;
console.log(x);
if
(false) {
var
x = 6;
console.log(x);
}
console.log(x);
//The Output will
be -
5
5
What will happen when the following code is executed?
var
x = 5;
function
a() {
var
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
5
6
What will happen when the following code is executed?
var
x = 5;
function
a() {
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
5
6
What will happen when the following code is executed?
var
x = 5;
function
a() {
let
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
5
6
What will happen when the following code is executed?
let
x = 5;
function
a() {
let
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
Uncaught
SyntaxError: Identifier 'x' has already been declared
at<anonymous>:
1: 1
What will happen when the following code is executed?
const
x = 5;
function
a() {
let
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
Uncaught
SyntaxError: Identifier 'x' has already been declared
at<anonymous>:
1: 1
What will happen when the following code is executed?
const x
= 5;
function
a() {
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
Uncaught
SyntaxError: Identifier 'x' has already been declared
at<anonymous>:
1: 1
What will happen when the following code is executed?
const x
= 5;
function
a() {
const
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
Uncaught
SyntaxError: Identifier 'x' has already been declared
at<anonymous>:
1: 1
What will happen when the following code is executed?
const
x = 5;
function
a() {
var
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
Uncaught
SyntaxError: Identifier 'x' has already been declared
at<anonymous>:
1: 1
What will happen when the following code is executed?
var
x = 5;
function
a() {
const
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
5
6
What will happen when the following code is executed?
let
x = 5;
function
a() {
var
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
Uncaught
SyntaxError: Identifier 'x' has already been declared
at<anonymous>:
1: 1
What will happen when the following code is executed?
var
x = 5;
function
a() {
var
x = 6;
return
x;
}
x;
a();
//The Output will
be -
6
What will happen when the following code is executed?
var x = 5;
function
a() {
var
x = 6;
return
x;
}
x = a();
//The Output will
be -
6
What will happen when the following code is executed?
var
x = 5;
function
a() {
var
x = 6;
return
x;
}
x
//The Output will
be -
5
What will happen when the following code is executed?
var
x = 5;
function
a() {
var
x = 6;
return
x;
}
console.log(x);
console.log(a());
//The Output will
be -
5
6
What will happen when the following code is executed?
var
x = 5;
function
a() {
var
x = 6;
}
console.log(x);
console.log(a());
//The Output will
be -
5
What will happen when the following code is executed?
var
x = 5;
function
a() {
let
x = 6; return x;
}
console.log(x);
console.log(a());
//The Output will
be -
5
6
The Results Looks like this.
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Anil Singh is an author, tech blogger, and software programmer. Book writing, tech blogging is something do extra and Anil love doing it. For more detail, kindly refer to this link..
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