Let Keyword -
The “let”
keyword declares a block-scoped variable. Let
and var declared outside any block, both are global.
The “let”
keyword allows you to declare a variable that are limited in scope to the
block, statement, or expression on which it’s used.
You can assign values to the variables when you
declare them or later in your script. If you do not initialize your variable in
the “let” statement, it is
automatically assigned the JavaScript value.
Let variables cannot be re-declared while var
variable can be re-declared in the same scope.
A variable declared by “let” can’t be used before its declaration or an error will result.
A variable declaration with “let” is helpful when we want the scope of a variable to be limited
to statement or expression level.
Let gives you the privilege to declare variables
that are limited in scope to the block, statement of expression unlike var.
The “let” statement declares a local variable in
a block scope. It is similar to var, in that we can optionally initialize the
variable.
Example looks like –
let
a = 10;
The “let” statement allows you to create a
variable with the scope limited to the block on which it is used.
It is similar to the variable we declare in other
languages like Java, .NET, etc.
Var Keyword -
The JavaScript variables statement is used to
declare a variable and, optionally, we can initialize the value of that
variable.
Var is keyword which defines a variable globally
regardless of block scope.
Example looks like –
var
a = 10;
The var keyword can be used everywhere. You can put
in all kinds of information like strings, Int, Object, Array and Function.
Variable declarations are processed before the
execution of the code.
The scope of a JavaScript variable declared with
var is its current execution context.
The scope of a JavaScript variable declared
outside the function is global.
For example 1,
function variable_Let_Var() {
console.log(i); // i is undefined due to hoisting
//console.log(j); // ReferenceError:
j is not defined
for( var i = 0; i < 3; i++ ) {
console.log(i); //Output- 0, 1, 2
};
console.log(i); // Output - 3
// console.log(j); //ReferenceError:
j is not defined
for( let j = 0; j < 3; j++ ) {
console.log(j);
};
console.log(i); // 3
// console.log(j); // ReferenceError:
j is not defined
}
For example 2 -
var a = 10;
var b = 20;
if (a === 10) {
var a = 11; // The scope is global
let b = 22; // The scope is inside the if-block
console.log(a);
// 11
console.log(b);
// 22
}
console.log(a); // 11
console.log(b); // 20
For example 3 -
function test_variable(){
var a = 33;
if (true) {
let a = (a + 55); // ReferenceError :
[ts] Block-scoped variable 'a' used before its declaration.
}
}
test_variable();
Should I replace var with let?
No, let is the new block scoping var.
The “var” variable scope is function level but “let”
variable is scoped in its enclosing block - ({}).
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