JavaScript - how to use let keyword

In this article, we would like to show you how to use the let keyword in JavaScript.

The let keyword has been introduced to JavaScript in ES2015. There are a few important things that should know for each programmer who wants to use let keyword.

let syntax is similar to var

1. variable declaration
   Copy

   xxxxxxxxxx

    

   1

   let a;

   2

   let b;

   3

   let c;
2. variable declaration with value assigning
   Copy

   xxxxxxxxxx

    

   1

   let a = 1;

   2

   let b = 2;

   3

   let c = 3;
3. if the variable is not assigned
   Copy

   xxxxxxxxxx

    

   1

   let a;

   2

   let b;

   3

   ​

   4

   console.log(a === undefined); // true

   5

   console.log(b === udeffined); // true

   Output:

   Copy

   xxxxxxxxxx

    

   1

   true

   2

   true
4. multiple variables declaration with one let keyword
   Copy

   xxxxxxxxxx

    

   1

   let a, b, c;

   2

   ​

   3

   console.log(a === undefined); // true

   4

   console.log(b === undefined); // true

   5

   console.log(c === undefined); // true

   Output:

   Copy

   xxxxxxxxxx

    

   1

   true

   2

   true

   3

   true
5. multiple variables declaration with value assigning
   Copy

   xxxxxxxxxx

    

   1

   let a = 1, b = 2, c = 3;

   2

   ​

   3

   console.log(a); // 1

   4

   console.log(b); // 2

   5

   console.log(c); // 3

   Output:

   Copy

   xxxxxxxxxx

    

   1

   1

   2

   2

   3

   3

    

6. multiple variables declaration, some with value assigning

   Copy

   xxxxxxxxxx

    

   1

   let a, b = 2, c;

   2

   ​

   3

   console.log(a); // undefined

   4

   console.log(b); // 2

   5

   console.log(c); // undefined

   Output:

   Copy

   xxxxxxxxxx

    

   1

   undefined

   2

   2

   3

   undefined

The scope of let is the nearest enclosing curly brackets.

xxxxxxxxxx

let a = 1;

​

{ // this brackets create new scope for let keyword variables

    let a = 2;

​

    console.log(a); // 2

}

​

console.log(a); // 1

Output:

xxxxxxxxxx

2

1

When let keyword is used with for loop the scope of variable is for whole for statement.

xxxxxxxxxx

for (let i = 0; i < 5; ++i) {

    console.log(i);

}

​

// i variable cannot be used here

// var keyword allowed to use variables here

Output:

xxxxxxxxxx

0

1

2

3

4

The difference has been described here.

• ALWAYS use let keyword.

1. JavaScript - ECMAScript / ES versions and features