How to use the let and const keywords of JavaScript

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ES2015 introduces two important new JavaScript keywords: let and const.

Variables defined by const are similar to let variables, but cannot be reassigned:

Example

Const PI = 3.141592653589793

PI = 3.14; / / error will occur

PI = PI + 10; / / can also make an error

Block scope

Variables declared using const within the block scope are similar to let variables.

In this case, x is declared in the block, unlike x declared outside the block:

Example

Var x = 10

/ / here, x is 10

{

Const x = 6

/ / here, x is 6

}

/ / here, x is 10

You can learn more about block scopes in the previous chapter JavaScript Let.

Assign a value on declaration

The JavaScript const variable must be assigned a value when declared:

Incorrect

Const PI

PI = 3.14159265359

Correct

Const PI = 3.14159265359

It's not a real constant.

The keyword const is misleading.

It does not define a constant value. It defines a constant reference to a value.

Therefore, we cannot change the original value of the constant, but we can change the properties of the constant object.

Original value

If we assign a primitive value to a constant, we cannot change the original value:

Example

Const PI = 3.141592653589793

PI = 3.14; / / error will occur

PI = PI + 10; / / can also make an error

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