ES6 let and const

Information drawn from

• Medium - These are the features in es6 that you should know
• Medium - Discover the power of closures in javascript

There are two ways for declaring a variable (let and const) plus one that has become obsolete (var).

let

let declares and optionally initializes a variable in the current scope. The current scope can be either a module, a function or a block. The value of a variable that is not initialized is undefined. Scope defines the lifetime and visibility of a variable. Variables are not visible outside the scope in which they are declared. Consider the next code that emphasizes let block scope:

let x = 1;
{ 
  let x = 2;
}
console.log(x); //1

In contrast, the var declaration had no block scope:

var x = 1;
{ 
  var x = 2;
}
console.log(x); //2

const

const declares a variable that cannot be reassigned. It becomes a constant only when the assigned value is immutable. An immutable value is a value that, once created, cannot be changed.

Primitive values are immutable, objects are mutable.

const freezes the variable, Object.freeze()  freezes the object.

The initialization of the const variable is mandatory.

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Last update on 20 Apr 2020

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