JavaScript: variables

JavasScript async Dynamic client-side scripting

description

📘 JavaScript: variables

Variables are containers for storing data, JavaScript Variables can be declared in 4 ways:

• Automatically
• Using var
• Using let
• Using const

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Note

• The var keyword was used in all JavaScript code from 1995 to 2015.
• The let and const keywords were added to JavaScript in 2015.
• The var keyword should only be used in code written for older browsers.

1 Var

Before the advent of ES6, var declarations ruled, var declarations are:

• globally scoped or
• function/locally scoped.

What is ES6? ES6 stands for ECMAScript 6: ECMAScript was created to standardize JavaScript, and ES6 is the 6th version of ECMAScript, it was published in 2015, and is also known as ECMAScript 2015

The scope is:

• global when a var variable is declared outside a function. This means that any variable that is declared with var outside a function block is available for use in the whole window.
• var is function scoped when it is declared within a function. This means that it is available and can be accessed only within that function.

App.js

  var greeter = "hey hi";
    
    function newFunction() {
        var hello = "hello";
    }

Here, greeter is globally scoped because it exists outside a function while hello is function scoped. So we cannot access the variable hello outside of a function. So if we do this:

App.js

    var tester = "hey hi";
    
    function newFunction() {
        var hello = "hello";
    }
    console.log(hello); // error: hello is not defined

We’ll get an error which is as a result of hello not being available outside the function.

It is considered good programming practice to always declare variables before use.

2 Let

let is now preferred for variable declaration. It’s no surprise as it comes as an improvement to var declarations. It also solves the problems with var.

let is block scoped

A block is a chunk of code bounded by {}. A block lives in curly braces. Anything within curly braces is a block.

So a variable declared in a block with let is only available for use within that block, let can be updated but not re-declared.

App.js

  let greeting = "say Hi";
   let times = 4;

   if (times > 3) {
        let hello = "say Hello instead";
        console.log(hello);// "say Hello instead"
    }
   console.log(hello) // hello is not defined

We see that using hello outside its block (the curly braces where it was defined) returns an error. This is because let variables are block scoped.

3 const

Variables declared with the const maintain constant values. const declarations share some similarities with let declarations.