JavaScript Variables and Constants

JavaScript Variables and Constants

In this tutorial, you will learn about JavaScript variables and constants, and also how to initialize and use them with the help of examples.

JavaScript Variables

In programming, a variable is a container (storage area) to hold data.


JavaScript Declare Variables

In JavaScript, we use the following keywords to declare variables: var and let. For example,

var x;
let y;

Here, x and y are variables.


JavaScript var Vs let

Both var and let are used to declare variables. However, there are some differences between them.

var let
var is used in the older versions of JavaScript let is the new way of declaring variables starting ES6 (ES2015).
var is function scoped (will be discussed in the later tutorials). let is block scoped (will be discussed in the later tutorials).
For example, var x; For example, let y;

Note: It is recommended to use let instead of var. However, there are a few browsers that do not support let. Visit JavaScript let browser support to learn more


JavaScript Initialize Variables

We use the assignment operator = to assign a value to a variable.

let x;
x = 5;

Here, 5 is assigned to variable x.

You can also initialize variables during its declaration.

let x = 5;
let y = 6;

In JavaScript, it's possible to declare variables in a single statement.

let x = 5, y = 6, z = 7;

If you use a variable without initializing it, it will have an undefined value.

let x; // x is the name of the variable

console.log(x); // undefined

Here x is the variable name and since it does not contain any value, it will be undefined.

You will learn about undefined and other data types in the next tutorial in detail.


Change the Value of Variables

It's possible to change the value stored in the variable. For example,

// 5 is assigned to variable x
let x = 5; 
console.log(x); // 5

// variable x is changed 3
x = 3; 
console.log(x); // 3

The value of a variable may vary. Hence, the name variable.


Rules for Naming JavaScript Variables

The rules for naming variables are:

  1. Variable names must start with either a letter, an underscore _, or the dollar sign $. For example,
    //valid
    let a = 'hello';
    let _a = 'hello;
    let $a = 'hello';
  2. Variable names cannot start with numbers. For example,
    //invalid
    Let 1a = 'hello'; // this gives an error
  3. JavaScript is case-sensitive. So y and Y are different variables. For example,
    let y = "hi";
    let Y = 5;
    
    console.log(y); // hi
    console.log(Y); // 5
  4. Keywords cannot be used as variable names. For example,
    //invalid
    let new = 5; // Error! new is a keyword.

Notes:

  • Though you can name variables in any way you want, it's a good practice to give a descriptive variable name. If you are using a variable to store the number of apples, it better to use apples or numberOfApples rather than x or n.
  • In JavaScript, the variable names are generally written in camelCase if it has multiple words. For example, firstName, annualSalary, etc.

JavaScript Constants

The const keyword was also introduced in the ES6(ES2015) version to create constants. For example,

const x = 5;

Once a constant is initialized, we cannot change its value.

const x = 5;
x = 10;  // Error! constant cannot be changed.
console.log(x)

Simply, a constant is a type of variable whose value cannot be changed.

Also, you cannot declare a constant without initiating it. For example,

const x;  // Error! Missing initializer in const declaration.
x = 5;
console.log(x)

Note: If you are sure that the value of a variable won't change throughout the program, it's recommended to use const. However, there are a few browsers that do not support const. Visit JavaScript const browser support to learn more.


Now that you know about variables, you will learn about different types of data a variable can store in the next tutorial.