In programming, a variable is a container (storage area) to hold data. For example,
let num = 5;
num is a variable. It's storing 5.
let keyword to declare variables. For example,
var x; let y;
Here, x and y are variables.
let are used to declare variables. However, there are some differences between them.
Note: It is recommended we use
let instead of
var. However, there are a few browsers that do not support
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;
let x = 5, y = 6, z = 7;
If you use a variable without initializing it, it will have an
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 // vaue of variable x is changed x = 3; console.log(x); // 3
The value of a variable may vary. Hence, the name variable.
The rules for naming variables are:
- 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';
- Variable names cannot start with numbers. For example,
//invalid Let 1a = 'hello'; // this gives an error
let y = "hi"; let Y = 5; console.log(y); // hi console.log(Y); // 5
- Keywords cannot be used as variable names. For example,
//invalid let new = 5; // Error! new is a keyword.
- 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.
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 initializing 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
Now that you know about variables, you will learn about different types of data a variable can store in the next tutorial.