Variables in JavaScript are used to store information. You can visualize them as boxes that hold something inside.

To declare a variable, there are three main ways.

Two keywords, let and const, belong to the modern version of JavaScript.

There’s also the keyword var, which is considered outdated and is not recommended for use.

let

The first method is using the keyword let.

To declare a variable, use the keyword let, followed by the variable name, then an equals sign "=", and finally the value to assign to that variable.

In programming, the equals sign "=" means assignment.

let name = "Cxd3";
// let — keyword for declaring a variable
// name — variable name
// = — assignment operator
// Cxd3 — value assigned to the variable
// ; — end of the code statement

const

The second method is using the keyword const.

const stands for “constant”, meaning the variable declared with const cannot be reassigned.

const dateOfBirth = 29.06.1999;

A variable name can contain letters, digits, the dollar sign $, and underscores _. However, it cannot start with a digit.

Also, variable names must not use reserved words in JavaScript, such as error, alert, prompt, and so on.

Use const for values that do not change later, such as names, dates of birth, etc.

Note: the value of a const variable can be modified if the variable holds an object.

const object = {
    age: 25
};
object.age = 35;
console.log(object.age); // output: 35

var

var is the old way of declaring variables. It’s found in legacy code.

Variables declared with var can also be reassigned like those declared with let.

The issue with var is that the variable exists even before it is declared in the code, making it visible everywhere.

If you try to access a var variable before its declaration, JavaScript will return undefined. For example:

console.log(name); // undefined
var name = "Cxd3";

This behavior is called “hoisting”.

Another feature of let and const is that they are scoped to the block (enclosed in curly braces).

{
  let result = 29;
}
console.log(result); // result is not defined

If declared with var, the variable result would be accessible outside the block.

To check what features are supported by different browsers, use the service Can I Use

“use strict”

This directive is placed at the beginning of a script and tells the browser to run the code in “strict mode”.

"use strict";
// this code runs in strict mode

Without "use strict", it’s possible to access undeclared variables. With "use strict", accessing such variables will result in “is not defined” errors.