JavaScript: Variables

Variables are used to store this information.

A variable is a “named storage” for data. We can use variables to store goodies, visitors, and other data.

To create a variable in JavaScript, use the let keyword.

We can put any value in the box.

We can also change it as many times as we want:

let message;

message = 'Hello!';

message = 'World!'; // value changed

alert(message);

When the value is changed, the old data is removed from the variable:

We can also declare two variables and copy data from one into the other.

let hello = 'Hello world!';

let message;

// copy 'Hello world' from hello into message
message = hello;

// now two variables hold the same data
alert(hello); // Hello world!
alert(message); // Hello world!

There are two limitations on variable names in JavaScript:

1. The name must contain only letters, digits, or the symbols $ and _.

2. The first character must not be a digit.

let $ = 1; // declared a variable with the name "$"
let _ = 2; // and now a variable with the name "_"

alert($ + _); // 3

Case matters

Variables named apple and AppLE are two different variables.

Non-Latin letters are allowed, but not recommended

It is possible to use any language, including cyrillic letters or even hieroglyphs, like this:

let имя = '...';
let 我 = '...';

Constants: to declare a constant (unchanging) variable, use const instead of let:

const myBirthday = '18.04.1982';

Summary

We can declare variables to store data by using the var, let, or const keywords.

• let – is a modern variable declaration. The code must be in strict mode to use let in Chrome (V8).

• var – is an old-school variable declaration. Normally we don’t use it at all, but we’ll cover subtle differences from let in the chapter The old "var", just in case you need them.

• const – is like let, but the value of the variable can’t be changed.

Variables should be named in a way that allows us to easily understand what’s inside them.