2.4 Variables

Most of the time, a JavaScript application needs to work with information. Here are 2 examples:

1. An online-shop – the information might include goods being sold and a shopping cart.

2. A chat application – the information might include users, messages, and much more.

Variables are used to store this information.

A variables

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, we need to use the let keyword.

The statement below creates (in other words: declares or defines) a variable with the name “message”:

let message;

Now we can put some data into it by using the assignment operator =:

let message;

message = 'Hello'; // store the string

The string is now saved into the memory area associated with the variable. We can access it using the variable name:

let message;
message = 'Hello!';

alert(message); // shows the variable content

To be concise we can merge the variable declaration and assignment into a single line:

let message = 'Hello!'; // define the variable and assign the value

alert(message); // Hello!

We can also declare multiple variables in one line:

let user = 'John', age = 25, message = 'Hello';

That might seem shorter, but it’s not recommended. For the sake of better readability, please use a single line per variable.

The multiline variant is a bit longer, but easier to read:

let user = 'John';
let age = 25;
let message = 'Hello';

Some people also write many variables like that:

let user = 'John',
    age = 25,
    message = 'Hello';

...Or even in the “comma-first” style:

let user = 'John'
  , age = 25
  , message = 'Hello';

Technically, all these variants do the same. So, it’s a matter of personal taste and aesthetics.

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var instead of let

In older scripts you may also find another keyword: var instead of let:

var message = 'Hello';

The var keyword is almost the same as let. It also declares a variable, but in a slightly different, “old-school” fashion.

There are subtle differences between let and var, but they do not matter for us yet. We’ll cover them in detail later, in the chapter The old "var".

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