What JavaScript Is and Where It Runs

What JavaScript Is and Where It Runs

JavaScript is a programming language. A programming language is a way to give a computer step-by-step instructions. JavaScript is special because it runs inside every web browser, like Chrome, Firefox, or Safari. When a page shows a pop-up, updates without reloading, or checks your form, that is usually JavaScript at work.

JavaScript also runs outside browsers, for example on servers with a tool called Node.js. In this tutorial we focus on the browser.

Your First JavaScript: Running Code in the Browser

You can try JavaScript right now, with no setup:

1. Open any web page in Chrome or Firefox.
2. Press F12 (or right-click, then choose "Inspect").
3. Click the Console tab.
4. Type 2 + 2 and press Enter.

The console is a place where you type code and see results. Let's print a message:

console.log('Hello, JavaScript!');

console.log is an instruction that prints text to the console.

Values and Data Types

A value is a piece of data, like a number or a piece of text. Each value has a type. These are the core types:

Strings use quotes: 'JavaScript' or "JavaScript".

Declaring Variables with let and const

A variable is a named box that stores a value. You create one with let (can change later) or const (cannot change):

const language = 'JavaScript';
let year = 1995;
year = year + 1;
console.log(`${language} was created in ${year - 1}`);

const means constant: fixed. let means "let this change". The line with backticks builds text by dropping values into placeholders marked with ${}. This is called a template literal. Rule of thumb: use const by default, and let only when the value must change.

Operators and Expressions

An operator is a symbol that does work on values, like + or *. An expression is code that produces a value, like 2 + 3 (which gives 5).

Common operators:

• +, -, *, / — do math
• % — remainder after dividing, like 7 % 2 gives 1
• === — compares two values, gives true or false
• !== — checks that two values are not equal
• && — and; || — or; ! — not

console.log(5 + 3);        // 8
console.log(10 % 3);       // 1
console.log(5 === 5);      // true
console.log(4 > 2 && 2 > 1); // true

Always use === to compare. A looser operator == exists but can give surprising results.