JavaScript

The primary programming language used for browser extensions is JavaScript. If you want to read more about JavaScript in general, there is some good info and tutorials here. The rest of this page is a very quick intro into the basic syntax of JavaScript.

If you’ve used C or a similar language before, and the example below makes sense to you, you can head to the page on Asynchronous JavaScript, which will probably be new to you, and read that in depth.

If not, you might want to read this page in a bit more carefully and follow the links, but don’t get too bogged down in the details of syntax—once you start writing code things will start to make sense.

JavaScript Example

Here is a basic example of JavaScript code:

function printFruits(fruits) {
	for (let i = 0; i < fruits.length; i++) {
		console.log(fruits[i], "are a fruit");
	}
}

const fruits = ["Apples", "Bananas", "Oranges"];
printFruits(fruits);

/* prints:
Apples are a fruit
Bananas are a fruit
Oranges are a fruit
*/

Basic Components

Variables

JavaScript is dyamically typed, interpreted language, meaning that as a programmer you don’t have to assign types (such as “string”, “integer”, or “array”) to variables when you declare them. Additionally, you don’t have to compile JavaScript—you just can run the uncompiled source. Lots of words have been written about the advantages and disadvantages of this, but it ends up making some things easier for smaller projects.

You can declare a variable using the let keyword, such as let x = 42; You also can use the const keyword to declare variables you don’t intend to change, and there also is the var keyword which has slightly different rules in terms of lexical scoping. Don’t get hung up on the differences between const, let, and var, it doesn’t really matter what you use, especially for small projects, but let is a good default choice.

More info:

• Grammar and types
• Let and var

Conditionals and Loops

if statements, and for and while loops look like they do in C:

if (x < y) {
	stuff;
}

for (let i = 0; i < n; i++) {
	stuff;
}

while(x < y) {
	x++;
}

More info:

• Control flow and error handling
• Loops and iteration

Functions

Functions are declared using the function keyword, followed by the name of the function, and the arguments in parentheses:

function square(x) {
  return x * x;
}

You can call functions the same way you do in C or similar languages:

square(5); // 25

More info:

• Functions

Printing

console.log is your print statement:

console.log("Hello world!");

More info

See the MDN JavaScript Guide for more information on the different components of JavaScript. There’s also another tutorial here that doesn’t assume as much prior programming experience.

Table of contents

• Asynchronous JavaScript