Programs store and transform values. In JavaScript you create variables using let or const. Use const for values that will not be reassigned, such as const taxRate = 0.2, and let for values that will change, such as a running total. An older keyword, var, still works but has confusing scoping rules, so modern code […]

Control flow determines which statements run and how many times. JavaScript's control structures will look familiar if you have used other languages, and they mirror the building blocks of every algorithm: sequence, selection and iteration. if, else if and else An if statement runs a block of code inside curly braces only when its condition […]

Functions are the main way to organise JavaScript code. They let you name a set of steps, reuse them, and pass behaviour around as values. JavaScript treats functions as first-class values, which means you can store them in variables, pass them to other functions and return them from functions. This idea powers much of the […]

Almost every real program works with collections of data. JavaScript's two essential structures are arrays, ordered lists of values, and objects, collections of named properties. Modern JavaScript provides expressive methods for working with both, and mastering them will make your code shorter and clearer. Arrays An array is written with square brackets, such as const […]

When the browser loads an HTML page, it builds an in-memory tree of objects representing every element, called the Document Object Model or DOM. JavaScript can read and change this tree, and the browser immediately updates what the user sees. This is how pages show messages, update totals, reveal menus and build lists without reloading. […]

Forms are how users give information to a web page: sign-ups, searches, checkouts and settings. JavaScript lets you improve forms by validating input before it is sent, giving instant feedback, and remembering preferences between visits using storage built into the browser. Reading form values Each input element has a value property containing what the user […]

Interactive pages respond to what users do, and modern applications constantly exchange data with servers. This final lesson covers events, which let your code react to user actions, and asynchronous programming, which lets your code wait for slow operations such as network requests without freezing the page. Listening for events The addEventListener() method attaches a […]