Eloquent JavaScript

eloquentjavascript.net

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Marijn Haverbeke's free book on JavaScript and programming, readable online with a built-in code sandbox so you can run and edit every example on the page.

Why it is useful

One of the best free introductions to programming itself, not just to JavaScript. It covers the language, the browser and Node.js, and finishes with real projects. The book is openly licensed, so there is nothing to sign up for or pay.