Four array methods replace most of the for-loops you'll ever write. What map, filter, reduce and find each do, step by step, how to chain them, and when a plain loop is still the better choice.

A hash function maps any input to a fixed-size number, the same input always to the same number. How a simple one works, why collisions are unavoidable, what makes a hash good, and how the same idea powers hash maps, caches and passwords.

The browser turns your HTML into a tree of objects, and that tree — not the HTML file — is what's on screen and what JavaScript changes. How to find, change, create and remove elements, and how events travel through the tree.

The two most basic ways to store a sequence. Arrays keep items side by side for instant access by position; linked lists chain scattered nodes for cheap inserts. What each operation costs, and why arrays win most of the time anyway.