JavaScript Array_fill Guide
Array.fill() and .with() for initializing and updating array elements without mutation. The guide walks through fill() & with(). `arr.fill(value, start, end)` — fills with value (mutates). `arr.with(index, value)` — returns new array with updated value (immutable, ES2023). Create initialized: `Array(5).fill(0)`. Create sequence: `Array.from({length: 5}, (_, i) => i)`. `with()` is useful for immutable state updates in React. The guide is organized into 1 sections that build on each other, each pairing a prose explanation with runnable code. Related guides: Arrays, Strings, Numbers, Math, Date. This is one of 145 JavaScript guides on JS Reference — see the full list on the guides page. Practice what you learn in the JavaScript playground, and look up any unfamiliar term in the JavaScript glossary.