fill() & copyWithin() in JavaScript
From the JavaScript Array Methods cheat sheet · Array Creation & Conversion · verified Jul 2026
fill() & copyWithin()
Fill array with values or copy elements within
javascript
// Fill array with value
[1, 2, 3].fill(0); // [0, 0, 0]
[1, 2, 3].fill(0, 1); // [1, 0, 0]
[1, 2, 3].fill(0, 1, 2); // [1, 0, 3]
// Copy within array
[1, 2, 3, 4, 5].copyWithin(0, 3); // [4, 5, 3, 4, 5]⚠️ Both methods MUTATE the array
💡 fill() sets multiple elements to same value
📌 copyWithin() copies part to another position
⚡ Useful for array initialization
🔴 Be careful with object references in fill()
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