Array(n).fill([]) shares one array

fill([]) evaluates [] once and puts that same array in every slot. Pushing to one pushes to all.

JavaScriptMutation and copying bugs

What it means

fill() copies the value you give it into each position. For numbers and strings that is fine. For an array or object, every position holds a reference to one shared object.

Common causes

1. Creating a list of empty groups

Every group is the same array.

Breaks

const teams = Array(3).fill([]);
teams[0].push("a");   // all three now contain "a"

Works

const teams = Array.from({ length: 3 }, () => []);

2. Filling with an object

Same trap with {}.

Breaks

const rows = new Array(5).fill({ done: false });

Works

const rows = Array.from({ length: 5 }, () => ({ done: false }));

How to find it in your own code

Use Array.from({ length: n }, () => newValue) so the factory function runs once per slot. Keep fill() for primitives such as 0, "" or false.

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Other common errors

list.sort() returns None TypeError: 'str' object does not support item assignment RuntimeError: dictionary changed size during iteration Mutable default argument keeps old values Changing one row of a 2D list changes them all TypeError: Cannot read properties of undefined ReferenceError: Cannot access before initialization [object Promise] in output