Why does array sort behave unexpectedly with numbers?

The default comparison is string-based, which is why [10, 2, 1].sort() produces [1, 10, 2]. This is one of the most common JavaScript surprises.

console.log([10, 2, 1].sort()); // [1, 10, 2]

console.log([10, 2, 1].sort((a, b) => a - b)); // [1, 2, 10]
console.log([10, 2, 1].sort((a, b) => b - a)); // [10, 2, 1]

How the comparator works

• Return a negative number when a should come before b
• Return a positive number when a should come after b
• Return zero when the order does not matter
• The comparator must be consistent or the sort order is undefined

sort mutates the array and returns the same reference. If you need a new array, copy first with slice() or spread, or use the newer toSorted() method.

const original = [3, 1, 2];
const sorted = original.toSorted((a, b) => a - b);

console.log(sorted); // [1, 2, 3]
console.log(original); // [3, 1, 2]