What is the difference between slice and splice?

The names are similar, but the mutation behaviour is opposite. This is a common interview question because it tests whether you know which array methods are pure.

const letters = ["a", "b", "c", "d"];

const slice = letters.slice(1, 3);
console.log(slice); // ["b", "c"]
console.log(letters); // ["a", "b", "c", "d"]

const removed = letters.splice(1, 2, "x");
console.log(removed); // ["b", "c"]
console.log(letters); // ["a", "x", "d"]

Arguments and return values

• slice(start, end) copies from start up to but not including end
• splice(start, deleteCount, ...items) removes elements and inserts new ones
• Negative indexes count from the end for both methods
• slice returns a new array; splice returns the removed elements

The copy from slice is shallow. Objects inside the new array are still the same references as the objects in the original array.

const original = [{ id: 1 }];
const copy = original.slice();

copy[0].id = 99;
console.log(original[0].id); // 99