Spread and Rest

The ... operator does two related jobs. As spread it expands an array or object; as rest it gathers many values into one.

const a = [1, 2];
const b = [...a, 3, 4];
const merged = { ...{ x: 1 }, y: 2 };
function total(...nums) { return nums.length; }

• Spread copies items into a new array or object
• Rest collects extra arguments into an array
• Useful for copying without mutating the original