The Spread Operator

const list = [1,1,2,3,5];
const bigger_list = [...list, 8, 13];
// [1, 1, 2, 3, 5, 8, 13];

const fn = (x, y, z) => x + y + z;
fn(bigger_list);
// "1,1,2,3,5,8,13ndefinedundefined" ([1,1,2,3,5,8,13] + undefined + undefined)
fn(...bigger_list);
// 4 (1 + 1 + 2)

const src = { foo: "bar" };
const cpy = { ...src }; // { foo: "bar" }
const tgt = { ...src, bar: "foo" }
// { foo: "bar", bar: "foo" }