Merge different arrays

We can use the ES6 spread operator to merge different arrays into one:
const a = [1, 2, 3];
const b = [4, 5, 6];
const c = [7, 8, 9];
const final = [...a, ...b, ...c]; // [1, 2, 3, 4, 5, 6, 7, 8, 9]
The spread operator is handy if we use the push method as well:
const a = [1, 2, 3];
const b = [4, 5, 6];
a.push(...b);
a; // [1, 2, 3, 4, 5, 6]