spread-operator.js

let first = [1,2,3];
let second = [4,5,6];

// If we do this
first.push(second);

// We get
console.log(first); // [1,2,3,[4,5,6]] that is not right

// Using the spread operator
first.push(…second);
console.log(first); // [1,2,3,4,5,6] that's what we wanted!