To extend an existing JavaScript array with another array, without creating a new array, we call the push method.
For instance, we write
const a = [1, 2];
const b = [3, 4, 5];
a.push(...b);
to call a.push with the items in b as arguments to append the items in b into a in the order they’re listed.
We use the spread operator to call push with the entries in b as arguments.
