What is it

Appends contents of another array to existing array

const array1 = [1, 2, 3]
const array2 = ["a", "b", "c"]

const resultOfConcat = array1.concat(array2);
// [1, 2, 3, "a", "b", "c"]

Alternative

Spread operator is recommended when concatenating multiple arrays

const arr1 = [0,1,2];
const arr2 = [3,4,5];

const arr3 = [...arr1, ...arr2];

console.log(arr3); // [0,1,2,3,4,5]