let arr1 = ['1', '2'];
let arr2 = ['3', '4'];

let combarr = [].concat(arr1); //define new variable, empty array then concatenating arr1 to it
combarr = combarr.concat(arr2); //combarr = itself, then concatenating arr2 to the end
console.log(combarr); //Expected output: ['1','2','3','4']