js array shift
HTML
<button onclick="runnerArr()">
press here
</button>JavaScript
var arr1 = ["x","x","x"];
var arr2 = ["a","b","c","d"];
function runnerArr() {
console.log(arr2);
for(var i =0; i <=arr1.length; i++) {
arr2[i] = arr1.shift();
}
console.log(arr2);
}