check multiple keys pressed at same time

multiple-keypress.js

window.keysOnHold = [];

window.onkeydown = function(e){
    if(this.keysOnHold.indexOf(e.keyCode) < 0){
        this.keysOnHold.push(e.keyCode);
    }
    console.log(this.keysOnHold);
};

window.onkeyup = function(e){
    this.keysOnHold.splice(this.keysOnHold.indexOf(e.keyCode), 1);
    console.log(this.keysOnHold);
};