Pass variable from array into a function

In ES6 you can do this:

getTotal(...numArray);

It called Spread syntax. For more info: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator

You can call the function using Function.prototype.apply. It will pass the Array as arguments to your function.

function getTotal () {
    var args = Array.prototype.slice.call(arguments);
    var total = 0;
    for (var i = 0; i < args.length; i++) {
        total += args[i];
    }

    return total;
}

var numArray = [1,2,3,4];

console.log( getTotal.apply( null, numArray ) );

Keep in mind you have a typo in your for loop. Should be args.length, instead of arg.length.

Please try this. Hope it helps

var sum = [1, 2, 3, 4, 5].reduce(add, 0);

function add(a, b) {
    return a + b;
}

If you use the spread operator, you may also want to ES6ify your code:

function getTotal(...vals){
  return vals.reduce((a,b)=>a+b,0);
}

getTotal(...[1,2,4]);

Why not just do this:

function getTotal (args) {
    var total = 0;
    for (var i = 0; i < args.length; i++) {
        total += args[i];
    }

    return total;
}

var numArray = [1,2,3];
console.log(getTotal(numArray)); //6