Javascript Array valueOf()

const arraySum = a => a.valueOf();
Array.prototype.valueOf = function() {
  return this.reduce((x, y) => x + (y.valueOf() === +y ? y : 0), 0);
};

/*//from  w w  w  . j  a v a2 s  .c o  m
Description:

Enable arithmetic operations on arrays, where the value of an array is given by the sum of its elements. Examples:

[1,2,3] + 4 = 10
[1,1,1] - 3 = 0
['a','b'] + 'c' = 'abc'
PS: Due to intrinsic uncertainty of empty arrays, in this kata an empty array is treated as 0. For hints see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/valueOf
*/

Array.prototype.valueOf = function() {
    
    if (typeof this[0] == "number"){
        var a = 0;
    } else {
        var a = '';
    }
    for (var i = 0; i < this.length; i++) {
        a += this[i]
    }
    
    return a;
};