How to get array with number from string

var letters = string.split(',');
var drp1 = {};
for (var i = 0; i < letters.length; i++) {
    drp1[letters[i]] = -1 - i;
}

var string = "A,B,C,D";
var res = {};
string.split(',').forEach(function(e, i) {
  res[e] = - (i + 1);
});
console.log(res);//prints {A: -1, B: -2, C: -3, D: -4}

The simple solution would be to use split to convert your string to an array, then use reduce to construct the object in the form you want:

var string = "A,B,C,D"
var drp1 = string.split(',').reduce(function(o,x,i){
    o[x] = -1 - i; 
    return o; 
}, {});

Note that reduce was introduced in ECMAScript 5.1, so it may not work in older browsers. If this is a concern, use the polyfill technique described in the linked documentation, or a simple for-loop as Barmar's answer shows.

You could use Array.map (see MDN)

var obj = {};
'A,B,C,D'.split(',').map(function (v, i) {this[a] = -(i+1);}, obj);