Split arrays into an object of arrays (by string before "_")

splitArrays.js

function splitArrays(arrayData) {
    var data = arrayData;

    var arrays = {};

    if(data) {
        data.forEach(function (str) {

            // Get id piece
            str_api = str.substring(0, str.indexOf('_'));

            // check if existing property for this id, if not initialize new array
            if (!arrays[str_api]) {
                arrays[str_api] = [];
            }

            // get value piece
            str_id = str.substring(str.indexOf() + 1);

            // add to that id's array
            arrays[str_api].push(str_id);

        });
    }

    return arrays;

}