groupBy.js

   /**
     * This will group a list of JSON objects (or any object) by a specific value defined by the user
     *
     * @example Example of a JSON object
     * let example = [
     *    {
     *      name: 'hello'
     *    },
     *    {
     *      name: 'team'
     *    }
     * ];
     * let grouped = groupBy(example, 'name.charAt.toUpperCase', [0,null]);
     * // Returns the object where 'H' and 'T' are valid keys.
     * Output = {
     *      H: [ { name: 'hello'}],
     *      T: [ { name: 'team'}],
     * }
     * @example Example using strings
     *  const l = ['hello', 'hellz', 'foobar', 'foo'];
     *
     * const expected = {
     *  HE: ['hello', 'hellz'],
     *  FO: ['foobar', 'foo']
     *};
     *
     * groupBy(l,'substring.toUpperCase',[[0,2],null]) == expected;
     * //returns true
     * @param {Array} list List of objects
     * @param {string} func Period seperated function calls or key calls
     * @param {Array} passIn list of values that will be passed into whatever functions are encountered in the func string
     * ORDER DOES MATTER
     * @returns {{}} Dictionary object with each key being the value you specified by user, and key values being the objects
     * that fall into that category.
     * @memberOf Utilities
     *
     */
    groupBy: (list, func, passIn) => {

        const isIterable = (object) => {

            return object !== null && typeof object[Symbol.iterator] === 'function';
        };

        return list.reduce((acc, val) => {

            const calls = func.split('.');
            let p = -1;
            const t = calls.reduce((a,c) => {

                if (typeof a[c] === 'function') {
                    const f = a[c].bind(a);
                    return f.apply(f, isIterable(passIn[++p]) ? passIn[p] : [passIn[p]]);
                }
                return a[c];
            }, val);
            if (t in acc){
                acc[t].push(val);
            }
            else {
                acc[t] = [val];
            }
            return acc;
        }, {});
    }