groupBy function to group a given array by a given accessor and return the grouped values as a js Map

groupArrayByAccessor.js

/**
* group an array by accessor
* @param arr array of elements
* @param accessor called on element, must return a value from the elements after which they are to be grouped
* @returns Map where keys are defined by accessor function and values are an array of elements
*/
const groupBy = (a, accessor) => {
    if (!Array.isArray(a)) throw 'param a is not an array';
    if (typeof(accessor) != 'function') throw 'param accessor is not a function';
    return a.reduce(
        (map, e) => map.set(accessor(e), [...map.get(accessor(e))||[], e]),
        new Map()
    );
}