implement add method to Map Object to handle collision when add different value to the same key.

add-to-map.js

// add method to Map object
Map.prototype.add =  function (key, value){
    // check first if the  key has value if not just set the value to the key
    if ( map.has(key)) {
        let oldValue = map.get(key); // get old value
        let tempArr = []; // create temp array to collect new and values together
        /* check if old value is array or not to make the values on one array which make 
        iteration over it when get the value command excute easier
        **/
        oldValue.constructor === Array ? tempArr = oldValue : tempArr.push(oldValue);
        tempArr.push(value); // push new value to temp array
        map.set(key,tempArr); // set temp array as new value to the key
        return; // return from the function
    } 
    map.set(key,value);
}