Finding the mode of an array
var mode = function(arr){


It's nice to name your functions for debugging purposes, if none other. You could replace this with a function declaration, e.g. function mode(arr) { or just name the function expression: var mode = function mode(arr) {

    var numMapping = {};
    for(var i = 0; i &lt; arr.length; i++){
        if(numMapping[arr[i]] === undefined){
            numMapping[arr[i]] = 0;
        }        
            numMapping[arr[i]] += 1;
    }


I would use arr.forEach(), since it lets you operate on the elements directly. You could also make the mapping a one-liner(but both of these things are more nitpicks/personal style choice)

    var greatestFreq = 0;
    var mode;
    for(var prop in numMapping){
        if(numMapping[prop] &gt; greatestFreq){
            greatestFreq = numMapping[prop];
            mode = prop;
        }
    }
    return parseInt(mode);
}


You can do this whole part during your first loop! Also, parseInt, while safe in this case, is not generally safe to use without a radix(you'd have linters complaining about this usage here). You could use parseInt(mode, 10), or some ToNumber() transformation like +mode(http://es5.github.io/#x11.4.6).

Here's a revised example:

var mode = function mode(arr) {
    var numMapping = {};
    var greatestFreq = 0;
    var mode;
    arr.forEach(function findMode(number) {
        numMapping[number] = (numMapping[number] || 0) + 1;

        if (greatestFreq &lt; numMapping[number]) {
            greatestFreq = numMapping[number];
            mode = number;
        }
    });
    return +mode;
}