Range number to range, find min, max and average number of arrays helper functions

jsHelperFunctions.js

var mapNumberToRange = function (num, in_min, in_max, out_min, out_max) {
    return (((num - in_min) * (out_max - out_min)) / (in_max - in_min) + out_min) || out_min;
}
var maxNum = function (arr) {
    return Math.max.apply(null, arr);
}
var minNum = function (arr) {
    return Math.min.apply(null, arr);
}
var sumArray = function (arr, key) {

    return arr.reduce(function (total, num) {
        return (key !== undefined) ? total + (num[key] || 0) : total + (num || 0);
    }, 0);
}
var avgNum = function (arr, key) {
    var notZeroElements = 1;
    return arr.reduce(function (total, num) {
        if (num != 0) notZeroElements++;
        return (key !== undefined) ? total + (num[key] || 0) : total + (num || 0);
    }, 0) / notZeroElements;
}
var getUniqueElementsByKey = function (arr, key) {
    var newArray = [];
    for (var i = 0; i < arr.length; i++) {
        var unique = true;
        var length = newArray.length;
        for (var j = 0; j < length; j++) {
            if (newArray[j][key] === arr[i][key]) {
                unique = false;
                break;
            }
        }
        if (unique) {
            newArray.push(arr[i]);
        }
    }
    return newArray;
}

// Checks of an object is empty and returns true/false
function isEmpty (obj) {
    var keys = Object.keys(obj);
    for (var i = 0; i < keys.length; i++) {
        if (Object.hasOwnProperty.call(obj, keys[i])) {
            return false;
        }
    }
    return true;
}