A set of Array utils which I use (It should be noted that some are dependent on others)

Array.prototypes.js

// Calculates the sum of the items in the array.
// The sum is the result of adding all the items together.
Array.prototype.sum = function(){
	return this.reduce((a, b) => a + b);
}

// Checks whether the specified item is present in the array.
Array.prototype.contains = function (item) {
	return this.find(x => x === item) !== undefined;
}

// Counts the number of times the specified item occurs in the array.
Array.prototype.count = function(item){
	return this.filter(x => x === item).length;
}

// Calculates the minimum value in the array.
// The minimum value is the smallest item in the array.
Array.prototype.min = function () {
	return Math.min.apply(null, this);
}

// Calculates the maximum value in the array.
// The maximum value is the largest item in the array.
Array.prototype.max = function () {
	return Math.max.apply(null, this);
}

// Calculates the statistical range of the array.
// The range is the difference between the largest and smallest values in the array.
Array.prototype.range = function(){
	return this.max() - this.min();
}

// Calculates the arithmetic mean of the array.
// The arithmetic mean is the average of the items in the array.
Array.prototype.mean = function(){
	return this.sum() / this.length
}

// Calculates the median for the specified percentile of the array.
Array.prototype.quartile = function (q) {
    let a = this.slice().sort((c, d) => c - d);
    let p = q * (a.length - 1), b = Math.floor(p);
    return a[b + 1] ? a[b] + (p - b) * (a[b + 1] - a[b]) : a[b];
};

// Calculates the median of the array.
// The median is the middle item in a sorted array.
Array.prototype.median = function(){
	return this.quartile(0.5);
}

// Calculates lower quartile median of the array.
// The lower quartile is the median of the lower half of the array.
Array.prototype.Q1 = function() {
	return this.quartile(0.25);
}

// Calculates upper quartile median of the array.
// The upper quartile is the median of the higher half of the array.
Array.prototype.Q3 = function() {
	return this.quartile(0.75);
}

// Calculates the interquartile range of the array.
// The interquartile range is the difference between the upper and lower quartiles.
Array.prototype.IQR = function(){
	return this.Q3() - this.Q1();
}

// Calculates the statistical mid-range of the array.
// The mid-range is the mean of the lower quartile and the upper quartile.
Array.prototype.midRange = function(){
	return (this.Q1() + this.Q3()) / 2;
}

// Calculates the lower outlier limit of the array.
Array.prototype.outlierMin = function(){
	return this.Q1() - (1.5 * this.IQR());
}

// Calculates the upper outlier threshold of the array.
Array.prototype.outlierMax = function(){
	return this.Q3() + (1.5 * this.IQR());
}

// Calculates the mode of the array.
// The mode is the most common item in the array.
Array.prototype.mode = function(){
	return this.reduce((a, b) => (a.count(b) > b.count(b) ? a : b));
}

// Calculates the variance of the array.
// The variance is the sum of the squared deviations from the mean.
Array.prototype.variance = function(){
	let mean = this.mean();
	return this.map((num) => Math.pow(num - mean, 2)).mean();
}

// Calculates the standard deviation of the array.
// The standard deviation is the square root of the variance.
Array.prototype.stdDev = function(){
	return Math.sqrt(this.variance());
}

// Calculates the absolute deviation of the array.
// The absolute deviation is the difference between the item and the mean.
Array.prototype.absDev = function(){
	let mean = this.mean();
	return this.map(num => Math.abs(num - mean)).mean();
}

// Calculates the z-scores for the items in the array.
// The z-score is the difference between the item and the mean divided by the standard deviation.
Array.prototype.zScore = function(){
	let mean = this.mean();
	let stdev = this.stdDev();
	return this.map((num) => { return (num - mean) / stdev; });
}

Array.prototypes.min.js

const prototypes = new Map();
prototypes.set('count', function (item) { return this.filter(x => x === item).length; });
prototypes.set('contains', function (item) { return this.find(x => x === item) !== undefined; });
prototypes.set('quartile', function (q) {
    let a = this.slice().sort((c, d) => c - d), p = q * (a.length - 1), b = Math.floor(p);
    return a[b + 1] ? a[b] + (p - b) * (a[b + 1] - a[b]) : a[b];
});
prototypes.set('sum', function () { return this.reduce((a, b) => a + b); });
prototypes.set('min', function () { return Math.min.apply(null, this); });
prototypes.set('max', function () { return Math.max.apply(null, this); });
prototypes.set('range', function () { return this.max() - this.min(); });
prototypes.set('mean', function () { return this.sum() / this.length });
prototypes.set('median', function () { return this.quartile(0.5); });
prototypes.set('mode', function () { return this.reduce((a, b) => (a.count(b) > b.count(b) ? a : b)); });
prototypes.set('Q1', function () { return this.quartile(0.25); });
prototypes.set('Q3', function () { return this.quartile(0.75); });
prototypes.set('IQR', function () { return this.Q3() - this.Q1(); });
prototypes.set('midRange', function () { return (this.Q1() + this.Q3()) / 2; });
prototypes.set('outlierMin', function () { return this.Q1() - (1.5 * this.IQR()); });
prototypes.set('outlierMax', function () { return this.Q3() + (1.5 * this.IQR()); });
prototypes.set('variance', function () { return this.map((num) => Math.pow(num - this.mean(), 2)).mean(); });
prototypes.set('stdDev', function () { return Math.sqrt(this.variance()); });
prototypes.set('absDev', function () { return this.map(num => Math.abs(num - this.mean())).mean(); });
prototypes.set('zScore', function () { return this.map((num) => { return (num - this.mean()) / this.stdDev(); }); });

Array.from(prototypes).forEach(([key, value]) => {
    Object.defineProperty(Array.prototype, key, {
        get: function () { return value.bind(this)(); }
    });
});