Some Array prototypes that I often find useful.

useful-array-prototypes.js

// Find the range of a numerical array
// Array.range()
Array.prototype.range = function() { return this.reduce((a, b) => Math.max(a, b)) - this.reduce((a, b) => Math.min(a, b)); };

// Find the sum of a numerical array
// Array.sum()
Array.prototype.sum = function() { return this.reduce((a, b) => a + b); };

// Find the average of a numerical array
// Array.mean()
Array.prototype.mean = function() { return this.reduce((a, b) => (a + b) / 2 )};

// [1, 2, 3, 4].sum() = 10
// [1, 2, 3, 4].range() = 3
// [1, 2, 3, 4].mean() = 3.125