Extract an array of differences by key from two arrays of objects

Array.prototype.extractDiff.js

Array.prototype.extractDiff = extractDiff;

var arr1 = [{ hash: 1234 }, { hash: "abcdefghijk" }];
var arr2 = [{ hash: 1234 }, { hash: "abcdefghijklmnopqrstuvwxyz" }];
var arr3 = [{ hash: 1234 }, { hash: "abcdefghijklmnopqrstuvwxy" }, { hash: "xyz1234" }];
var arr4 = [{ hash: "abcdefghijklmnopqrstuvwxy" }, { hash: "xyz1234" }, { hash: 1234 }];

var diff1 = arr1.extractDiff(arr2, "hash");
var diff2 = arr3.extractDiff(arr1, "hash");
var diff3 = arr4.extractDiff(arr3, "hash"); // returns [] because 3 and 4 are just different arrangements of same objects

/**
 * @description Find difference between two Arrays of Objects
 * @param Comparison {Object[]} - Array of objects in which to find differences
 * @param Key {String} - Key to perform match against in array's objects
 * @returns Differences {Array}
 */
function extractDiff (comparisonArr, key) {
	if (!key) {
		return new Error("Requires a key for comparison");
	}
	
	var longer, shorter;
	if (this.length < comparisonArr.length) {
      		longer = comparisonArr;
      		shorter = this;
	} else {
      		longer = this;
      		shorter = comparisonArr;
	} 
	
	return longer.map(findMatchesInArray, longer)
		.filter(trimUndefined);

	function findMatchesInArray (currentValue) {
		var matches = shorter.filter(function (comparison) {
			return comparison[key] === currentValue[key];
		});

		if (!matches.length) {
			return currentValue;
		}
	}

	function trimUndefined (val) {
		return val !== undefined;
	}

}