sets.js

#!/usr/bin/env node

function intersect(x, y) {
  const setX = new Set(x);
  const setY = new Set(y);

  let intersected = [];
  for (let value of setX) {
    // I worked with Sets because of an assumption that this is
    // cheaper than Array#includes.
    if (setY.has(value)) {
      intersected.push(value);
    }
  }

  return intersected;
}

function union(x, y) {
  return Array.from(new Set([...x, ...y]));
}

function difference(x, y) {
  const setX = new Set(x);
  const setY = new Set(y);

  let complements = [];
  for (let value of setY) {
    if (!setX.has(value)) {
      complements.push(value);
    }
  }

  return complements;
}

// Apparently also known as a disjunctive union.
function symmetricDifference(a, b) {
  const c = union(a, b);

  // Probably there's a more efficent way of doing this
  // but this is at least expressive.
  return difference(intersect(a, b), c);
}

console.log(intersect([1, 2, 3, 4, 11], [3, 4, 5, 6, 7, 8, 9]));
console.log(union([1, 2, 3, 4, 11], [3, 4, 5, 6, 7, 8, 9]));
console.log(difference([1, 2, 3, 4, 11], [3, 4, 5, 6, 7, 8, 9]));
console.log(symmetricDifference([1, 2, 3, 102], [2, 3, 4, 5, 99, 100]));

function createDifference(identity) {
  return function difference(xs, ys) {
    const xsMap = new Map(xs.map(x => [identity ? identity(x) : x, x]));
	const ysMap = new Map(ys.map(y => [identity ? identity(y) : y, y]));
	  
	let complements = [];
	for (let [key, value] of xsMap) {
	  if (!ysMap.has(key)) {
	    complements.push(value);
	  }
	}
  
    return complements;
  };
}

But what about when we want to get out all items with the same identity but a changed inner property?

function createChanged(identity) {
  return function changed(xs, ys) {
    const xsMap = new Map(xs.map(x => [identity ? identity(x) : x, x]));
	const ysMap = new Map(ys.map(y => [identity ? identity(y) : y, y]));
	  
	let items = [];
	for (let [key, value] of xsMap) {
	  if (ysMap.has(key) && isEqual(ysMap.get(key), value) === false) {
	    items.push(ysMap.get(key));
	  }
	}
  
    return items;
  };
}