an equals function for comparing two arrays in JavaScript

array-equals.js

// 14 June 2023
// an equals function for comparing two arrays in JavaScript

// how we compare two arguments for equality:
// both are arrays
// with same length
// with same items at same indexes

// why this does not exist in the JavaScript built-in space: sorting.

// our algorithm assumes that if they differently sorted they are not equal,
// because sometimes insertion order matters.

Array.equals = function (a, b) {
  return Array.isArray(a)
    && Array.isArray(b)
    && a.length === b.length
    && a.every(function (v, i) {
      return v === b[i]
    });
};

// In real life we won't append `equals` as a function to the `Array` namespace.

// A better approach in JS land would be an `equals` method in the `Array` *prototype*
// so we could use `a.equals(b)`, as with this implementation...

Array.prototype.equals = function(a) {
  if (!Array.isArray(a) || this.length !== a.length) {
    return false;
  }

  // micro-optimization:
  // instead of using the `every` method, use a faster fail loop
  // in case we have big arrays to compare...

  for (var i = 0, j = this.length - 1; i <= j; i++, j--) {
    if (this[i] !== a[i] || this[j] !== a[j]) {
      return false;
    }
  }

  return true;
}


/* test it out */

var a = ['1', 0];
var b = ['1', '3'];
var c = Array.from(a);

var s = a.slice().sort();
var t = c.slice().sort();

var o = {name: 'o'}
var p = o;
var q = Object.create(p);

console.group("Array.equals");
  console.assert(!Array.equals(1, 1), "primitives");
  console.assert(!Array.equals([], [1]), "unequal lengths");

  console.assert(!Array.equals(a, b), "should not equal");
  console.assert(Array.equals(a, c), "should equal");

  console.assert(!Array.equals(a, s), "differently sorted");
  console.assert(Array.equals(s, t), "equal sort");

  console.assert(!Array.equals([o], [q]), "different items");
  console.assert(Array.equals([o], [p]), "same items");
console.groupEnd("Array.equals");

console.group("instance.equals");
  console.assert(!a.equals(1), "primitives");
  console.assert(!a.equals([]), "unequal lengths");

  console.assert(!a.equals(b), "should not equal");
  console.assert(a.equals(c), "should equal");

  console.assert(!a.equals(s), "differently sorted");
  console.assert(s.equals(t), "equal sort");

  console.assert(![o].equals([q]), "different items");
  console.assert([o].equals([p]), "same items");
console.groupEnd("instance.equals");

/* should see no errors */