One way we can challenge ourselves to grow as JavaScript developers is to practice with quiz questions! The following questions are intended to be challenging and instructive. If you know exactly how to answer each one, that’s great, but if you get some wrong and learn why you got it wrong, I contend that’s even better!

Question 1: Array Sort Comparison

Consider the following arrays. What gets logged in various sorting conditions?

const arr1 = ['a', 'b', 'c'];
const arr2 = ['b', 'c', 'a'];

console.log(
  arr1.sort() === arr1,
  arr2.sort() == arr2,
  arr1.sort() === arr2.sort()
);

Answer and Explanation

Answer: true, true, false

There are a couple concepts at play here. First, the array sort method sorts your original array and also returns a reference to that array. This means that when you write arr2.sort(), the arr2 array object is sorted.

It turns out, however, the sort order of the array doesn’t matter when you’re comparing objects. Since arr1.sort() and arr1 point to the same object in memory, the first equality test returns true. This holds true for the second comparison as well: arr2.sort() and arr2 point to the same object in memory.

In the third test, the sort order of arr1.sort() and arr2.sort() are the same; however, they still point to different objects in memory. Therefore, the third test evaluates to false.

Question 2: A Set of Objects

Consider the following Set of objects spread into a new array. What gets logged?

const mySet = new Set([{ a: 1 }, { a: 1 }]);
const result = [...mySet];
console.log(result);

Answer and Explanation

Answer: [{a: 1}, {a: 1}]

While it’s true a Set object will remove duplicates, the two values we create our Set with are references to different objects in memory, despite having identical key-value pairs. This is the same reason { a: 1 } === { a: 1 } is false.

It should be noted if the set was created using an object variable, say obj = { a: 1 }, new Set([ obj, obj ]) would have only one element, since both elements in the array reference the same object in memory.

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