Understanding the Prototype Chain: Why Functions and Objects Yield Different Comparison Results

Exploring the Prototype Chain and Comparison in JavaScript

As someone who often dives deep into JavaScript’s intricacies, I recently stumbled upon a curious case involving comparisons within the prototype chain of objects and functions. Let me share my experience and clarification on this matter.

Firstly, to understand the problem, it’s essential to grasp the concept of prototype chains in JavaScript. In JavaScript, almost every object is linked to another object, the prototype, which provides inherited properties and methods. This linkage continues until an object’s prototype is null, indicating the end of the chain.

Let’s start by exploring the prototype chain for a function-based object and a plain object. Consider a simple function cat() used as a constructor to create an instance kitty. Also, let’s compare it with a direct object creation using object literal syntax, stored in myObj.

function cat() {}
let kitty = new cat();
let myObj = {};

When checking their prototypes, you’ll notice that kitty, an instance of cat, inherits from cat.prototype, and since cat is a function, cat.prototype itself has its prototype, which is Object.prototype. On the other hand, myObj directly inherits from Object.prototype.

console.log(typeof kitty.__proto__.__proto__); // Output: object
console.log(typeof myObj.__proto__); // Output: object
console.log(typeof Object.prototype); // Output: object

All appear as ‘object’, which is consistent since in JavaScript, both function prototypes and object prototypes are objects.

Further, if we check if these prototypes are exactly the same object, not just similar or equivalent objects, the results are affirmative:

console.log(kitty.__proto__.__proto__ === Object.prototype); // true
console.log(myObj.__proto__ === Object.prototype); // true 
console.log(myObj.__proto__ === kitty.__proto__.__proto__); // true

Each log statement above returns true, demonstrating that both kitty.__proto__.__proto__ and myObj.__proto__ reference the same object, Object.prototype.

However, a curious thing happens when attempting a chained comparison:

console.log(kitty.__proto__.__proto__ === myObj.__proto__ === Object.prototype); // false

This statement returns false, which might initially seem counterintuitive given our earlier results. To understand why, we need to decipher how JavaScript evaluates this expression. It’s due to the way the equality operator === works in JavaScript, following the associative property, which in simple terms, processes from left to right.

The expression kitty.__proto__.__proto__ === myObj.__proto__ === Object.prototype is evaluated as:

(kitty.__proto__.__proto__ === myObj.__proto__) === Object.prototype

Since the first comparison (kitty.__proto__.__proto__ === myObj.__proto__) evaluates to true, the resulting expression becomes:

true === Object.prototype

Clearly, true is not the same object as Object.prototype, which explains why the final result is false.

So, the takeaway from this exploration is that while kitty.__proto__.__proto__, myObj.__proto__, and Object.prototype all refer to the same underlying object, the way JavaScript evaluates chained equality comparisons can lead to unexpected results. Such nuances of JavaScript can be fascinating and, at times, a bit tricky, making the language a continuous learning journey.