Immutable.js & Jest-mock-extended Incompatibility Fix

Hey guys! Today, we're diving deep into a tricky issue that some of you might have encountered while using Immutable.js with jest-mock-extended. Specifically, we're talking about an incompatibility that arises when trying to use mergeDeep with mocks generated by jest-mock-extended. This can lead to some unexpected behavior, and we're here to break down the problem, understand the root cause, and explore potential solutions. So, buckle up, and let's get started!

Understanding the Bug: A Code Example

Let's start with a concrete example to illustrate the problem. Imagine you have a test setup like this:

import { mergeDeep } from 'immutable';
import { mock } from 'jest-mock-extended';

descibe('immutable + jest-mock-extended', () => {
  test('mergeDeep', () => {
    const myMock = mock();

    const merged = mergeDeep({ myMock }, { myMock });

    console.log(merged.myMock); // undefined!

    expect(merged.myMock).toBe(myMock); // fails because merged.myMock is undefined
  });
});

In this seemingly simple test, we're creating a mock object using jest-mock-extended, and then attempting to merge it deeply using mergeDeep from Immutable.js. You'd expect merged.myMock to be the same as myMock, but surprise! It turns out to be undefined. This is definitely not the behavior we want, and it highlights a core incompatibility issue between these two libraries.

Diving Deeper: The Unexpected undefined

So, what's going on here? Why is merged.myMock turning into undefined? To understand this, we need to delve into the inner workings of Immutable.js and how it determines if an object is immutable. The key lies in the isImmutable functions within Immutable.js, which are used to check if a given object is considered an Immutable.js data structure. This check is crucial for methods like mergeDeep to function correctly, as they need to know how to handle immutable objects.

The Root Cause: Symbol Shenanigans

The issue stems from how Immutable.js checks for immutability. The isImmutable functions (like isCollection, isMap, etc.) often rely on checking for the presence of specific symbols on the object. Let's take a look at a simplified version of this check:

function isImmutable(obj) {
  return new Boolean(obj && obj[SYMBOL]);
}

This code snippet checks if the object obj has a property associated with a particular SYMBOL. The problem here is the way this check is performed. Instead of strictly checking the value associated with the symbol, it simply checks for a truthy value. This is where things get interesting with jest-mock-extended.

jest-mock-extended and the Proxy Magic

jest-mock-extended uses JavaScript Proxies to create its mocks. Proxies are powerful tools that allow you to intercept and customize operations on an object. In the case of jest-mock-extended, when you access a property on a mock object, the Proxy might automatically populate that property with a function if it doesn't already exist. This is a clever mechanism that allows mocks to be flexible and handle various access patterns.

Consider this example:

test('isImmutable', () => {
  const myMock = mock();
  isImmutable(myMock);
  console.log(myMock); // it now has `'@@__IMMUTABLE_ITERABLE__@@': [Function: mockConstructor] { ... }` property
});

Here, we create a mock object myMock. When we call isImmutable(myMock), the Immutable.js check accesses a symbol on the mock. This access triggers the Proxy in jest-mock-extended to populate the mock with a function associated with that symbol. Since a function is a truthy value in JavaScript, the isImmutable check incorrectly identifies the mock as an Immutable.js object.

The Domino Effect: mergeDeep and Beyond

This misidentification has a cascading effect. When mergeDeep (or other Immutable.js methods) encounter what they believe to be an Immutable.js object, they apply specific logic for merging immutable structures. This logic doesn't play well with the jest-mock-extended mocks, leading to the undefined result we saw earlier. The core issue is that Immutable.js is treating the mocks as if they were created within its own ecosystem, which they are not.

In essence, Immutable.js's loose check for immutability combined with jest-mock-extended's Proxy-based mocks creates a perfect storm of incompatibility.

Potential Solutions: Navigating the Incompatibility

Now that we understand the problem, let's explore some potential solutions to bridge this gap between Immutable.js and jest-mock-extended. Here are a couple of approaches we can consider:

1. Strict Symbol Value Check

One solution is to make the Immutable.js immutability checks more stringent. Instead of simply checking for a truthy value associated with the symbol, we can perform a more precise check. This means verifying that the value associated with the symbol is exactly what Immutable.js expects, either a literal true or even a unique symbol.

Here's how this might look in code:

function isImmutable(obj) {
  return obj && obj[SYMBOL] === true; // or obj[SYMBOL] === SOME_UNIQUE_SYMBOL
}

By using a strict equality check (===), we ensure that we're only identifying objects as immutable if they truly originate from Immutable.js. This would prevent the jest-mock-extended proxies from being misidentified.

2. hasOwnProperty to the Rescue

Another approach involves leveraging the hasOwnProperty method in JavaScript. Instead of directly accessing the symbol on the object, we can use hasOwnProperty to check if the object explicitly owns the symbol property.

function isImmutable(obj) {
  return obj && obj.hasOwnProperty(SYMBOL);
}

This method avoids triggering the Proxy's auto-population behavior in jest-mock-extended. By checking if the object directly has the symbol property, we bypass the Proxy's intervention and get a more accurate assessment of immutability.

Choosing the Right Solution

Both of these solutions offer a way to address the incompatibility issue. The best approach might depend on the specific needs and constraints of your project. A strict symbol value check provides a clear and direct way to verify immutability, while using hasOwnProperty offers a more robust way to avoid Proxy interference.

Implications and Broader Impact

This incompatibility issue highlights the importance of understanding how libraries interact with each other, especially when dealing with advanced JavaScript features like Proxies and symbols. It also underscores the need for careful consideration when designing immutability checks to avoid false positives.

While we've focused on mergeDeep in this discussion, it's likely that this issue affects other Immutable.js methods as well. The underlying problem lies in the misidentification of mocks as immutable objects, so any method that relies on accurate immutability checks could be impacted.

Wrapping Up: A Path Forward

In conclusion, the incompatibility between Immutable.js and jest-mock-extended when using mergeDeep stems from how Immutable.js checks for immutability and how jest-mock-extended uses Proxies. By understanding this interaction, we can implement solutions like strict symbol value checks or using hasOwnProperty to ensure accurate immutability detection. This allows us to use these powerful libraries together effectively.

Hopefully, this deep dive has been helpful in understanding this issue and potential solutions. Keep an eye out for updates in either library that might address this further. In the meantime, these workarounds should help you keep your tests running smoothly. Happy coding, guys!