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Brian Baliach

In a previous blog, [JavaScript Equality Under the Lens: Enhancing React’s Dependency Checks](https://www.balysnotes.com/javascript-equality-checks), we delved into the nuances of JavaScript data types and their equality checks. We explored values with identity and those without, examining how JavaScript stores objects by reference rather than by value. To address object comparison within React's dependency checks, we devised a method to fetch keys from an object and compare their values, though this method was limited to one level deep.

In this blog, we'll talk about a new way to do equality checks that will allow us to go n levels deep (rather than just one level) and will still be applicable not only to simple data types like numbers and strings, but also complex structures like objects and arrays. For this approach, we'll rely on an advanced but straightforward technique for value-based comparison of objects using `JSON.stringify` and `JSON.parse`.

### Revisiting Object Equality: The Challenge

JavaScript’s intrinsic method of object comparison works by comparing memory references. This means two distinct objects with identical properties and values are considered unequal:

```javascript
const objectOne = { key: "value" };
const objectTwo = { key: "value" };

console.log(objectOne === objectTwo); // returns 'false'
```

In our previous blog, we created a function to check equality by manually comparing object keys and values. However, this function was limited to a shallow comparison:

```javascript
function shallowEqual(obj1, obj2) {
 const keys1 = Object.keys(obj1);
 const keys2 = Object.keys(obj2);

 if (keys1.length !== keys2.length) return false;

 for (let key of keys1) {
 if (obj1[key] !== obj2[key]) return false;
 }

 return true;
}
```

While this approach was somewhat effective, it fell short when dealing with nested objects and arrays. This is where `JSON.stringify` and `JSON.parse` come into play.


### The Magic of `JSON.stringify` and `JSON.parse`

In JavaScript, `JSON.stringify` converts a JavaScript object to a JSON string, while `JSON.parse` parses a JSON string to construct the JavaScript object. By converting objects into strings and comparing these string representations, we can bypass the idiosyncrasies of object reference comparison and handle nested structures efficiently:

```javascript
const objectOne = { key: "value" };
const objectTwo = { key: "value" };

console.log(JSON.stringify(objectOne) === JSON.stringify(objectTwo)); // returns 'true'
```
And at the same time still return the original object using `JSON.parse` (if there's this use case).

### Applying JSON Techniques in React

This approach is pivotal for React components' dependency checks. Let’s consider an advanced example where we use this technique in a custom hook:

```typescript
import { useCallback, useEffect, useMemo, useRef, useState } from "react";

export type ObjType = {
 [key: string | number | symbol]: any;
};

/**
 * This hook checks for equality even for nested objects or arrays. Only causes a re-render if the [obj] param changes.
 */
const useObjectEqualityChecker = (obj: ObjType) => {
 const renders = useRef(0);
 if (renders.current >= 10) {
 throw new Error(
 `Total Renders: ${renders.current}. A tad bit too much ey?`
 );
 }
 const _internalStringify = useCallback((o: any) => {
 return JSON.stringify(o);
 }, []);

 const _objString = _internalStringify(obj);
 const currentObjRef = useRef(_internalStringify(obj));
 const [currentObj, setCurrentObj] = useState<string>(_internalStringify(obj));

 useEffect(() => {
 if (_objString) {
 try {
 if (currentObjRef.current !== _objString) {
 renders.current += 1;
 currentObjRef.current = _objString;
 setCurrentObj(_objString);
 }
 } catch (e) {
 // left blank intentionally. No need for any re-render.
 }
 }
 }, [_objString]);

 return useMemo(() => {
 return JSON.parse(currentObj);
 }, [currentObj]);
};

export default useObjectEqualityChecker;
```

This hook can come in handy when building components that take arrays or objects as props, where a developer might have forgotten to use `useCallback` or `useMemo` therefore returning different memory references every render.

### Why This Approach is Better

1. **Value-Based Comparison**:
 - Unlike reference-based comparison, the JSON-based approach guarantees that objects with the same properties and values will be considered equal, thereby avoiding unexpected re-renders in React.

2. **Depth Coverage**:
 - This method seamlessly handles nested objects and arrays, ensuring accurate and deep comparisons without the need for custom deep equality functions.

3. **Simplicity and Clarity**:
 - The technique is straightforward to implement, making your code more readable and less error-prone. It reduces the cognitive load for developers trying to understand object equality logic.

4. **Consistency**:
 - Standardized behavior of `JSON.stringify` ensures consistency across different environments and usages, avoiding potential pitfalls of manually deep-checking objects or relying on third-party libraries.

### Performance Implications

Though the JSON approach is powerful, it comes with performance trade-offs, especially for larger objects or frequent comparisons:

1. **Serialization Overhead**:
 - Serializing and deserializing objects involve significant overhead compared to reference-based comparisons. For large and complex data structures, this could lead to performance bottlenecks.

2. **Frequency of Operation**:
 - If used excessively in performance-critical sections of your application, the cumulative impact might be noticeable. However, for infrequent or isolated checks, the performance impact might be negligible.

```javascript
const largeObject1 = { /* a large complex structure */ };
const largeObject2 = { /* a similarly large complex structure */ };

// Comparing using reference
console.time('Reference Comparison');
console.log(largeObject1 === largeObject2);
console.timeEnd('Reference Comparison');

// Comparing using JSON
console.time('JSON Comparison');
console.log(JSON.stringify(largeObject1) === JSON.stringify(largeObject2));
console.timeEnd('JSON Comparison');
```

In most cases, the reference comparison will be significantly faster.

### Conclusion

No one-size-fits-all solution exists in software development. Understanding the trade-offs and careful application of these techniques will greatly enhance your ability to manage and optimize equality checks in JavaScript. This method of using `JSON.stringify` and `JSON.parse` for object comparison, especially in React, guarantees accurate value-based comparisons, making your components more reliable and efficient.

Until next time, happy coding!

JavaScript Equality Under the Lens: Enhancing React’s Dependency Checks - Part 2

JavaScript, a language of many faces, often presents concepts that are deceptively simple yet profound in their implications. Among these, understanding equality checks and the nuanced landscape of JavaScript types stands paramount. Whether you're just dipping your toes into the vast ocean of JavaScript or you're a seasoned sailor navigating through the stormy quirks of the language, grasping these concepts is crucial. 

This blog post sets out to demystify a concept that's often sidelined but deserves a spotlight: JavaScript's identity and equality checks. Despite their critical importance, details on how these checks align with ECMAScript standards are not thoroughly addressed in either React's documentation or the MDN web docs. The ECMAScript Language Specification doc however, does a great job in explaining these concepts. You can find it [here](https://tc39.es/ecma262/multipage/#sec-intro) and hopefully build the next version of Chrome. 

Starting with foundational principles and progressively delving into complex nuances, we aim to explore this topic thoroughly, concluding with a hands-on example to reinforce your grasp on the subject.

### JavaScript Types: The Foundations
JavaScript, at its core, categorizes values into several types — `boolean`, `undefined`, `null`, `number`, `string`, `object`, and `symbol`. Each type serves a unique purpose:
 

##### Boolean: 
Represents a logical entity, having two values: true or false.
 

##### Undefined: 
Denotes a variable that has been declared but not yet assigned a value.
 

##### Null: 
Represents the intentional absence of any object value. It's often used to indicate that a variable should hold an object but currently does not. 
 

##### Number: 
Holds numeric values. JavaScript uses floating-point arithmetic for its numbers.
 

##### String: 
Represents textual data. It's a sequence of characters. Practically every primitive has string representation of itself;.
 

##### Object: 
Unlike the other types that are primitive, objects are collections of properties.
 

##### Symbol: 
Introduced in ES6, symbols are unique and immutable data types used as identifiers for object properties.
 

##### Hear ye, hear ye!
##### Understanding these types is the foundation upon which we build the concept of equality in JavaScript.
 

### The Enigma of Null and Undefined

Both `null` and `undefined` denote the absence of value in a variable, but they are used in subtly different contexts. `undefined` is the JavaScript engine's way of saying, "This variable not only has no value, but also might not even exist (no name has been reserved in memory for that variable)." In contrast, `null` is a deliberate assignment by the developer, saying, "This variable presently has no value but specifically holds an empty or non-existent object. So `null` is an actual value, like `number` or `string`, the only difference being that the variable has no value by design (a name will be reserved in memory for that variable). "
 

### Equality Checks: "== vs ==="
JavaScript offers two operators for equality checks: `==` (abstract equality) and `===` (strict equality).

- `==` checks for equality after performing any necessary type conversions. It means if the values being compared are of different types, JavaScript will try to convert them to a common type before making the comparison. So `"1" == 1` will return true since the value 1 which is a number (no double quotes), will be converted to the value 1 as a string (with double quotes). 
- `===`, or strict equality, checks for value equality without any type conversion. If the values being compared are of different types, the equality check immediately returns false. So in the previous example `"1" === 1` will return false since `string` and `number` are different types.

Understanding these distinctions is vital as they often lead to unexpected results, especially for beginners.
 

### Identity vs. Equality: The Subtlety
In JavaScript, values can either have identity (complex data types like objects, where the value is a reference to a location in memory) or be without identity (primitive values). PAUSE.

What in Sam's hill does it mean to have a value with or without identity? Well, let's dive into that mystery together.

A value with identity basically means a value that includes an extra property whose value is unique to that specific value. Same way your GitHub username is unique to you, and there can never be a situation where someone else might happen to have another account using your username.

A value without identity is a value doesn't have this extra property that declares its uniqueness.

So to put this into even better context, when you declare a variable whose value is an object the object automatically becomes a value with identity, the unique property being the memory location of the variable:
```javascript
const variable = {name: "I am an object"};
```

When you however declare a variable whose value is a primitive, the value itself is the unique property (instead of the memory location):
```javascript
const variable = 1;
```

When comparing primitive values (`string`, `number`, `boolean`, `undefined`, `null`, `symbol`), JavaScript checks their equality by value (which now in this case, is the value itself). However, when comparing objects, JavaScript checks whether they reference the same location in memory — which now is the property that uniquely identifies the object.

So when checking for equality for objects, comparing two objects with the exact same properties and the exact same values for those properties will still return false since these objects have different locations in memory.

```javascript
const myObjectOne = {name: "I am unique!"};
const myObjectTwo = {name: "I am unique!"}
console.log(myObjectOne === myObjectTwo); // returns 'false' since each variable has it's own memory location.
console.log("-----------Boundary----------");

const myObjectOneAgain = {name: "I am unique!"};
const myObjectTwoAgain = myObjectOneAgain;
console.log(myObjectOneAgain === myObjectTwoAgain);// returns 'true' since the second variable's value is the first variable's memory location.
// So basically, the value of [myObjectTwoAgain] resolves to the memory address of [myObjectOneAgain].
console.log("-----------Boundary----------");; // prints out 'false' since each variable has it's own memory location.
```
 

### Practical Application: useObjectEqualityChecker
Now, let's blend theory with practice through a React hook, `useObjectEqualityChecker`, designed to check object equality by comparing their key-value pairs rather than their memory reference.

```typescript
import { useMemo, useRef } from "react";

export type ObjType = { [key: string | number | symbol]: any; };

const useObjectEqualityChecker = (obj: ObjType) => {
 const refObject = useRef<ObjType>({});

 return useMemo(() => {
 if (obj) {
 const keys = Object.keys(obj);
 let valuesMatch = true;
 keys.forEach((k) => {
 if (refObject.current[k] !== obj[k]) {
 valuesMatch = false;
 }
 });
 if (!valuesMatch) {
 refObject.current = { ...refObject.current, ...obj, };
 }
 }
 return refObject.current;
 }, [obj]);
};

export default useObjectEqualityChecker;
```

This hook demonstrates a way to check equality beyond mere reference checking. It iteratively compares each key-value pair of the current and previous objects, updating the reference only when a discrepancy is found. This approaches equality checking with more nuance, especially valuable in React, where re-renders should be minimized for performance.

If you want even more sophisticated equality checks like deep comparisons, there's the: [lodash.iseQual](https://lodash.com/) library that'll do this for you out of the box. At least now you do have a bit of clarity on what's happening under the hood, so there's no harm in using a library.



### Wrapping Up

Equality in JavaScript is a multifaceted concept, laced with subtleties that can lead to nuanced bugs and performance issues if misunderstood. By unraveling these intricacies, from different types to identity checks, and applying them in practical scenarios like the `useObjectEqualityChecker` hook, developers can write more efficient and bug-free code. Understanding equality deeply not only aids in mastering JavaScript but also in harnessing the full power of frameworks such as React.

And there you have it — a journey from the foundational to the advanced, all within the realm of JavaScript's equality checks. May this guide serve as a beacon, illuminating the path for both neophytes and veterans of JavaScript alike. 

Cheers!

JavaScript Equality Under the Lens: Enhancing React's Dependency Checks with an Innovative Hook Example

Showing posts from javascript category

JavaScript Equality Under the Lens: Enhancing React’s Dependency Checks - Part 2

JavaScript Equality Under the Lens: Enhancing React's Dependency Checks