Lodash isEqual vs JSON.stringify in JavaScript

Using JSON.stringify to compare objects is a common shortcut in JavaScript, but it introduces subtle bugs that can break application logic. While string serialization appears to offer an easy way to check deep equality, it fails to handle structural nuances, non-JSON data types, and cyclic references. In contrast, Lodash’s _.isEqual performs true semantic deep comparisons, correctly inspecting object shapes and diverse data types. This article breaks down the architectural flaws of using JSON.stringify for equality checks and explains why _.isEqual is the superior, production-ready solution.

Key Order Sensitivity

JavaScript objects represent unordered collections of key-value pairs. However, JSON.stringify serializes keys based on their property enumeration order. If two objects contain identical keys and values created in different sequences, JSON.stringify produces different strings:

const objA = { name: "Alice", role: "Admin" };
const objB = { role: "Admin", name: "Alice" };

JSON.stringify(objA) === JSON.stringify(objB); // false
_.isEqual(objA, objB); // true

_.isEqual traverses keys structurally, ignoring insertion order and comparing only whether the properties and their corresponding values match.

Loss of Non-JSON Data Types

JSON format strictly supports strings, numbers, booleans, arrays, objects, and null. When JSON.stringify encounters other common JavaScript types, it either converts them or omits them entirely:

• undefined, Functions, and Symbols: Omitted when found in objects, or converted to null inside arrays.
• NaN and Infinity: Both serialize directly to null.
• Date Objects: Serialized to ISO strings, losing their Date prototype and type information.
• Map and Set: Serialized as empty object literals ({}).
• RegExp: Serialized as an empty object ({}).

const user1 = { id: 1, action: () => {}, created: new Date("2024-01-01") };
const user2 = { id: 1, action: undefined, created: "2024-01-01T00:00:00.000Z" };

JSON.stringify(user1) === JSON.stringify(user2); // true (False Positive)
_.isEqual(user1, user2); // false (Accurate)

_.isEqual handles full JavaScript type checking. It compares Date objects by their millisecond timestamps, verifies RegExp pattern and flag parity, checks Map and Set members, and recognizes distinct prototypes.

Handling Circular References

If an object directly or indirectly references itself, passing it to JSON.stringify throws an unhandled runtime exception:

const nodeA = {};
nodeA.self = nodeA;

JSON.stringify(nodeA); // Uncaught TypeError: Converting circular structure to JSON

_.isEqual maintains an internal cache of visited objects during its recursive traversal. It safely resolves cyclical graphs without blowing the call stack or throwing errors.

Performance and Early Exit Optimization

JSON.stringify must serialize the entire object graph into a continuous string before the equality comparison even begins. If two large objects differ only by their first property, stringification still processes the entire data structure from start to finish and allocates significant memory for both serialized strings.

_.isEqual short-circuits. It performs immediate checks, failing fast if:

1. Object references match identically via reference equality (===).
2. Array lengths or object key counts differ.
3. Any single nested property does not match.

This localized, fail-fast evaluation avoids unnecessary traversals and memory allocations on non-matching objects.