Why Lodash _.reverse Modifies the Original Array

This article explains why the _.reverse method in the Lodash JavaScript library mutates the original array instead of returning a new one. While Lodash is widely known for promoting functional programming and immutable data handling, _.reverse intentionally alters the source array to mirror JavaScript's native behavior, optimize runtime performance, and minimize unnecessary memory allocation.

Direct Alignment with Native JavaScript

The primary reason _.reverse mutates the original array is consistency with ECMAScript standards. In native JavaScript, Array.prototype.reverse() operates in place. Lodash was designed to supplement and wrap native JavaScript functionality, not necessarily reinvent standard array behaviors unless specifically providing functional utilities. When developers call _.reverse(array), Lodash delegates the operation directly to the native Array.prototype.reverse under the hood.

Memory and Performance Optimization

Reversing an array in place is an \(O(n)\) operation with \(O(1)\) auxiliary space complexity. By swapping elements within the existing memory allocation, the engine avoids the overhead of creating a new array, copying references, and triggering garbage collection. For large datasets, creating copies automatically on every reverse call could lead to substantial memory spikes and performance degradation.

Functional vs. Utility Focus

Many developers assume every Lodash method is pure and immutable. However, Lodash is a utility library rather than a strict immutable data library (like Immutable.js). Lodash methods are categorized based on their intended use case. Methods designed to mirror built-in JavaScript mutating operations often maintain those mutation semantics for predictability relative to the native language.

How to Reverse Without Mutating

If you require an immutable reverse operation, you have several straightforward alternatives:

Clone before reversing with Lodash:

const original = [1, 2, 3];
const reversed = _.reverse(_.slice(original));
// or
const reversed = _.reverse([...original]);

Use modern native JavaScript (toReversed): Modern JavaScript environments (Node.js 20+, modern browsers) provide the native Array.prototype.toReversed() method, which creates a reversed copy without altering the original:
```
const original = [1, 2, 3];
const reversed = original.toReversed();
```

Spread syntax with native reverse:

const original = [1, 2, 3];
const reversed = [...original].reverse();