When enabled, the plugin overrides these array methods:

• Mutating: push, pop, shift, unshift, splice, reverse, sort
• Non-mutating: filter, slice, concat, flat, find, findIndex, findLast, findLastIndex, some, every, indexOf, lastIndexOf, includes, join, toString, toLocaleString

Our benchmarks show that the overridden methods (plus the other perf changes in Immer 10.2 and 11.0) are 50-80% faster than the baseline behavior of Immer 10.1.

The plugin adds about 1.5-2K minified to Immer's bundle size.

It's important to note that the plugin does change the "safe to mutate a draft" semantics of Immer. Any of these methods that receives an array item as a callback argument will not automatically wrap that item in a Proxy!. That means that if you try to mutate an argument in a method such as filter, it will actually mutate the real underlying object, which will cause bugs in your app. This is an intentional design tradeoff. Semantically, all of these methods imply read-only access to array values, so if your code tries to mutate an array item in a callback, that is a bug in your code.

Note that this does not override map, flatMap, forEach, or reduce / reduceRight. Those methods do imply either side effects and potential mutations, or returning arbitrary values. Given that, we determined it was both safest and simplest to keep their behavior as-is.

See the Array Methods Plugin docs page for further details on the behavior of the overridden methods.

What's Changed

• Override array methods to avoid proxy creation while iterating and updating by @markerikson in https://github.com/immerjs/immer/pull/1184