One Cool Trick to Simplify Reducer Functions

Eric Elliott
Jun 4, 2020 · 2 min read

Redux is an amazing tool if you take the time to get to know it. One of the things about Redux that commonly trips people up is that reducers must be pure functions.

A pure function is a function which:

  1. Given same arguments, always returns the same result, and
  2. Has no side-effects (e.g., it won’t mutate its input arguments).

The problem is that sometimes a reducer needs to make a complicated change to some input state, but you can’t just mutate the state argument without causing bugs.

The solution is a handy tool called Immer. In this video, I’ll introduce you to Immer and show you how to use it to reduce the complexity of your reducer code. With one or two small reducers, the difference is pretty subtle, but on a large project, it can significantly simplify your application code.

Here’s an example. Imagine you’re building a social network, and you need to keep track of posts that a user has liked. When they like a post, you add a like object to the user’s likes collection. That might look something like this:

Notice what we’re returning from the like.type case: Mixing bits of the payload into a nested property of the state object using the JavaScript object spread syntax: ...state and ...state.likes on lines 19–25.

After immer, you can simplify that part to a one-liner (line 21):

The produce function returns a partial application (a function which has been partially applied to its arguments) which then takes the arguments of the reducer function. You pass it a callback function which takes a draft of the state object instead of the real state object. You’re free to mutate that object as if it were any other mutable object in JavaScript. No more spreading nested properties to avoid mutating the input argument.

After your callback function runs, Immer compares the draft to the original state and then builds a new object with your changes applied, so your function feels like it’s mutating, but still behaves like a pure function. You get the best of both worlds: The simplicity of mutation with the benefits of immutability.

Next Steps has in-depth lessons on topics like pure functions, immutability, partial applications, and other functional and object oriented programming concepts.

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Eric Elliott is a tech product and platform advisor, author of “Composing Software”, cofounder of and, and dev team mentor. He has contributed to software experiences for Adobe Systems, Zumba Fitness, The Wall Street Journal, ESPN, BBC, and top recording artists including Usher, Frank Ocean, Metallica, and many more.

He enjoys a remote lifestyle with the most beautiful woman in the world.

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