除异步操作外，Redux中“副作用”的准确定义是什么？

Great questions—let’s break these down clearly, since wrapping your head around Redux’s definition of side effects is crucial for writing predictable, maintainable Redux code.

First, let’s ground this: Redux expects reducers to be pure functions. A pure function’s output depends only on its input, and it doesn’t alter anything outside its own scope. Any operation that violates this is a side effect. Beyond async calls (like API requests), common examples include:

• Non-deterministic value generation: Calling Math.random(), Date.now(), or grabbing user input directly in a reducer. These produce different results every time, making your state changes unpredictable.
• Direct external state modifications: Writing to localStorage, modifying global variables (not part of the Redux store), or mutating the arguments passed to the reducer (like changing a property on an input object instead of returning a new state).
• DOM manipulations: Directly updating the DOM (e.g., document.querySelector('.count').textContent = 5) inside a reducer. Redux’s pattern is that state changes should drive UI updates, not the other way around.
• Sync external API calls: Even non-async calls that interact with external systems (like reading from localStorage or calling a browser API that changes some external state) count.
• Note: Logging to the console is technically a side effect, but it’s widely considered "safe" because it doesn’t alter state or break predictability—Redux docs are focused on effects that impact your app’s behavior.

The key here is that async logic is the most common side effect developers run into with Redux, so the docs call it out explicitly. But it’s just one type of side effect.

The docs’ wording is emphasizing that reducers must avoid all operations that break pure function rules. By listing async logic first (since it’s a top pain point) and then saying "other 'side effects'", they’re clarifying:

Async operations are a big one, but don’t forget all the other things that make your reducer impure.

To formalize Redux’s definition of a side effect in this context: any operation that either:

1. Produces a different output for the same input (non-deterministic), or
2. Modifies state or behavior outside the reducer’s own execution scope (external state changes, DOM updates, etc.)

So "other" is just a way to highlight that async isn’t the only thing to watch out for—there’s a broader category of impure operations that don’t belong in reducers.

内容的提问来源于stack exchange，提问作者rahulserver