How do I cancel a `this.setState`?

The idiom for "tell React not to do anything" is "return the previous state". It's up to React's internal management to determine that there are no actual changes to the DOM, and nothing will be visible to the user. In particular, the default shouldComponentUpdate will return false if the two objects are identical.

So the only thing you need is:

return previousState;

Sorry for answering my own question here, but newer React 16 allows you to cancel a setState.

Minor changes to setState behavior:

• Calling setState with null no longer triggers an update. This allows you to decide in an updater function if you want to re-render.
• Calling setState directly in render always causes an update. This was not previously the case. Regardless, you should not be calling setState from render.
• setState callback (second argument) now fires immediately after componentDidMount / componentDidUpdate instead of after all components have rendered.

source

---

The important distinction between returning null and returning previousState is that returning null does what like PureComponent does and prevents a the render method from being called at all.

Starting from React 16, calling setState always makes the render method fire even if you return previousState.

---

To @bennygenel's point though, you should use shouldComponentUpdate to prevent re-renders.

---

Update: As of React 16.8, if you're using React Hooks (this is unrelated to class components), then returning previous state from useState does, in fact, prevent the re-render altogether. See here for source