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JavaScript React Components (2018) Stateful Components and Lifecycle Methods Stopwatch State

Chris Holder
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Chris Holder
Full Stack JavaScript Techdegree Graduate 19,841 Points

Why don't we mutate prevState instead of this.state when we change the value of isRunning?

I thought we weren't supposed to mutate this.state with setState because React sometimes batches state changes and so we should pass a function with prevState to setState instead?

1 Answer

Gabbie Metheny
Gabbie Metheny
33,778 Points

It looks like the React docs recommend setting state using a callback and the previous state, but they do mention you can do it the way Guil has here, too (as a "shallow merge of stateChange into the new state")-- and looking back, we're using the same method of setting state with an object (as opposed to a callback) in AddPlayerForm. It seems like generally, for simpler, smaller state updates (i.e. just change one value, as opposed to updating a large array), this shallow merge approach is sufficient because you're unlikely to get out of sync with the UI. You certainly could refactor this to use a callback, though!

*** EDIT ***

In the next video, we end up adding the callback to update based on prevState as we add more functionality to the Stopwatch!