Will componentDidMount always be called before componentWillUnmount?

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Is it guaranteed that the componentDidMount lifecycle method will always be executed (and complete) before componentWillUnmount?

For example, in my componentDidMount method I am registering a subscription:

componentDidMount() {
    this.unsubscribe = subscribe();
}

componentWillUnmount() {
    this.unsubscribe();
}

If componentDidMount is not guaranteed to execute and complete before componentWillUnmount then this.unsubscribe is potentially undefined and attempting to invoke it will throw a runtime error.

reactjs
1 Answers

React class component documentation about componentDidMount lifecycle method (https://reactjs.org/docs/react-component.html#componentdidmount) states:

This is a good place to set up any subscriptions. If you do that, don’t forget to unsubscribe in componentWillUnmount().

Based on this, I would say it is guaranteed that componentDidMount will always be executed before componentWillUnmount, as otherwise most of the unsubscriptions would throw the error you mention.

user_33553690
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