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    Handle componentDidMount and componentWillUnmount in React Component Refactor to Hooks

    Kent C. DoddsKent C. Dodds
    reactReact
    ^16.7.0-alpha

    Let's refactor our GitHubClientProvider class component to a function component that uses hooks. This one's pretty interesting because we can use useEffect to encapsulate everything we need for a single effect, truly separating that concern within our component.

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    Transcript

    Transcript

    Instructor: Let's go ahead and refactor this GitHub client provider. I'm going to make a function called githubClientProvider. It's going to take some props, and then we need to initialize our state. Now, this one's a little bit interesting, because we've got some if statements.

    We're conditionally initializing our state, but we're even reaching into local storage, which is something that we don't really want to do every single time we render. We'll take a look at that here in a sec. For our error state, though, that's pretty easy.

    Let's go ahead, and I'm going to go up here the top. We'll pull in useState, and we'll come back down here. We'll get our error and setError. That'll be useState. We want to initialize it to the same thing that we had before, null.

    Perfect. Then let's go ahead, and I'm just going to move this up right here. We could do this, and initialize our client to this. I just don't really like this code right in here. I feel like this could be a bit of a performance problem, reaching into local storage every single time the GitHub client provider is rendered.

    Maybe it's not a big deal, but I would rather avoid that. Let's go ahead and get our client, and set client functions here with useState. useState can actually take an initializer. We're going to provide it a function, and whatever this function returns will be used as the new initial value for our client.

    React will only call this function the first time a given component is rendered. That's perfect for our use case, because we only really want to reach into local storage the first time, which is what we were doing in the constructor.

    It only reached into local storage that first time, when the component is actually being initialized. This is great. Let's move this up inside of that function. Instead of this.props, anywhere you see this-dot in a function component, that's wrong, and you want to get rid of it.

    We're going to do props.client. We're going to get the client from the props. If there is a props client, then we'll simply return it. That props client is what's going to be used as our initial state for the client. Perfect.

    If there's not a props client, we'll reach into local storage. If there is a token, then we're going to get the client from this.getClient. Let's return that instead. We've got a this-dot in here, so we're going to need to have that get client move that up here.

    Let's do that. I'm going to just reach down here. We'll cut that out, and we'll paste that right here. Then we'll make that a function. Perfect. Instead of this.getClient, we'll just call getClient, because it's right in the same closure. No need for this.

    So far, so good. Let's go ahead and take a look at the rest. We got rid of our constructor. Now, we have a componentDidMount, and that's going to be a useEffect. Inside of our useEffect, we'll do all the same things that we did in our componentDidMount.

    We'll just move that right up. It's important to remember that useEffect is not an exact match to componentDidMount. useEffect is actually a combination between componentDidMount and componentWillUnmount, as well as componentDidUpdate.

    Another difference here is that our useEffect callback will actually run sometime after the render has happened, and not synchronously. Whereas our componentDidMount will run synchronously right after the render has happened.

    Those subtle differences are important to remember as you're making these refactors. Sometimes, you may actually need to use layoutEffect, rather than useEffect, depend on your use case. Go ahead and start with useEffect, because it will generally be better for performance.

    We've moved our componentDidMount up into our useEffect. Let's go ahead and bring in useEffect, because that red underline is bothering me. We'll bring in useEffect. Now, we've got some this-dot going on in here.

    Let's get rid of all that stuff. this.state, we can just get rid of entirely, because we have got our client that we can reference directly from our useState call up here. Perfect.

    Then we're doing a this.unsubscribeHistory is assigned to that history.listen. The only reason that we did this this.unsubscribeHistory is so that we could reference it in our componentWillUnmount. Thankfully, hooks allow you to do your setup and your tear down or your clean up in the same callback.

    We could actually just do const unsubscribeHistory, and then we'll return a function called cleanup that doesn't accept any parameters. We can call unsubscribeHistory right there. No reason to make an instance property anymore here.

    Great. We're making good progress. We took care of our componentDidMount and our componentWillUnmount. Now, an important thing here is we don't have a componentDidUpdate. We only want our effect to run on mount and unmount.

    We don't want to have that effect callback run on every update. That makes sense, because we only want to do all of this stuff one time. We only need to set up a subscription to history once. We only need to check the client on mount, and then history.

    Our history lesson callback will take care of the rest of the time as the user's navigating around to redirect them to the homepage if the user's not logged in, so we don't have a client. To make this only run mount and componentWillUnmount, we'll provide it an array of inputs where that array is empty.

    Nothing in that array can possibly change. Therefore, our callback will only be called once, and our clean up will only be called on unmount. Now, we've simulated the same thing that we had in componentDidMount and componentWillUnmount.

    Next here, we've got log out and log in. Those are functions that were being referenced in our Git client. Let's go ahead and pull those out. We'll pull both of these, just cut them out, and put them right up here.

    We'll convert both of these into a function. Log in is an async function. Then instead of this-dot, which you never want to have in a function component, we'll just use a reference to those variables directly. We'll use object shorthand as well.

    Let's take a look at these. We have a this.setState to set the client and error to null. Now, we'll just say setClient null, and setError null. We can get rid of that call to this.setState. Then we have a this.setState here to set error to that error object that we get when we're logging in.

    We have a this.setState to set the client. Let's do setClient is getClient with the data token. We've successfully moved log out and log in, and all we have left is the render. The render is probably the easiest one to pull.

    We'll just take it from here to here, and we'll return that same thing. Then we can get rid of the class, we'll save, and we're all set. Let's take a look at this children here. We need to get that from props.children.

    Now, we'll take a look at your app. If I go to Amza Brisky, we've loaded the data successfully with the client that the GitHub client provider provided for us. That's awesome, and we didn't have to actually change how this component is being used.

    This is just a clean refactor, taking all of the stuff from the class, and changing it into using hooks and a function component. It's arguably a lot simpler code, because now our useEffect that used to be represented by a componentDidMount and componentWillUnmount is all represented in this single useEffect callback.

    Our log in, log out and getClient can all be right here, just referencing those functions directly. We can call useState with a function to ensure that this code only runs the first time this GitHub client provider is being rendered.