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A surprising behavior of React's useEffect

Since React 16.8 came out earlier this week, I’ve been playing around with hooks in personal projects so that I can better understand their ins and outs.

Many of the hooks that accept callbacks also take an optional array of values that are used for optimization purposes: if none of the values in the array change between renders, the callback is skipped.

The docs contain a tantalizing hint about the future:

The array of inputs is not passed as arguments to the effect function. Conceptually, though, that’s what they represent: every value referenced inside the effect function should also appear in the inputs array. In the future, a sufficiently advanced compiler could create this array automatically.

Until “the future” comes, determining which values should appear in the inputs array seems like a really tedious (and error prone!) code review task, so I tried writing a utility method to remove some of the “sharp edges”:

With this utility method, the values are passed to the callback. If the callback is defined outside of the component, then it becomes impossible for it to use any unlisted values, making code review much simpler.

Unfortunately, it has a critical flaw when used with refs.

Suppose I have a component that needs to attach an event handler directly to a DOM node:

Aside: that this is more than just a hypothetical example: React attaches all event handlers at the document level. One unfortunate consequence of this is that Chrome will force any touch event handlers to be passive. This is undesirable for things like drawing controls which may need to call preventDefault to stop undesirable scrolling.

As written, this effect will never be attached to the canvas element unless an ancestor of CanvasElement triggers a re-render.

This is the case because the value provided to addTouchListener gets bound at render time: before canvasRef.current is set to an actual canvas element. It’s not until the second render–when canvasRef.current is set to the value from the first render–that addTouchListener is called with a DOM element.

This leads to another interesting discovery. Suppose we abandon useBoundEffect so that we can pass the correct value to addTouchListener:

In this version, the effect attaches the event handler after the first render, as desired, but it also cleans up and re-attaches on the second render. This happens for the same reason that the event handler was not attached the first time: the initial value of canvasRef.current is what is used to determine whether the effect needs to be re-run.

React itself could provide a solution to this by allowing the second argument to be specified as a function that returns an array, instead of the array itself. If React called this function immediately before applying the effect, it would enable a much more precise calculation of whether it needs to be re-applied.

On the other hand, I’m not keen to wait on such a feature, so I decided to see if I could emulate it. This is what I came up with:

In this version, we have to use two effects. The first runs on every render to determine whether the inputs have changed. The second is necessary to clean up when the component is unmounted: it runs only on the first render and sets up a cleanup method.

Aside #2: if you try to use this in your code and supply the second argument in the form () => [value1, value2], you may get an error from the Typescript compiler complaining that the return type is not compatible with the expected type. This happens because Typescript is really conservative about inferring arrays as tuples. If you want to avoid adding a bunch of explicit types everywhere, you can define a method like this to give the compiler the necessary hint:

With this in place, the second argument would change to the form () => createTuple(value1, value2). I’m hopeful that future versions of Typescript will make this simpler, as there are a number of GitHub Issues related to tuple inference; on the other hand, there are no concrete proposals, so I’m not holding my breath.

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