Author avatar

Jake Trent

Re-render a React Component on Window Resize

Jake Trent

  • Jun 27, 2019
  • 6 Min read
  • 6,309 Views
  • Jun 27, 2019
  • 6 Min read
  • 6,309 Views
Web Development
React

Introduction

Most of the time, we attempt to create React apps that have flexible UIs, responding to the available visual space. Sometimes, however, this is neither possible or practical. In such instances, it can be useful to re-render a React component explicitly when the window or viewport size changes.

Listen for Resize

React doesn't have a resize event baked into it, but we can listen to the native browser window resize event from within our React component:

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import React from 'react'
function MyComponent() {
  React.useEffect(() => {
    function handleResize() {
      console.log('resized to: ', window.innerWidth, 'x', window.innerHeight)
    

    window.addEventListener('resize', handleResize)
  })
  return <div>w00t!</div>
}
javascript

This code will simply listen for the window resize event and console log something like "resized to: 1024 x 768".

Re-render on Resize

But the above code will not yet re-render anything when the resize event is detected. We still have to tell React itself that something has changed in order to trigger a re-render.

Under normal conditions, React will re-render a component when its props or state changes. To trigger a re-render of MyComponent in the example, we'll set internal state on the component when the event is detected:

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import React from 'react'
function MyComponent() {
  const [dimensions, setDimensions] = React.useState({ 
    height: window.innerHeight,
    width: window.innerWidth
  })
  React.useEffect(() => {
    function handleResize() {
      setDimensions({
        height: window.innerHeight,
        width: window.innerWidth
      })
    

    window.addEventListener('resize', handleResize)
  })
  return <div>Rendered at {dimensions.width} x {dimensions.height}</div>
}
javascript

Now we have set up some internal state, dimensions, that has height and width properties. Inside handleResize, we no longer simply console.log, but instead set new state when the resize is detected, using setDimensions. If we only cared about height or width resizes exclusively, we could track only what we needed.

Additionally, to show that a re-render is actually occurring, we've changed the output to print something like "Rendered at 1024 x 768".

Cleanup Listeners

When adding an event listener, such as we are for the resize event, we should make sure to clean up after ourselves. In the example so far, we haven't, and that could cause our app problems later.

React executes components multiple times, whenever it senses the need. And in each re-render, useEffect is going to be called again. This will create n new event bindings of handleResize to the resize event. If this component is re-rendered often, this could create a serious memory leak in our program. We only ever need or want one event listener. If we always clean up established event listeners before creating new ones, we'll ensure a single listener.

React gives us a way to do this with useEffect. When passing a function to useEffect, if that function also returns a function, that returned function will be called to perform any needed cleanup. We can put our removeEventListener code there:

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import React from 'react'
function MyComponent() {
  const [dimensions, setDimensions] = React.useState({ 
    height: window.innerHeight,
    width: window.innerWidth
  })
  React.useEffect(() => {
    function handleResize() {
      setDimensions({
        height: window.innerHeight,
        width: window.innerWidth
      })
    

    window.addEventListener('resize', handleResize)

    return _ => {
      window.removeEventListener('resize', handleResize)
    
  })
  return <div>Rendered at {dimensions.width} x {dimensions.height}</div>
}
javascript

Now we're cleaned up nice and responsibly.

Resizing Less Often

Currently, our example code is set up to call handleResize as often as the window resizes. We're setting state and re-rendering for every single pixel change as often as the event loop will let us.

But what if there's a good reason to handling the resizing less often than that? We might want to be less aggressive in our re-rendering for performance reasons, such as in the case of a slow or expensive-to-render component.

In such a case, we can debounce the resize handling and thus the re-rendering. This will mean to throttle or wait between calls to our handleResize function. There are solid debounce implementations. Let's add a short and simple one to our example:

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import React from 'react'

function debounce(fn, ms) {
  let timer
  return _ => {
    clearTimeout(timer)
    timer = setTimeout(_ => {
      timer = null
      fn.apply(this, arguments)
    }, ms)
  };
}

function MyComponent() {
  const [dimensions, setDimensions] = React.useState({ 
    height: window.innerHeight,
    width: window.innerWidth
  })
  React.useEffect(() => {
    const debouncedHandleResize = debounce(function handleResize() {
      setDimensions({
        height: window.innerHeight,
        width: window.innerWidth
      })
    }, 1000)

    window.addEventListener('resize', debouncedHandleResize)

    return _ => {
      window.removeEventListener('resize', debouncedHandleResize)
    
  })
  return <div>Rendered at {dimensions.width} x {dimensions.height}</div>
}
javascript

Note that we wrap handleResize in a debounce() call and bind the new function that it returns to the debouncedHandleResize variable. Then we use this variable instead in both the event listener setup and cleanup.

The debounce() call has as its second parameter 1000ms, meaning that we are ensuring the handleResize code is called a maximum of once per second.

Conclusion

Bringing together the ability to listen to the native resize event, clean up after those event bindings, and control how often the event handler runs, we can now confidently re-render our React component in response to any viewport resize event.

To see this code in action, check out this running example.

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