Ensure an image upload is smaller than a specific size

Feb 16, 2023

Learn how to make sure an image upload stays under a size limit like 3MB by checking the files size in a React onChange handler and rejecting bigger files.

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I had a form with a file input box, to let people upload an image:

<input
  name='image'
  type='file'
  accept='image/*'

I needed this image to be smaller than 3MB.

So here’s what I did to implement this requirement in a React app, using the onChange event:

<input
  name='image'
  type='file'
  accept='image/*'
  onChange={(event) => {
    if (event.target.files && event.target.files[0]) {
      if (event.target.files[0].size > 3 * 1000 * 1024) {
        alert('Maximum size allowed is 3MB')
        return false
      }
      setImage(event.target.files[0])
      setImageURL(URL.createObjectURL(event.target.files[0]))
    }
  }}
/>

Notice the 3 * 1000 * 1024 part: mixing 1000 and 1024 is easy to get wrong. KB and KiB are different things, and I made a byte size converter to sort that out.

Tagged: JavaScript · All topics

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