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]))
    }
  }}
/>