JSFiddle - React, Tailwind, and code Playground

HTML

<input id="file_input" type="file" />

JavaScript

document.getElementById('file_input').addEventListener('change', (event) => {
  const file = event.target.files[0];
  if (file) {
    const reader = new FileReader;
    reader.onload = () => { 
        const img = new Image;
        img.onload = () =>  alert(img.width + ' x ' + img.height); 
        img.src = reader.result; 
    };
    reader.readAsDataURL(file);
  }
});