-- HTML
<input type="file" accept=".json" />


-- JavaScript
document.querySelector("input").addEventListener('change', (ev)=>{
  let mijnLezer = new FileReader();
  
  mijnLezer.addEventListener('load', (event)=>{
    console.dir(event.target);
	let parsed = JSON.parse(event.target.result);
	console.dir(parsed);
  });

  mijnLezer.readAsText(ev.target.files[0]);
});