Edit in JSFiddle

document.getElementById('dl0').addEventListener('click', () => {
  const blob = new Blob(['hoge'], { type: 'text/plain' });
  saveAs(blob, 'dl0.txt');
});

document.getElementById('dl1').addEventListener('click', () => {
  const blob = new Blob(['hoge'], { type: 'text/plain' });
  const a = document.createElement('a');
  a.href = window.URL.createObjectURL(blob);
  a.download = 'dl1.txt';
  a.target = '_blank';
  
  a.click();
});

document.getElementById('dl2').addEventListener('click', () => {
  const blob = new Blob(['hoge'], { type: 'application/octet-stream' });
  saveAs(blob, 'dl2.txt')
});

<button id="dl0">FileSaver.js</button>
<button id="dl1">a tag download</button>
<button id="dl2">application/octet-stream</button>