For a zip file that looks like:

• a.png
• subfolder/
  • b.png
  • c.png
  • d.png

var zip = new JSZip();
zip.load(arrayBuffer);
var subfolder = zip.folder('subfolder');
var content = subfolder.generate({type: "blob"});

As JSZip.folder sets the root, I expected that content would be restricted to just the files within subfolder, i.e.:

• b.png
• c.png
• d.png

Instead I just get the original zip contents. I have managed to produce the expected behaviour with:

var subfolder = zip.folder('subfolder');
subfolder.files = subfolder.file(/*/);
var content = subfolder.generate({type: "blob"});