expressjs.com/_includes/api/en/3x/res-set.md

res.set(field, [value])

Set header field to value, or pass an object to set multiple fields at once.

res.set('Content-Type', 'text/plain')

res.set({
  'Content-Type': 'text/plain',
  'Content-Length': '123',
  ETag: '12345'
})

Aliased as res.header(field, [value]).