Difference between res.setHeader and res.header in node.js

res.setHeader() is a native method of Node.js and res.header() is an alias of res.set() method from Express framework.

Documentation: res.setHeader(), res.set()

This two methods do exactly the same thing, set the headers HTTP response. The only difference is res.setHeader() allows you only to set a singular header and res.header() will allow you to set multiple headers. So use the one fit with your needs.

Perhaps an example can clarify more:

// single field is set 
res.setHeader('content-type', 'application/json');

// multiple files can be set
res.set({
     'content-type': 'application/json',
     'content-length': '100',
     'warning': "with content type charset encoding will be added by default"
  });