How can I inject a custom HTTP Header into every request that SuperAgent makes?

Question

Clearly SuperAgent supports custom HTTP headers:

request
   .post('/api/pet')
   .send({ name: 'Manny', species: 'cat' })
   .set('X-API-Key', 'foobar')
   .set('Accept', 'application/json')
   .end(function(err, res){
     if (res.ok) {
       alert('yay got ' + JSON.stringify(res.body));
     } else {
       alert('Oh no! error ' + res.text);
     }
   });

My Question:

• If I'm pulling down SuperAgent via npm, how can I inject my own HTTP header across all requests that SuperAgent makes?
• Note: I'm entire willing to create a new npm package that extends SuperAgent if necessary.

Answer 1

I'd just make a separate module with something like this:

myagent.js

var superagent = require('superagent');

var defaultHeaders = {};
function isObject(obj) { return Object(obj) === obj; };

function request(method, url) {
   return superagent(method, url).set(defaultHeaders);
}

request.set = function (field, value) {
   if (isObject(field)) {
      for(var key in field) this.set(key, field[key]);
      return this;
   }
   defaultHeaders[field] = value;
   return this;
}
module.exports = request;

Usage

var request = require('./myagent');
request.set({'X-My-Header': 'foo'}); // sets the default

request.get('/bar').send() // will send the default header

The module behaves the same way as superagent but sets default headers before returning the Request object. See here