http.get()解读

上源码

exports.get = function(options, cb) {
  var req = exports.request(options, cb);
  req.end();
  return req;
};

//get 本质上是简化的 request， 没有body 自带了request.end()

exports.request = function(options, cb) {
  return new ClientRequest(options, cb);
};

request调用的ClientRequest的instance

const client = require('_http_client');
const ClientRequest = exports.ClientRequest = client.ClientRequest;

//ClientRequest is coming from module _http_client
client.ClientRequest 具体看源码， options里主要检查是否给了domain protocol port method agent 等参数。

对于重点解决的cb – callback 如下

if (cb) {
    self.once('response', cb);
  }

once 是event中定义的

//doc解释

Adds a one time listener for the event. This listener is invoked only the next time the event is fired, after which it is removed.

EventEmitter.prototype.once = function once(type, listener) {
  if (typeof listener !== 'function')
    throw new TypeError('listener must be a function');

  var fired = false;

  function g() {
    this.removeListener(type, g);

    if (!fired) {
      fired = true;
      listener.apply(this, arguments);
    }
  }

  g.listener = listener;
  this.on(type, g);

  return this;
};

//once 主要对cb是不是function判断然后 主要是一次执行的逻辑判断， 后面调用 on

EventEmitter.prototype.on = EventEmitter.prototype.addListener;

//继续addListener
重点code

existing = events[type];

判断 response 是 可以找到的event类型

if (typeof existing === 'function') {
     // Adding the second element, need to change to array.
     existing = events[type] = [existing, listener];
   }

并变为array类型

后续如何触发event未跟踪到