Just move "free" listener into "init()" method.
And replace var to let and const.

@TimothyGu revert all replacement.

https://github.com/request/tunnel-agent/blob/master/index.js#L47 Now TunnelingAgent can inherits from http.Agent more easily.

function TunnelingAgent extends http.Agent {
  constructor(options = {}) {
    super(options);

    this.proxyOptions = this.options.proxy || {}
    this.maxSockets = this.options.maxSockets || http.Agent.defaultMaxSockets
  }

  init() {
    this.on('free', function onFree(socket, host, port) {
      // override the free socket listen
    });
   }
  
  // others methods
  addRequest() {}
}

ping @nodejs/http

@fengmk2 is this superseded by #11567?

@lpinca I don't think so, #11567 relaxed the constraint of extended agents, this PR makes the initialization part of the agents customizable via the init() method. Although the ongoing #13005 also tries to make HTTP agents more overridable as well.

This needs a rebase before anyone familiar with this part of the codebase can give a review...@fengmk2

@joyeecheung :)

I fixed the conflicts and push again. Anyone can review now!

PTAL @nodejs/collaborators @nodejs/http

oy! completely missed this one, sorry @fengmk2

This likely needs a doc update. Code looks fine on first read through, needs a CI

CI: https://ci.nodejs.org/job/node-test-pull-request/9861/

@jasnell The ci is still running for a long time.

the ci completed, the results just are not posting appropriately back to Github.
Here's a fresh run: https://ci.nodejs.org/job/node-test-pull-request/9966/

Ping @fengmk2

@fengmk2 do you still want to pursue this?

I am closing this due to long inactivity. @fengmk2 if you would like to pursue this, please reopen.