remember-this

Easily create functions bound to a specific object.

Purpose

JavaScript changes the value of the this variable depending upon how a function is invoked. For example:

var person = {
  name: "Brian",
  getName: function() {
    return this.name;
  }
}

// Prints "Brian"
console.log(person.getName());


var getNameDetached = person.getName;

// Prints "undefined"
console.log(getNameDetached());

Why did it print "undefined"? Because getNameDetached was not invoked through the person object. Instead, it was invoked as a stand-alone function, so its this variable was bound to JavaScript's global object. To fix this, you would instead need to do:

// Manually bind the function back to person
var getNameAttached = person.getName.bind(person);

// Prints "Brian"
console.log(getNameAttached());

In other words: you need to get the function from person, then remember to bind it back to person. This is easy to forget, and it can introduce some very bizarre bugs into your code.

But, good news! remember-this provides a couple of functions that encapsulate this pattern so that your life can be awesome!

Installation

npm install remember-this

Usage

Let's pretend that I'm wiring together my routes for an Express.js application. I want the routes to call the methods (erm... functions) on my usersController object. I can achieve this as follows:

var router           = express.Router();
var usersControllers = new UsersController(...);


// rt stands for "Remember This"
var rt = require('remember-this').rt;

// Creates the function c(funcName).  It grabs
// usersController[funcName] and binds it back to
// usersController.
var c = rt(usersController);


// Equivalent to:
//   router.get('/', usersController.index.bind(usersController));
//
router.get('/', c('index'));


// Equivalent to:
//   router.get('/:id', usersController.show.bind(usersController));
//
router.get('/:id', c('show'));

In other words: I used the rt(...) function to create the c(...) function. The c(...) function grabs methods (erm... functions) from usersController and binds them back to usersController.

If you want to keep the number of local variables down to a minimum, then you can pass a callback to the rt(...) function instead. For example:

var router           = express.Router();
var usersControllers = new UsersController(...);


// rt stands for "Remember This"
var rt = require('remember-this').rt;


rt(usersController, function(c) {
  router.get('/', c('index'));
  router.get('/:id', c('show'));
});

Or, maybe your usersController only contains 1 method that you want to pass to your router. In that case, you can use the bound function instead:

var router           = express.Router();
var usersControllers = new UsersController(...);

var bound = require('remember-this').bound;

router.get('/:id', bound(usersController, "show"));