js_function_context.js

'use strict';

function MyType() {
  this.id = null;
}

MyType.prototype.handler = function () {
  console.log('handle ' + this.id);
};

var t = new MyType();
t.id = 42;

function caller(fn) { // same goes for setTimeout(), etc...
  fn();
}

caller(t.handler); // this throw an error because t.handler will be called without the context
caller(t.handler.bind(t)); // this works because bind(ctx) returns a function with the given "ctx"