bad

bad.js

function FancyThing(options) {
  this.message = 'hello';
  this.options = options;
}
FancyThing.prototype.doSomething = function() {
  // yea, don't do this:
  this.options.callback.call(this);
  // this is better:
  // this.options.callback(this)
};

(new FancyThing({
  callback: function() {
    console.log(this.message); // 'hello'
  }
})).doSomething();

(new FancyThing({
  callback: function() {
    console.log(this.message); // undefined
  }.bind({ other: 'object' })
})).doSomething();