javascript - Why does storing a reference to a function then calling the function cause 'this' to have a context of window? -

javascript - Why does storing a reference to a function then calling the function cause 'this' to have a context of window? -

i reading tutorial @ can understand _.bind , _bindall: http://blog.bigbinary.com/2011/08/18/understanding-bind-and-bindall-in-backbone.html

the website has next code

function developer(skill) { this.skill = skill; this.says = function(){ alert(this.skill + ' rocks!'); } } var john = new developer('ruby'); john.says(); //ruby rocks!

vs

function developer(skill) { this.skill = skill; this.says = function(){ alert(this.skill + ' rocks!'); } } var john = new developer('ruby'); var func = john.says; func();// undefined rocks!

why storing reference function calling function cause have context of window?

when execute

a.b();

then a context of execution of b (this within b) unless b bound function.

if don't have a, if have

b();

then it's same as

window.b();

so window context of execution of b.

note that

a.b();

is same as

b.call(a);

and

b();

is same as

b.call(); // phone call replaces first argument global object if it's null or undefined

if want bind context, can (on modern browsers)

var func = john.says.bind(john); func();

or (more classically) utilize closure :

var func = function(){john.says()}; func();

javascript