Javascript method and function

Because method is part of the window object, this refers to window.

Consider this example

var name = 'foo';
method();

logs...

> hello,foo

obj.sayHello()'s scope is within obj. method is a global property, so assigning it to the sayHello function makes the sayHello function look for this.name within the global scope.

To understand this, assign a global property of name directly in the script

var obj = {
    name:'Tom',
    sayHello:function() {
        console.log("hello," + this.name);
    }
}

obj.sayHello();

//Assign name to the global scope
this.name = "Jerry";
var method = obj.sayHello;

method.call(this); //Calling from the global scope, same as method()

//Then call method from the obj scope
method.call(obj);

The scope of method is the global window object

You'd have to do:

method.call(obj);  //instead of method()

or

var method = obj.sayHello.bind(obj);

to achieve the same efect

Info for the call method: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call

Info for the bind method: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind

Cheers

var method's scope is global. it is equals to just define a function

var method = function() {
    console.log("hello," + this.name);
}