CODEWITHSUNDEEP
javascriptscope
Linlin
Linlin

Reputation: 2257

Javascript method and function

As shown below, the method() will output hello,undefined. What is the scope of the method()? Thank you.

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

obj.sayHello();

var method = obj.sayHello;
method();

output

hello,Tom
hello,undefined

Upvotes: 2

Views: 53

Answers (4)

Edgar Villegas Alvarado
Edgar Villegas Alvarado

Reputation: 18354

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

Upvotes: 2

SomeShinyObject
SomeShinyObject

Reputation: 7811

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);

Upvotes: 2

Nikita U.
Nikita U.

Reputation: 3618

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

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

Upvotes: 1

Phil
Phil

Reputation: 164831

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

Consider this example

var name = 'foo';
method();

logs...

> hello,foo

Upvotes: 3

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