JS add property to object via function call from construcator

Question

I can't figure out why the following code:

 function Foo() {
    this.a = "a";                               
    this.c = add();
}
var add = function() {
    this.b = "added";
}

obj2 = new Foo();
alert("obj2.b is " + obj2.b); //obj2.b is Undefined

Won't create 'b' property to obj2 while this code:

function Foo() {
    this.a = "a";                               
    this.func1 = add;
    this.c = this.func1();
}
var add = function() {
    this.b = "added";
}

obj2 = new Foo();
alert("obj2.b is " + obj2.b); // obj2.b is added

Will create the 'b' property. I'm new to JS, I tried reading about using function as constructor and basicly I got the idea but I guess there is still something I'm missing.
Thank you, Noam

The value of this is set based on how a function is called. If you call a function as a property of an object, this in the function becomes the object. — user1106925
– user1106925, Commented Jun 26, 2016 at 12:17

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

the value of this is determined by how a function is called.

If no caller, then this will point to default/global scope(window).

If you need to pass context to another function, you should use one of following: .apply, .bind

Sample

function Foo() {
    this.a = "a";                               
    this.c = add.apply(this);
}
var add = function() {
    this.b = "added";
}

obj2 = new Foo();
console.log("obj2.b is " + obj2.b); //obj2.b is Undefined

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jun 26, 2016 at 12:21

Rajesh Dixit

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2 Comments

Rajesh Dixit Over a year ago

@Downvoter, please find updates and if something is missing or incorrect, please point it out.

Noam Over a year ago

Got it. Thank you :)

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