Issue in understanding 'This' keyword

Question

Why does in the following code, i don't see output as T12345 ABC and instead i see undefined undefined.

The reason i am expecting T12345 ABC because myCarDetails now refer to global context and global context have registrationNumber and brand defined.

var registrationNumber = "T12345";
var brand =  "ABC";

var car = { 
    registrationNumber: "T12345",
    brand: "ABC",

    displayDetails: function(){
        console.log(this.registrationNumber + " " + this.brand);
    }
}

var myCarDetails =  car.displayDetails;
myCarDetails();

Possible duplicate of [What is the scope of variables in JavaScript?](https://stackoverflow.com/questions/500431/what-is-the-scope-of-variables-in-javascript) — Steven Stark, Mar 11 '19 at 20:29
May i know why this is duplicate, The question you referred is an overview of this operator whereas i am talking about specific problem under 'this' umbrella ! — simbada, Mar 11 '19 at 20:31
What execution environment are you using? Your code works exactly as you expect it to work in Firefox via the console. — Pointy, Mar 11 '19 at 20:32
While `this` refers to the global object, `var registrationNumber` and `var brand` may not be defined in global scope. It really depends on how the whole piece of code is executed. — Felix Kling, Mar 11 '19 at 20:33
On Mozilla, there is live javascript code run option. right? i am using that — simbada, Mar 11 '19 at 20:34
@simbada you are asking about scope of 'this', which is a very common and often duplicated question on SO that has been answered countless times. — Steven Stark, Mar 11 '19 at 20:34
There are likely not evaluating the code in global scope. If you want to ensure that `registrationNumber` and `brand` are defined in global scope, do `window.registrationNumber = ...;` instead. — Felix Kling, Mar 11 '19 at 20:34
Ah well it must be the case that what appears to be the global (`window`) scope is actually *not* the global scope in that environment. — Pointy, Mar 11 '19 at 20:34
@pointy great i see your point and now getting close to find the reason, By the way if it has been global scope window then the output should be what i am expecting. Correct? — simbada, Mar 11 '19 at 20:36
Yes, exactly — if you copy the exact code you posted and paste it into your browser developer console, it should work (it did for me in Firefox) — Pointy, Mar 11 '19 at 20:41
You can do a simple test: `var simbada1 = "Hello World!"; console.log(window.simbada1);` If that says `undefined` then the outer scope of the execution environment is not really the global scope. — Pointy, Mar 11 '19 at 20:42

score 0 · Answer 1 · answered Mar 11 '19 at 20:53

When we try to access any property of an Object which is a function itself, in this case way to access this property is -

var myCarDetails = car.displayDetails();

myCarDetails

Complete Working code is as Follows -->

var car = { registrationNumber: "T12345",

         brand: "ABC",

         displayDetails:  function() {

         console.log(this.registrationNumber + " " + this.brand);
         }
       }

var myCarDetails = car.displayDetails();

myCarDetails;

Additional Resource --> https://www.w3schools.com/jS/js_objects.asp

Issue in understanding 'This' keyword

1 Answers1