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JavaScript Understanding "this" in JavaScript

Tommaso Poletti
Tommaso Poletti
5,205 Points
Posted by Tommaso Poletti
Tommaso Poletti
5,205 Points

I can't understand why use " = " instead " : " inside a object

I can't understand the mechanism that converts 

this.name = name || anonnymus;

in 

{ name: anonymus }
Dennis Brown
Dennis Brown
28,742 Points
Dennis Brown
Dennis Brown
28,742 Points

What this is doing is simply assigning the value to the object property directly with dot notation. By doing so when it's read it checks to see if name exists or if 'Portland' exists to use as a default.

We have to use a property in order to use this, because without it we have nothing to reference the parent of. Since we are referencing City with the this keyword, we want to make sure to assign properties to the City object. If we were to use the object literal notation, we would be assigning objects instead of values to the properties.

var City function(name, state) {
  this.name = { name: name || 'Portland' }  // we end up creating another object in this.name with the property 'name'
}

Another deal-breaker is even though we can use comparison operators in object literals, we cannot use logical operators (or, and, not), at least not effectively. 

var newObj = {
  numOne: 2 && 3  // shows 3
  numTwo: 4 || 5  // shows 4 again
  numThree: !6    // shows 'false'
}

I hope that clears things up, and if not, I'll be happy to revise what's needed.

2 Answers

Erik Nuber
Erik Nuber
20,629 Points
Erik Nuber
Erik Nuber
20,629 Points 
this.name = name || anonnymus;

{ name: anonymus }

You are only giving part of the code. This.name is apart of a constructor function.

If the value name being passed into that function was not passed in, anonymous would then take it's place. Else the name passed into the function would become this.name

var Cat = function(name, furType) {
   this.name = name || "anonymous";
   this.furType = furType || "to much fur...help! *koff**koff*";

}

blackCat = new Cat("jasper", "some fur");
whiteCat = new Cat();

Here blackCat would return blackCat.name to be jasper and blackCat.furType would be some fur

And whiteCat.name would be anonymous because nothing was passed in and, whiteCat.furType would be to much fur...help! koffkoff for the same reason

You can copy paste the example I gave into the console and test it yourself to see...

Steven Parker
Steven Parker
229,644 Points
Steven Parker
Steven Parker
229,644 Points

The first line looks like it might come from a constructor function. It would assign an explicit (if defined) or default value to the "name" in a new object being constructed when it is used. But this line by itself doesn't create an object.

The second line is a complete object literal. It exists without being made by a constructor, and has a predefined value for "name".

These lines represent two different ways of assigning a value to a "name" inside an object, but one doesn't "convert" to the other.