Incomprehensible object reassignment in javascript

Question

I've been looking at the examples of this game engine and I really don't understand why it reassigns the object, I code in java and I'm learning javascript, it may be a concept error.

example https://blacksmith2d.io/Docs/Examples/Arcade-Physics/CollisionEvents

onassetsLoaded() event he is setting object properties and then reassign it again below.

this.arcade = arcade;
this.circle = circle;
this.box = box;

also i don't understand this part on if statement

!this.circle

trincot · Accepted Answer · 2020-12-20T17:22:19.800

2

About these statements:

this.arcade = arcade;
this.circle = circle;
this.box = box;

Earlier in that block of code, all variables that occur at the right side were defined as local variables (local to the method onAssetsLoaded):

const arcade = ...
const circle = ...
const box = ...

So the assignments that have you wondering are in fact copying references from local variables to instance variables (i.e. properties). This is needed to not lose this information once the onAssetsLoaded call has completed.

As to this expression:

!this.circle

The not-operator (!) can be used on any expression. It evaluates to true when the operand is "falsy". In JavaScript values like undefined, null, 0, "", NaN, are considered "falsy", while all objects are considered "truthy". Here it is used to detect that this.circle has not been assigned a proper value, i.e. it would mean that onAssetsLoaded had not yet been called, and this.circle was still undefined.

edited Dec 20 '20 at 17:22

answered Dec 20 '20 at 14:13

trincot

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using this in javascript means local variable applies globally, is this correct? , I was thinking in java. – G-programmer Dec 20 '20 at 15:02
1

`this` is an object. It can get properties. `this.a = 1` will give a value to a property of the `this` object. If you have also a variable called `a`, then realise that this is *not* a synonym for `this.a`. If that `a` variable is local, and you want to not lose that information after it goes out of scope, then you need to copy it to something that can be accessed from elsewhere, like `this.a`. There is a lot to say about `this`, as it doesn't always act the same way as in Java. Check out the very enlightening [How does the this keyword work](https://stackoverflow.com/q/3127429/5459839). – trincot Dec 20 '20 at 17:17
1

ok, i'm checking this url , anyways i give your answer as correct. – G-programmer Dec 20 '20 at 18:05

Eloims · Answer 2 · 2020-12-20T14:23:56.770

Java and Javascript are as similar as a Car and a Carpet https://stackoverflow.com/a/245068/1897495 :-). The knowledge that you have acquired in Java cannot be directly transposed.

Unlike in Java, access to objects properties are explicit in Javascript: the use of this. is mandatory. The same goes when calling methods from the same class (you need to use this.)

class SomeClass {
   constructor() {
       this.param1 = 42;
   }

   localAndParameter(param1) {
       console.log(this.param1); // prints "42"
       console.log(param1); // prints "666"
   }

   localAndGlobal() {
       console.log(this.param1); // still "42"
       console.log(param1); // prints 123
   }

   callMethods() {
       this.emptyMethod(); // this works
       emptyMethod(); // This would raise an exception if called
       // "emptyMethod" is undefined or something like that
   }

   emptyMethod() {
   }
}

const param1 = 123;
const obj = new SomeClass();
obj.localAndParameter(666);
obj.localAndGlobal();

Also the language is dynamic.

Properties can be added to objects at all times, not only when declared/constructed

class SomeClass {
   constructor() {
       // no properties created.
   }

   someMethod() {
       // everything is dynamic! I can create a property here
       this.newProperty = 23;
   }
}

const obj = new SomeClass()
console.log(obj.newProperty); // prints "undefined"
obj.someMethod();
console.log(obj.newProperty); // prints "23"

so basically if he doesn't use this when he was adding these properties , that's why is he reassignment the object? — G-programmer, Dec 20 '20 at 14:10
I think the confusion of the OP just comes from not understanding than `this.something` and `something` are not the same in Javascript (which is the case in Java) — Eloims, Dec 20 '20 at 14:24
yes, you are correct , i was thinking in java , that's why i don't understand this concept — G-programmer, Dec 20 '20 at 15:03

Incomprehensible object reassignment in javascript

2 Answers2