## What is `this` inside foo.bar()?

## The problem

![Code showing - method which is passed as callback loses reference to original obj as this, but calling foo.bar() directly works](https://cdn.hashnode.com/res/hashnode/image/upload/v1585425858545/Ut1FVXRIJ.png)

Calling a method directly works as expected.  
But when we pass a method as callback, it loses reference to the original object (as `this`) when called.

Why is that? **Can the spec help us explain this difference?**

------

## The explanation

## For methods,

⭐️ `foo.​bar()` translates to `foo.bar.call(foo)`.

That is, when a method (function which is accessed as a object property) is called, the object gets passed as `this` inside the function.

So, in this case -  
**What** →  foo is passed as `this` inside bar  
**When** →  if bar is a function and *accessed as object property* (i.e. foo.bar).  

(*Note* - We are using the [Function.call](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call) method above to estimate what the language is doing internally, by passing a custom `this` value)

## For functions, 
⭐️ `fn()` translates to `fn.​call(undefined)`   

In case of a normal function call - `this` value within the function will be undefined.

But, there is a slight catch here. In case of non-strict mode, if `this` is set to `undefined` or `null` (as above), then it is internally replaced with the global object.  

Effectively -

| **mode**       | **this value**  |
|------------|---------------|
| strict     | undefined     |
| non-strict | global object |


## Method to function, 

⭐️ Now, if we were to rewrite the method call as a function call, then the value of `this` will change from `foo` (object) to either `undefined` or `global`.

One example of this is rewriting `foo.bar()` using a intermediate variable - ` fn = foo.​bar;  fn()`.

If `bar` was referencing other values from foo using `this`, those values will become undefined or resolve to a wrong variable.

🧠 This is exactly the reason why passing methods as callback changes the value of `this` (passed within it). Instead of calling a method directly, callback is passed as a function and called later by some other code.

## In other words,

⭐️ When `foo.bar` is called, the function `bar` is not aware that it is "attached" to the object foo. Based on the exact syntax of a function call, if the language can figure out a clear `someThing.someFunction()` structure, then it will happily forward someThing as this.

But, if you take out a function from a object and call it separately, there is no way to figure out which object it was originally attached to. Hence, `this` will be undefined.

-------

## 📖 What does the spec say? 

TLDR - If you would like to see me actually go through the spec, this video might be more interesting.

%[https://youtu.be/RV27K79PDuI]
[Part 1](https://youtu.be/j894fgpStZI) of the video covers previous sections.

-----

![EvaluateCall- if IsPropertyReference, set this to base object](https://cdn.hashnode.com/res/hashnode/image/upload/v1584625813403/gPjSBUHIZ.png)

![IsPropertyReference- reference where base is object or primitive](https://cdn.hashnode.com/res/hashnode/image/upload/v1584625822167/ZmBSyVj95.png)

Link - 1. https://tc39.es/ecma262/#sec-evaluatecall, 2. https://tc39.es/ecma262/#sec-ispropertyreference

### PropertyReference

This `foo.bar` structure is defined in the spec as a [PropertyReference](https://tc39.es/ecma262/#sec-ispropertyreference).  
Think of PropertyReference as anything that is valid as the left side of an assignment expression, if you were assigning to the property of a object or primitive.  

PropertyReference = Reference + base is object or primitive

![PropertyReference illustration](https://cdn.hashnode.com/res/hashnode/image/upload/v1585431425436/5XBWmcI5y.png)

Some examples of valid and invalid PropertyReference 👇

%[https://gist.github.com/bendtherules/3467664a9c567617342b73c3681c1dc7]

### Steps
1. If it is a PropertyReference, then set `thisValue` to base of the PropertyReference (i.e. `foo` in `foo.bar`).  
2. Or else, take thisValue from surrounding Environment Record.

The obtained `thisValue` is forwarded to the abstract operation [Call](https://tc39.es/ecma262/#sec-call), which in turn verifies that the resolved value of `foo.bar` is a function and then calls the internal method [function.[[Call]]](https://tc39.es/ecma262/#sec-built-in-function-objects-call-thisargument-argumentslist) with the same thisValue.  
This [[call]] interface is very similar to the `function.call` public interface and can be approximated with that. One small diff. we noted earlier is that [[call]] changes `this` value from undefined to global object in non-strict mode.

Note - `foo.bar` is resolved as a string or symbol "bar" within the object `foo` - and goes through the [usual resolution process](https://tc39.es/ecma262/#sec-getvalue) honoring getters, proxies and prototype chains.

## Bound function and Arrow function -
The function `foo.bar` doesn't need to be a  [simple function object](https://tc39.es/ecma262/#sec-ecmascript-function-objects), but it can be anything with a [[call]] interface like a exotic [bound function](https://tc39.es/ecma262/#sec-bound-function-exotic-objects-call-thisargument-argumentslist) or a proxy.  

**Bound functions** ignore the `thisValue` that was passed in and instead uses internal [[BoundThis]] as the actual `this`. [[BoundThis]] is the custom thisValue that was passed while binding using `Function.bind`.  
That is why one solution to this callback problem is to bind a function to the object before passing it as a callback.


**Arrow function** is a type of function object whose `this` value is resolved from the lexical scope.  
Its [[call]] method ignores the received thisValue (from Call abstract method) and always resolves `this` from its lexical scope, like any other free variable in a closure.

![thisMode in function objects](https://cdn.hashnode.com/res/hashnode/image/upload/v1585433951432/jnD8M_NB3.png)
**👇 thisMode in function objects**

It is a function object with internal [[ThisMode]] slot set to lexical.

This can be another solution to the callback problem - creating a arrow function inside a object constructor, will ensure that `this` always resolves to the object.

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🎬 That's all! Hope you had a interesting read.  
I would really appreciate if you leave some feedback 🌀