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81 lines
4.8 KiB
Markdown
81 lines
4.8 KiB
Markdown
## What is `this` inside foo.bar()?
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## The problem
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When we pass a method as callback, it loses reference to the original object (as `this`) when it is called.
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But calling a method directly works as expected.
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Why is that? **Can the spec help us explain this difference?**
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*(For simplicity, let's assume the caller doesn't intend to pass a different `this` value here, and calls onClick() directly)*
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------
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## The explanation
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##### For methods,
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⭐️ `foo.bar()` translates to `foo.bar.call(foo)`.
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That is, when a method (function which is accessed as a object property) is called, the object gets passed as `this` inside the function.
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So, in this case -
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*What* → foo is passed as `this` inside bar
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*When* → if bar is a function and *accessed as object property* (i.e. foo.bar).
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##### For functions,
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⭐️ `fn()` translates to `fn.call(this)`
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(where `this` is taken from caller's scope)
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##### Method to function,
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Now, if we were to rewrite `foo.bar()` using a intermediate variable - ` bar1 = foo.bar; bar1()`,
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then the value of `this` inside bar will change from `foo` to the `outer this`.
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If `bar` was referencing other values from foo using `this`, those values will become undefined or resolve to a wrong variable.
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🧠 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.
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##### In other words,
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⭐️ 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.
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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 set from caller's scope.
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-------
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## 📖 What does the spec say?
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Link - 1. https://tc39.es/ecma262/#sec-evaluatecall, 2. https://tc39.es/ecma262/#sec-ispropertyreference
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This `foo.bar` structure is defined in the spec as a [PropertyReference](https://tc39.es/ecma262/#sec-ispropertyreference).
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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.
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**PropertyReference = Reference + base is object or primitive**
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Some examples of valid and invalid PropertyReference -
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%[https://gist.github.com/bendtherules/3467664a9c567617342b73c3681c1dc7]
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⭐️ If it is a PropertyReference, then set `thisValue` to base of the PropertyReference (i.e. `foo` in `foo.bar`).
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⭐️ Or else, take thisValue from surrounding Environment Record.
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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.
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This [[call]] interface is very similar to the `function.call` public interface and can be approximated with that.
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#### A few interesting things to note -
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1. `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.
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2. 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 exoctic [bound function](https://tc39.es/ecma262/#sec-bound-function-exotic-objects-call-thisargument-argumentslist) or a proxy.
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Bound functions, for example, ignore the `thisValue` that was passed in and instead uses internal [[BoundThis]] as the actual this (which is stored from when it was created). That is why one solution to this callback problem is to bind a function before passing it as a callback.
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----
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*That's all! Hope you had a interesting read.
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I would really appreciate if you leave some feedback.*
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*If you want to start reading ecmascript specification yourself, https://timothygu.me/es-howto/ is a great starting point.* |