Move sentence and explain constructor rules better in Classes.md - #214
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| This example covers quite a few of the inheritance features in TypeScript that are common to other languages. | ||
| Here we see the `extends` keywords used to create a subclass. You can see this where `Horse` and `Snake` subclass the base class `Animal` and gain access to its features. | ||
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| Derived classes that contain constructor functions must call `super()` which will execute the constructor function on the base class. |
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Can you make this part of the preceding paragraph? I would add two sentences after this one, something like:
You have to call
super()before accessingthisin the derived constructor.
BothHorseandSnaketechnically do this since neither one actually referencesthis.
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Nathan Shively-Sanders (@sandersn) I have added those two sentences. Since you seemed to want to clarify restrictions about constructors, I also added other sentences to explain another restriction from the TypeScript spec about when the 'super' call had to be the first statement in the constructor.
I moved all that into its own paragraph since it got a bit long. It might be too long now, so feel free to suggest further improvements.
I also added a blank line to split up the earlier paragraph, since the sentence about "This example covers quite a few of the inheritance features..." now leads into the three paragraphs that follow which each explain a different concern (extends, constructor restrictions, and overriding).
I left the changes as two separate commits -- the original one to fix an issue and the second one as an enhancement. Let me know if you would prefer them squashed together into one commit though.
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| Here we see the `extends` keywords used to create a subclass. You can see this where `Horse` and `Snake` subclass the base class `Animal` and gain access to its features. | ||
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| TypeScript has some restrictions about constructors intended to help ensure instances of a class have their properties correctly initialized before the properties are used. Derived classes that contain constructor functions must call `super()` which will execute the constructor function on the base class. You have to call `super()` before accessing `this` in the derived constructor. Both `Horse` and `Snake` technically do that since neither one actually references `this`. A 'super' call must also be the first statement in the constructor of a derived class when the derived class contains initialized properties or has parameter properties. Neither `Horse` or `Snake`, however, have those kind of properties right now. So, as long as those two classes stay that way, they could have console logging or other statements before the `super` call in their constructors without generating compile-time errors. |
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One sentence per line please. We do this for easy diff-ability.
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Use backticks around "super" in "A 'super' call"
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Thanks for the feedback. I made those improvements and also added parentheses for each super call reference to be consistent.
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Paul D. Fernhout (pdfernhout)
changed the title
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👍 I like the changes. We don't care much about commits, so just leave them as-is. |
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| Here we see the `extends` keywords used to create a subclass. You can see this where `Horse` and `Snake` subclass the base class `Animal` and gain access to its features. | ||
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| TypeScript has some restrictions about constructors intended to help ensure instances of a class have their properties correctly initialized before the properties are used. |
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it is not really a TypeScript restriction. it is a JS one. and enforced by the fact that the super, and the this share the same object, and that the object is not well formed, until super call has returned.
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The text did not make sense with the sentence about calling super() between a sentence that leads into an example (with a colon) and the example.
This issue was introduced in this commit: d0fa464