Support override keyword on class methods

[rwyborn]

(Update by @RyanCavanuagh)

Please see this comment before asking for "any updates", "please add this now", etc.. Comments not meaningfully adding to the discussion will be removed to keep the thread length somewhat reasonable.

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(NOTE, this is not a duplicate of Issue #1524. The proposal here is more along the lines of the C++ override specifier, which makes much more sense for typescript)

An override keyword would be immensely useful in typescript. It would be an optional keyword on any method that overrides a super class method, and similar to the override specifier in C++ would indicate an intent that "the name+signature of this method should always match the name+signature of a super class method". This catches a whole range of issues in larger code bases that can otherwise be easily missed.

Again similar to C++, it is not an error to omit the override keyword from an overridden method. In this case the compiler just acts exactly as it currently does, and skips the extra compile time checks associated with the override keyword. This allows for the more complex untyped javascript scenarios where the derived class override does not exactly match the signature of the base class.

Example use

class Animal {
    move(meters:number):void {
    }
}

class Snake extends Animal {
    override move(meters:number):void {
    }
}

Example compile error conditions

// Add an additional param to move, unaware that the intent was 
// to override a specific signature in the base class
class Snake extends Animal {
    override move(meters:number, height:number):void {
    }
}
// COMPILE ERROR: Snake super does not define move(meters:number,height:number):void

// Rename the function in the base class, unaware that a derived class
// existed that was overriding the same method and hence it needs renaming as well
class Animal {
    megamove(meters:number):void {
    }
}
// COMPILE ERROR: Snake super does not define move(meters:number):void

// Require the function to now return a bool, unaware that a derived class
// existed that was still using void, and hence it needs updating
class Animal {
    move(meters:number):bool {
    }
}
// COMPILE ERROR: Snake super does not define move(meters:number):void

IntelliSense

As well as additional compile time validation, the override keyword provides a mechanism for typescript intellisense to easily display and select available super methods, where the intent is to specifically override one of them in a derived class. Currently this is very clunky and involves browsing through the super class chain, finding the method you want to override, and then copy pasting it in to the derived class to guarantee the signatures match.

Proposed IntelliSense mechanism

Within a class declaration:

1. type override
2. An auto complete drop down appears of all super methods for the class
3. A method is selected in the drop down
4. The signature for the method is emitted into the class declaration after the override keyword.

[stephanedr]

Indeed a good proposal.

However what about the following examples, which are valid overrides to me:

class Snake extends Animal {
    override move(meters:number, height=-1):void {
    }
}

class A {...}
class Animal {
    setA(a: A): void {...}
    getA(): A {...}
}

class B extends A {...}
class Snake extends Animal {
    override setA(a: B): void {...}
    override getA(): B {...}
}

Additionally I would add a compiler flag to force the override keyword to be present (or reported as a warning).
The reason is to catch when renaming a method in a base class that inherited classes already implement (but not supposed to be an override).

[rwyborn]

Ah nice examples. Generally speaking I would expect the use of the override keyword to enforce exact matching of signatures, as the goal of using it is to maintain a strict typed class hierarchy. So to address your examples:

1. Adding an additional default param. This would generate a compile error: Snake super does not define move(meters:number):void. While the derived method is functionally consistent, client code calling Animal.move may not expect derived classes to also be factoring in height (as the base API does not expose it).
2. This would (and should always) generate a compile error, as it is not functionally consistent. Consider the following addition to the example:

class C extends A {...}
var animal : Animal = new Snake();
animal.setA(new C());
// This will have undefined run-time behavior, as C will be interpreted as type B in Snake.setA

So example (2.) is actually a great demo of how an override keyword can catch subtle edge cases at compile time that would otherwise be missed! :)

And I would again stress that both examples may be valid in specific controlled/advanced javascript scenarios that may be required... in this case users can just choose to omit the override keyword.

[NoelAbrahams]

This will be useful. We currently work around this by including a dummy reference to the super method:

class Snake extends Animal {
    move(meters:number, height?:number):void {
         super.move; // override fix
    }
}

But this only guards against the second case: super methods being renamed. Changes in signature do not trigger a compilation error. Furthermore, this is clearly a hack.

I also don't think that default and optional parameters in the derived class method's signature should trigger a compilation error. That may be correct but goes against the inherent flexibility of JavaScript.

[stephanedr]

@rwyborn
It seems we don't expect the same behaviour.
You would use this override keyword to ensure a same signature, whereas I would use it more as a readibiliy option (so my request to add a compiler option to force its usage).
In fact what I would really expect is that TS detects invalid overriding methods (even without the use of override).
Typically:

class Snake extends Animal {
    move(meters:number, height:number):void {}
}

should raise an error, because it's really an override of Animal.move() (JS behaviour), but an incompatible one (because height is not supposed to be optional, whereas it will be undefined if called from an Animal "reference").
In fact using override would only confirm (by the compiler) that this method really exists in the base class (and so with a compliant signature, but due to the previous point, not due to the override keyword).

[rwyborn]

@stephanedr , speaking as a single user I actually agree with you that the compiler should just always confirm the signature, as I personally like to enforce strict typing within my class hierarchies (even if javascript doesn't!!).

However in proposing that this behavior is optional via the override keyword I am trying to keep in mind that ultimately javascript is untyped, and hence enforcing the strict signature matching by default would result in some javascript design patterns no longer being expressible in Typescript.

[kungfusheep]

@rwyborn I'm glad you mentioned the C++ implementation because that's exactly how I imagined it should work before I got here - optionally. Although, a compiler flag that forced the use of the override keyword would go down well in my book.

The keyword would allow compile time errors for a developers clumsy typing, which is what worries me most about overrides in their current form.

class Base {
    protected commitState() : void {

    }
}


class Implementation extends Base {
    override protected comitState() : void {   /// error - 'comitState' doesn't exist on base type

    }
}

Currently (as of 1.4) the Implementation class above would just declare a new method and the developer would be none the wiser until they notice their code isn't working.

[RyanCavanaugh]

Discussed at suggestion review.

We definitely understand the use cases here. The problem is that adding it at this stage in the language adds more confusion than it removes. A class with 5 methods, of which 3 are marked override, wouldn't imply that the other 2 aren't overrides. To justify its existence, the modifier would really need to divide the world more cleanly than that.

[hdachev]

Please excuse the whining, but honestly, while your argument does apply to the public keyword in a language where everything is public by default, quite world dividing indeed, having things like abstract and an optional override keyword will simply help developers feel safer, make less mistakes and waste less time.

Overriding is one of the few remaining highly-typo-sensitive aspects of the language, because a mistyped overriding method name is not an obvious compile time problem. The benefit of override is obvious, as it you allows you to state your intention to override - if the base method doesn't exist its a compile time error. All hail the type system. Why would anyone not want this?

[rwyborn]

I concur 100% with @hdachev , the small inconsistency referred too by @RyanCavanaugh is easily out weighed by the benefits of the keyword in bringing compile time checks to method overrides. I would again point out that C++ uses an optional override keyword successfully in exactly the same way as suggested for typescript.

I can not emphasize enough how much of a difference override checking makes in a large scale code base with complex OO trees.

Finally I would add that if the inconsistency of an optional keyword really is a concern, then the C# approach could be used, that is the mandatory use of either the "new" or "override" keywords:

class Dervied extends Base {

    new FuncA(newParam) {} // "new" says that I am implementing a new version of FuncA() with a different signature to the base class version

    override FuncB() {} // "override" says that I am implementing exactly the same signature as the base class version

    FuncC() {} // If FuncC exists in the base class then this is a compile error. I must either use the override keyword (I am matching the signature) or the new keyword (I am changing the signature)
}