Index signature arguments are not type checked based on the specialized key type they are constrained to

[malibuzios]

TypeScript Version:

1.8.7

Code

type OnlyNumberKeys = {[key: number]: any};
let a: OnlyNumberKeys = {};

a["abcd"] = 1; // No error?
let x = a["abcd"]; // No error?

type OnlyStringKeys = {[key: string]: any};
let b: OnlyStringKeys = {};

b[1234] = 1; // No error?
let y = b[1234]; // No error?

Expected behavior:
Should error

Actual behavior:
Doesn't error

(Sorry if this is a duplicate but a search did not yield anything on this)

[RyanCavanaugh]

The first example is an error under noImplicitAny.

The second is by design -- indexing by a number is a subset of indexing by a string, so indexing by a number when a string would have been valid is also valid.

[malibuzios]

@RyanCavanaugh

I'm not sure exactly what the logic is on why the first one errors on noImplicitAny? and why not in standard mode?

I renamed the title to a more accurate one, as the keys are of course checked if they receive something like an object or a boolean, but not if the key type is one of the generally valid one (i.e. string, number, Symbol).

I also tested in ES6 mode for Symbol, and it also did not error on both cases:

type OnlyNumberKeys = { [key: number]: any };
let a: OnlyNumberKeys = {};

a[Symbol("abcd")] = 1; // No error?
let x = a[Symbol("abcd")]; // No error?

type OnlyStringKeys = { [key: string]: any };
let b: OnlyStringKeys = {};

b[Symbol("abcd")] = 1; // No error?
let y = b[Symbol("abcd")]; // No error?

[jods4]

Indexing is special because it's the built-in JS way to perform "reflection", i.e. to access a field dynamically at runtime.

type X = { length: number; [key: number]: string };
let some: X;
let len = some['length']; // equivalent to some.length;
let dynamic = "length";
len = some[dynamic]; // works at runtime, treated as any at compile time

And because the result of dynamic access has to be any, doing so is an error under --noImplicitAny. But it turned out to be an important enough scenario to be allowed even in no implicit any code, with its sibling option --suppressImplicitAnyIndexErrors.

I think a similar argument can be made for Symbol.

[malibuzios]

@jods4 @RyanCavanaugh

I'm aware that square bracket notation was designed to become a "back door" where anything can be accessed. I do use it myself occasionally. However, I don't remember a case I needed it when a type had an explicit index signature. The reason is very simple:

Since an index signature was designed to "open up" access to what is usually a large amount of key space, such as "all the strings", "all the numbers" and even "all the symbols" (though that is not supported yet, I opened issue #7667 for that) or a combination of them. One can always write something like:

type OnlyStringOrNumberKeys = { [key: string | number]: any };
type OnlyStringOrSymbolKeys = { [key: string | symbol]: any };
type OnlyNumberOrSymbolKeys = { [key: number | symbol]: any };
type AllPossibleKeys = { [key: string | number | symbol]: any };

I mean what really is the benefit of allowing the programmer to specify a parameter type if it is not actually enforced?

There's a very good reason I'm mentioning this at this particular time. The reason is that in light of a proposal like #5683 (also see the duplicate #7656) which is currently in discussion, where the key would be constrained to a finite set of string literals or possibly numbers (with numeric literals), there's not much point of implementing it if key types are not fully checked. E.g. you had something like:

type OnlySomeStringLiteralKeys = { [key: "A" | "B" | "C"]: any };
x: OnlySomeStringLiteralKeys = {};

x["D"] = 1; // Error, specified key type is not assignable to "A" | "B" | "C"

I can't see how that constraint would be truly realized if you can still, "by design" do things like:

x[4385] = "a";
x[Symbol("abcd")] = 12;

?

[jods4]

I mean what really is the benefit of allowing the programmer to specify a parameter type if it is not actually enforced?

You make a good point.

But I think disallowing dynamic access completely on those objects is too much collateral damage.

In noImplicitAny code, but with suppressImplicitAnyIndexErrors, I have seen this done:

let arr: number[];

let x = arr[4]; // array indexer -> x: number
// dynamically augment the array with metadata or additional infos
arr['priority'] = 'high';  // ok...

This is certainly not the best code, and you probably should use an intersection type to describe your array number[] & Metadata but just sayin' I've seen things like that.

Not sure what the best approach is here.
Some thoughts:

• If an indexed access is specific (with upcoming string or symbol literals support), then it should be OK and properly typed.
• Otherwise, if you run without noImplicitAny or with suppressImplicitAnyIndexErrors I don't think you really can forbid a dynamic access?

Perhaps some real-world examples could help better define a solution?

[malibuzios]

@jods4

I'm not sure if internally Array<T> type is bound by the rules of the { [x: number]: T } interface. I think it may be a type that is handled separately (though I could be wrong). I can understand that in other types like Node's Buffer that may actually be the case. So you're right that for something like:

interface NodeBuffer {
    [index: number]: number;
        ... Lots of properties follow ...
}

It would break the ability to "back door" into, say, undocumented properties. The simplest workaround that one would need to either cast it to any:

let buf: Buffer;
(<any> buf)["undocumentedProperty"] = 12;

Or use some sort of a intersection type (though I don't think that would be something that would appeal or even be understood by beginners).

But I can see this becoming a bit tedious when it is frequently used, and it would break some existing code, so I agree this would be a major problem if key checking was stricter. Unfortunately I don't really have a good alternative solution for that at this point.

Anyway, as for a real-world example (other the ones described in #7656 for the string literal keys). In the case of "symbols keys only" signatures I could see an example where I have a function that only cares about the metadata hidden inside some object so ideally it would be nice to have a type that basically sees anything thrown at it as just a bunch of symbols, and no more:

function iOnlyCareAboutTheSymbolsInWhateverYouGiveMe(x: any) {
  let symbolsOnlyHere: { [key: symbol]: any } = x;
  // ... do something with the hidden metadata ...
}

[jods4]

Nitpicking maybe, but it's not just a "backdoor". It's how you can access members dynamically at runtime.
There are good use cases for that. E.g. try to write a serializer, a string format or an ORM without it.
It's like saying Reflection in .net is a "backdoor", although it's the backbone of many frameworks and features.

Regarding examples:

• symbolsOnlyHere is not very compelling. The value over using x: any is quite low.
• From allow string literals (unions of string literals?) as a type of an indexer #7656 I feel like some examples over there would be better served with properly typed members or using a real Map, now that they are commonplace.

Probably the most compelling example so far is this one of yours:

type RequestMethod = "GET" | "SET" | "PUT" | "UPDATE" | "DELETE";
type HandlerObject = { [key: RequestMethod]: (requestObject: RequestObject) => boolean; };

I think this is desirable but I'm not sure what the best way to proceed here is.

Another thing I haven't seen mentionned yet: how should it work with generics?

function dump<T>(obj: T) {
  for (let prop in obj)
    console.log(obj[prop]);  // <-- trouble spot
}

let hi = { greetings: "hello world" };
dump(hi);  // sure.

interface SymbolsOnly { [key: symbol]: any };
dump<SymbolsOnly>(hi);  // ok or not? there's indexed access in dump, which may not be a symbol.

[malibuzios]

@jods4

I'm trying to figure out what is the actual effect of having index signature key types at the first place, also considering what would happen if string literals are allowed as keys. First:

let x: { [key: number] : Array<number> };

There seem to be two different interpretations of what this should mean:

1. You can only index x using a number when using square brackets, which would give you a value of type Array<number> or error otherwise.
2. If you index x using using a number key using square brackets, you get a value of type Array<number>, but if you index something else you get any (this is how this works today).

Now let's take this to string literals:

let x: { [key: "A" | "B" | "C" ]: Array<number> };

1. You can only index x using a "A", "B" or "C" using square brackets, which would give you a value of type Array<number> or error otherwise.
2. If you index x using using "A", "B" or "C" using square brackets, you get a value of type Array<number>, but if you index something else you get any.

The second option here wouldn't seem to be be very useful in this case as I feel it is not strict enough, so I'm afraid that if the semantics of indexer key types were generally intended to be this way then I would personally recommend against implementing #5683 at this point. I'm also considering the fact that { [key: "A" | "B" | "C" ]: Array<number> } seem like it may be assignable from anything if interpreted as an all-optional interface type.