keyof T and T[K]

Note about terminology

• keyof is a type operator. Note, name change from Keysof type operator #10425 to keyof; since the type name represents one value and not the whole value domain. this is inline with the other names of types e.g. we call number and not numbers,
• Henceforth, formal names of these type operators:
  • keyof T => Index Type Query
  • T[K] => Indexed Access Type

Details

keyof T

• known properties and index signatures (number | string for string indexers and number for numeric indexers)

• Examples:

  • keyof { a: number, b:string} => "a" | "b"
  • keyof { [x: string] : number } => string | number
  • keyof { [x: number] : number } => number
  • keyof {}[] => "toString" | "pop" |...| number
  • keyof { a: number, b:string } | { a: number, c:string} => "a"
  • keyof { a: number, b:string } & { a: number, c:string} => "a" | "b" | "c"

• What about symbols?

  • symbols not supported yet. we need to make symbols literal types first.

• private and protected
  A piece of trivia, accessing private and protected properties are allowed for index access today

  class C {
  private x;
  }
  
  let c: C;
  c.x; // not allowed
  c["x"];  // is allowed today

  • should this apply to the new type operator:
    ts type T = C["x"]; // OK or not?
  • for
    • nameof in C# does not do this either
    • it exists at run time
  • against
    • we should not carry past misdeeds in new features
    • it exposes implementation details
    • not valid for declaration emit, since we strip out the type
  • Decision
    • keyof T is public-only known properties, private and protected properties are not included

T[K]

• only index with a type K parameter iff it is constraint to the keyof T
• this grantees that the output is type of a property
• can be used with literal types

class Thing {
    name: string;
    width: number;
}

type q1 = Thing["name"]; // string
type q2 = Thing["name" | "width"]; //string | number
type q3= Thing["name" | "foo"]; // Error "foo" is not a property on Thing

Putting them together

function getProperty<T, K extends keyof T>(obj: T, key:K) {
     return obj[key]; // T[K]
}

function setProperty<T, K extends keyof T>(obj: T, key:K, value: T[K]) {
    obj[K] = value;
}

var thing: Thing;
var thingList: Thing[];

getPropeprty(thing, "foo") // Error
getPropeprty(thing, "name") // string
getPropeprty(thingList, 1) // Thing
getPropeprty(thingList, 1 + 2) // Thing

setPropoerty(thing, "name", "my name"); // OK
setPropoerty(thing, "name", 3); // Error

Index access unification

• The way we do T[K] for a type should be the same as that for an expression t[k].
• so

thing[condition ? "name" : "width"]; // should be string | number

Issues

Variance

Consider this:

setPropoerty(thing, condition? "name" : "width", "string");  

• this is allowed today, but it is unsafe... Keyof T should be a union for types of all properties for read, but an intersection for write.
• This is the same behavior we have today for indexers anyways:

interface I {
     a: number;
     b: string;
     [x: string] : number | string;
}

const k = condition? "a" : "b";

x[k] // number | string;
x[k] = "bar";  // allowed but not safe

• We are getting close to the "safe variance" glass ceiling
• solving these all would need to add variance in the type system
• a readonly, writeonly, and readwrite types
• this applies to class properties, parameters, everything
  • this would be a large braking change, and have to be done holistically
  • the safer it would be the tighter it would be for users to use

Where we can land:

• For expressions, it would be an error i.e.

thing[condition ? "name" : "width"] = "bar";  /// Error, we know unsafe 

• but not for generic type argument

setProperty(thing, condition ? "name" : "width", "bar"); // OK 

Note about readonly:

• you can write a readonly propoerty using setPropoerty.
• there is not much we can do here. the time where this would be observed is at instantiation time, which is too late at that point to report errors.
• and again it goes back to variance, you need readonly keyof T and readwrite keyof T.