Narrower (literal) type for Function.prototype.name if function is const

Search Terms

ES6 function name, const assertions, as const, function as const

Suggestion

I would like to see the name property of functions to be available at compile time if the given function is a const expression, e.g.:

class Test {
    public a() { console.log("Test.a"); }
    public b() { console.log("Test.b"); }
}

type FunctionWithName<N> = (...args: any[]) => any & { readonly name: N };

function replaceFunction<T, K extends keyof T>(target: T, fn: FunctionWithName<K>) {
    target[fn.name] = fn;
}

// proposed inline syntax
let testInstance = new Test();
replaceFunction(testInstance, function a() { console.log("Replacement.a"); })

// proposed "as const" syntax
const newB = function b() {
    console.log("Replacement.b");
} as const;
replaceFunction(testInstance, newB);

// example of a compile time error since "c" is not assignable to "a" | "b"
replaceFunction(testInstance, function c() { console.log("Replacement.a"); })

Use Cases

My use case is a function like shown above, where I would like to have static type checking available, without having to specify the extra parameter that specifies the function name to replace, e.g. compare:

// current syntax:
replaceFunction(testInstance, "a", function a() { /* ... */ });
// proposed syntax:
replaceFunction(testInstance, function a() { /* ... */ });

Examples

let myObject = {
    a: () => "a",
    b: () => "b"
}

// fictional "mocking" library:
// this syntax can only validate that "a" is in "myObject" through the const "a" parameter
mockingLibrary.mock(myObject, "a", function a() { return "different value" });

// proposed style, using Function.prototype.name
mockingLibrary.mock(myObject, function a() { return "different value" });

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Beginnen Sie mit der Überprüfung der Beispiele des Vorschlags zu Function.prototype.name, const expressions und generischen replaceFunction-Aufrufen. Bestimmen Sie die Auswirkungen auf das Typsystem und die Kompatibilität, und definieren Sie anschließend Tests, die zeigen, dass literale Funktionsnamen erhalten bleiben und ungültige Namen abgelehnt werden, ohne das ausgegebene JavaScript zu ändern.