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

[RAnders00]

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:

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

[AnyhowStep]

I had this idea before, too. But this will not play nicely with minifiers. They'll mangle your function names and your run time and compile time values will be different.

So, it'll work okay for specific cases but not in general.

You can, of course, preserve your function names after minification by disabling the option to mangle function names, or using Object.defineProperty()

Also, in some environments, I think the name is always an empty string, no matter what. I can't remember if my memory is hazy or if it is actually true. I couldn't find anything after a minute of Google. I'm going to assume my memory is bad

---

Regarding minifiers,

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name#JavaScript_compressors_and_minifiers