从对象返回访问的属性名称（键）

Question

Is it possible in Javascript/Typescript to write a function that returns the arguments name/key as string?

function foo(arg) {...}

let user = new User();
foo(user.userId) // => returns string: "userId"
foo(user.name) // => returns string: "name"

For my purposes it would be also okay if the function could return the whole expression, means:

foo(user.userId) // => returns string: "user.userId"
foo(user.name) // => returns string: "user.name"

Answer 1

One option is not exactly a function, but a wrapper, if that's acceptable - you could use a Proxy which intercepts property access and returns the key that was attempted to be accessed:

 function User() {} const userProxy = new Proxy( new User(), { get: (obj, prop) => prop } ); console.log(userProxy.userId) // => returns string: "userId" console.log(userProxy.name) // => returns string: "name" 

Answer 2

There is no nameof operator in typescript (like in C#). There is the keyof type operator which allows you to specify that a string must be a key of a type. So we could rewrite your function to:

class User {
    userId: number;
    name: string
}

function foo<T, K extends keyof T>(arg: T, key: K) {
    return key
} 

let user = new User();
foo(user, "userId") // => returns string: "userId"
foo(user, "name") // => returns string: "name"
foo(user, "namee") // error

Note There is a nameof implementation for Typescript by fellow SO user David Sherret you can find it here . I have not personally tried it but it may be useful.