Safe way to extract property names

Question

I'm looking for a way to get an object property name with typechecking that allows to catch possible regressions after refactoring.

Here's an example: the component where I have to pass the property names as strings and it will be broken if I'll try to change the property names in the model.

interface User {
   name: string;
   email: string;
}

class View extends React.Component<any, User> {

   constructor() {
      super();
      this.state = { name: "name", email: "email" };
   }

   private onChange = (e: React.FormEvent) => {
      let target = e.target as HTMLInputElement;
      this.state[target.id] = target.value;
      this.setState(this.state);
   }

   public render() {
      return (
         <form>
            <input
               id={"name"}
               value={this.state.name}
               onChange={this.onChange}/>
            <input
               id={"email"}
               value={this.state.email}
               onChange={this.onChange}/>
            <input type="submit" value="Send" />
         </form>
      );
   }
}

I'd appreciate if there's any nice solution to solve this issue.

There are currently some suggestions on github for helping with this (See [#1579](https://github.com/Microsoft/TypeScript/issues/1579), [#394](https://github.com/Microsoft/TypeScript/issues/394), and [#1003](https://github.com/Microsoft/TypeScript/issues/1003)). You could check out [this](http://stackoverflow.com/a/32542368/188246), but beware it might not work once the code is minified. — David Sherret, Nov 05 '15 at 15:11
@DavidSherret your `this` solution is the only answer that I can come up with as well. Please post as answer — basarat, Nov 05 '15 at 23:21

nzjoel · Accepted Answer · 2020-08-25T04:55:32.670

In TS 2.1 the keyof keyword was introduced which made this possible:

const propertyOf = <TObj>(name: keyof TObj) => name;

or

const propertiesOf = <TObj>(_obj: (TObj | undefined) = undefined) => <T extends keyof TObj>(name: T): T => name;

or using Proxy

export const proxiedPropertiesOf = <TObj>(obj?: TObj) =>
  new Proxy({}, {
    get: (_, prop) => prop,
    set: () => {
      throw Error('Set not supported');
    },
  }) as {
    [P in keyof TObj]?: P;
  };

These can then be used like this:

propertyOf<MyInterface>("myProperty");

or

const myInterfaceProperties = propertiesOf<MyInterface>();
myInterfaceProperties("myProperty");

or

const myInterfaceProperties = propertiesOf(myObj);
myInterfaceProperties("myProperty");

or

const myInterfaceProperties = proxiedPropertiesOf(myObj);
myInterfaceProperties.myProperty;

This will give an error if myProperty is not a property of the type MyObj.

This is great. here is a sample of how it can be added to a class https://gist.github.com/anonymous/5d5d041b4671480855070af478eb3fc2 — David Wilton, Feb 15 '18 at 04:47
Yes, this raises compile time error! Which is wonderful. I had to remove this from the interface though: [key: string]: any; because that made all properties/strings valid. — AL - Divine, Jun 11 '20 at 14:23

score 32 · Answer 2 · edited May 23 '17 at 11:47

32

Right now there's not really a great way of doing this, but there are currently some open suggestions on github (See #1579, #394, and #1003).

What you can do, is what's shown in this answer—wrap referencing the property in a function, convert the function to a string, then extract the property name out of the string.

Here's a function to do that:

function getPropertyName(propertyFunction: Function) {
    return /\.([^\.;]+);?\s*\}$/.exec(propertyFunction.toString())[1];
}

Then use it like so:

// nameProperty will hold "name"
const nameProperty = getPropertyName(() => this.state.name);

This might not work depending on how the code is minified so just watch out for that.

Update

It's safer to do this at compile time. I wrote ts-nameof so this is possible:

nameof<User>(s => s.name);

Compiles to:

"name";

edited May 23 '17 at 11:47

Community

1
1

answered Nov 05 '15 at 23:31

David Sherret

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for a => a.property I found that i needed to remove the '\}' from the regex – George Onofrei May 11 '17 at 10:47
I'd very much suggest you make a separate answer for your **very awesome `ts-nameof`** package. It gets somewhat lost here in between the hacky function-to-string solution. Or just remove that part? Either way, your post shouldn't start the way it does, because there now *is* a great way of doing this. – panepeter Jan 15 '21 at 13:07
1

Don't use `ts-nameof`. It is deprecated by the author: https://github.com/dsherret/ts-nameof/issues/121 – Viacheslav Dobromyslov Dec 23 '21 at 19:15

score 2 · Answer 3 · edited Nov 06 '18 at 22:26

This is specifically for React/React-Native developers.

To safely get property-name, I use the below class:

export class BaseComponent<P = {}, S = {}> extends Component<P, S> {
  protected getPropName = (name: keyof P) => name;
  protected getStateName = (name: keyof S) => name;
}

And replaced extends React.Component<PropTypes> with extends BaseComponnent<PropTypes,

Now, with in the Component you can call, this.getPropName('yourPropName') to get the property name.

score 0 · Answer 4 · answered Jun 03 '22 at 23:24

You can extract property name as string using keyof and Pick:

interface Test {
  id: number,
  title: string,
}

type TitleName = keyof Pick<Test, "title">;
     //^? type TitleName = "title"

const okTitle: TitleName = "title";
const wrongTitle : TitleName = "wrong";
     // Error: Type '"wrong"' is not assignable to type '"title"'

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Safe way to extract property names

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