Typescript. How to get object's property name from its value?

Question

I'm writing a typescript class for using with immutable map

class NavigableObject<T> {
    constructor(private obj: T, private path: string[] = []) { }

    To<R>(p: (x: T) => R): NavigableObject<R> {
        return new NavigableObject<R>(p(this.obj),
                       this.path.concat(this.getPropName(p(this.obj))));
    }

    getPath() {
        return this.path;
    }

    private getPropName(value) {
        for (var item in this.obj) {
            if (this.obj[item] === value) {
                return item;
            }
        }
    }
}

let test = {
    a: {
        a1: 1,
        a2: 1
    },
    b: {
        b1: 1,
        b2: 2
    }
}

let navigableTest = new NavigableObject(test);

navigableTest.To(m => m.b).To(m => m.b2).getPath(); // = ["b", "b2"]

navigableTest.To(m => m.a).To(m => m.a2).getPath(); // = ["a", "a1"] <-- I selected a2, tho

There is a problem with getPropName method. When obj has two properties with same value, only the first property will be matched.

Does anyone know how to work around this?

What do you want to happen? An array of keys? An error for non-uniqueness? What? — Niet the Dark Absol, May 05 '16 at 10:35
An object's keys should be unique. Can you give an example of how/why you'd have an object with more than key of the same name? — Anthony E, May 05 '16 at 10:35
I mean, an object has two properties with same value (not same key). — Hoang Hiep, May 05 '16 at 10:38
@NiettheDarkAbsol I want to get strong typing when selecting properties and then return full-path for using with [immutable map](http://facebook.github.io/immutable-js/docs/#/Map/setIn). — Hoang Hiep, May 05 '16 at 11:09

score 3 · Accepted Answer · edited May 23 '17 at 11:54

You could use this way of getting the property name:

class NavigableObject<T> {
    constructor(private obj: T, private path: string[] = []) { }

    To<R>(p: (x: T) => R): NavigableObject<R> {
        return new NavigableObject<R>(p(this.obj),
                       this.path.concat(this.getPropName(p)));
    }

    getPath() {
        return this.path;
    }

    private static propertyRegEx = /\.([^\.;]+);?\s*\}$/;

    private getPropName(propertyFunction: Function) {
        return NavigableObject.propertyRegEx.exec(propertyFunction.toString())[1];
    }
}

let test = {
    a: {
        a1: 1,
        a2: 1
    },
    b: {
        b1: 1,
        b2: 2
    }
}

let navigableTest = new NavigableObject(test);

navigableTest.To(m => m.b).To(m => m.b2).getPath(); // = ["b", "b2"]

navigableTest.To(m => m.a).To(m => m.a2).getPath(); // = ["a", "a2"]

score 3 · Answer 2 · answered Jul 06 '17 at 20:40

The future is here. As per Tim Perry's link, TypeScript has now added keyof, which is a great feature for getting the available properties of a class.

Usage, as per the TypeScript documentation:

interface Person {
    name: string;
    age: number;
    location: string;
}

type K1 = keyof Person; // "name" | "age" | "location"
type K2 = keyof Person[];  // "length" | "push" | "pop" | "concat" | ...
type K3 = keyof { [x: string]: Person };  // string

Tim Perry · Answer 3 · 2016-05-05T14:31:15.153

There's nothing easy you can do here with your current querying approach. It's ambiguous which property you're selecting, so there's going to be no easy way to get the right path. It's not that your code is 'wrong' now as such, it's just that there's two possible correct answers, and it's picked one arbitrarily.

You can change the rules on which of the possible keys it picks, but there are no sensible rules that will reliably get you the right single answer. Alternatively you could return all the possible answers, and have an ambiguous path, but it doesn't seem like that does what you're looking for.

There might be one option by doing crazy things like parsing the function provided with Esprima or even regular expressions to work out which properties are being grabbed, but it's generally going to be a bad idea. That's likely to be complex and fragile, unreliable unless you can guarantee the exact shape of the code you'll be provided in To(), and run pretty slowly too.

If you do want to be able to select properties like this and know the path used for certain, you'll have to give your To function the key of the property, not just a function to get the value it holds. It's a less elegant API, but it's the only sensible way you're going to get the behaviour you're looking for:

class NavigableObject<T> {
    constructor(private obj: T, private path: string[] = []) { }

    To<R>(p: string): NavigableObject<R> {
        return new NavigableObject<R>(this.obj[p],
                       this.path.concat(p));
    }

    getPath() {
        return this.path;
    }
}

let test = {
    a: {
        a1: 1,
        a2: 1
    },
    b: {
        b1: 1,
        b2: 2
    }
}

let navigableTest = new NavigableObject(test);

navigableTest.To('b').To('b2').getPath();

navigableTest.To('a').To('a2').getPath();

In future it might be possible to do this with type safety in TypeScript, but not right now. This PR is looking into exactly these issues, but it's still under some discussion yet, so there'll be a while until it's implemented. Note that it's still proposing the string-based approach from the example above, it's just the type system will check that string constants are valid property names for the types used.

score 0 · Answer 4 · answered May 05 '16 at 10:38

Don't terminate the for on the first value found, and return an array of names with matching values. I leave to you the propblem of how to handle multiple names.

private getPropName (value) {
    var items = [];
    for (var item in this.obj) {
        if (this.obj[item] === value)
            items.push (item);
    return items;
}

Typescript. How to get object's property name from its value?

4 Answers4