Typescript string dot notation of nested object

UPDATE for TS4.1. String concatenation can now be represented at the type level through template string types, implemented in microsoft/TypeScript#40336. Now you can take an object and get its dotted paths right in the type system.

Imagine languageObject is this:

const languageObject = {
    viewName: {
        componentName: {
            title: 'translated title'
        }
    },
    anotherName: "thisString",
    somethingElse: {
        foo: { bar: { baz: 123, qux: "456" } }
    }
}

First we can use recursive conditional types as implemented in microsoft/TypeScript#40002 and variadic tuple types as implemented in microsoft/TypeScript#39094 to turn an object type into a union of tuples of keys corresponding to its string-valued properties:

type PathsToStringProps<T> = T extends string ? [] : {
    [K in Extract<keyof T, string>]: [K, ...PathsToStringProps<T[K]>]
}[Extract<keyof T, string>];

And then we can use template string types to join a tuple of string literals into a dotted path (or any delimiter D🙂

type Join<T extends string[], D extends string> =
    T extends [] ? never :
    T extends [infer F] ? F :
    T extends [infer F, ...infer R] ?
    F extends string ? 
    `${F}${D}${Join<Extract<R, string[]>, D>}` : never : string;    

Combining those, we get:

type DottedLanguageObjectStringPaths = Join<PathsToStringProps<typeof languageObject>, ".">
/* type DottedLanguageObjectStringPaths = "anotherName" | "viewName.componentName.title" | 
      "somethingElse.foo.bar.qux" */

which can then be used inside the signature for translate():

declare function translate(dottedString: DottedLanguageObjectStringPaths): string;

And we get the magical behavior I was talking about three years ago:

translate('viewName.componentName.title'); // okay
translate('view.componentName.title'); // error
translate('viewName.component.title'); // error
translate('viewName.componentName'); // error

Amazing!

Playground link to code

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Pre-TS4.1 answer:

If you want TypeScript to help you, you have to help TypeScript. It doesn’t know anything about the types of concatenated string literals, so that won’t work. My suggestion for how to help TypeScript might be more work than you’d like, but it does lead to some fairly decent type safety guarantees:

---

First, I’m going to assume you have a languageObject and a translate() function that knows about it (meaning that languageObject was presumably used to produce the particular translate() function). The translate() function expects a dotted string representing list of keys of nested properties where the last such property is string-valued.

const languageObject = {
  viewName: {
    componentName: {
      title: 'translated title'
    }
  }
}
// knows about languageObject somehow
declare function translate(dottedString: string): string;
translate('viewName.componentName.title'); // good
translate('view.componentName.title'); // bad first component
translate('viewName.component.title'); // bad second component
translate('viewName.componentName'); // bad, not a string

Introducing the Translator<T> class. You create one by giving it an object and a translate() function for that object, and you call its get() method in a chain to drill down into the keys. The current value of T always points to the type of property you’ve selected via the chain of get() methods. Finally, you call translate() when you’ve reached the string value you care about.

class Translator<T> {
  constructor(public object: T, public translator: (dottedString: string)=>string, public dottedString: string="") {}

  get<K extends keyof T>(k: K): Translator<T[K]> {    
    const prefix = this.dottedString ? this.dottedString+"." : ""
    return new Translator(this.object[k], this.translator, prefix+k);
  }

  // can only call translate() if T is a string
  translate(this: Translator<string>): string {
    if (typeof this.object !== 'string') {
      throw new Error("You are translating something that isn't a string, silly");
    }
    // now we know that T is string
    console.log("Calling translator on \"" + this.dottedString + "\"");
    return this.translator(this.dottedString);
  }
}
    

Initialize it with languageObject and the translate() function:

const translator = new Translator(languageObject, translate);

And use it. This works, as desired:

const translatedTitle = translator.get("viewName").get("componentName").get("title").translate();
// logs: calling translate() on "viewName.componentName.title"

And these all produce compiler errors, as desired:

const badFirstComponent = translator.get("view").get("componentName").get("title").translate(); 
const badSecondComponent = translator.get("viewName").get("component").get("title").translate(); 
const notAString = translator.get("viewName").translate();

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Hope that helps. Good luck!