Does typescript allow interface and string union types?

Question

I'm trying to implement a method like that takes a key argument that is either a string or an instance of the indexable type interface IValidationContextIndex. The implementation looks like this:

  /**
   * Gets all ValidationContext container values.
   * @returns An array of ValidationContext instances contained in the cache.
   */
  public static getValidationContextValues(key: IValidationContextIndex | string ): Array<ValidationContext> {
    if (key instanceof IValidationContextIndex ) [
      return Object.values(<any> key);
    ]
    else {
      const vci = ValidationContainer.cache[<string>key];
      return Object.values(<any> vci);
    }
  }

Typescript give the following error for the if block:

[ts] 'IValidationContextIndex' only refers to a type, but is being used as a value here.

Any ideas on how to fix this?

For most interface I think it's possible to add a type property ( type: 'IValidationContextsIndex'; ), but that does not work in this case since the the interface is an indexable type interface ....

If I do `key: any` and perform an instanceof check I still get the same error ... — Ole, Jun 28 '18 at 03:47
Remember that interfaces and types just disappear once you transpile into javascript. Those are for your code-checking and linting. Not for your end-result. — ippi, Jun 28 '18 at 03:47
Yup I understand that part - basically the interface is gone, so Typescript is like "Dude I got nottin ..." — Ole, Jun 28 '18 at 03:48
Maybe this will help then: https://stackoverflow.com/a/40718205/1497533 — ippi, Jun 28 '18 at 03:50
Just check your first if statement it should be curly braces — Indrakumara, Jun 28 '18 at 03:55

score 2 · Accepted Answer · answered Jun 28 '18 at 03:51

2

There isnt a way to check the type in typescript at run time as almost everything becomes an object once transpiled, so you may have to something along what is defined as user-defined typed guards

answered Jun 28 '18 at 03:51

Jaya

3,721
4
32
48

Ole · Answer 2 · 2018-06-28T04:01:18.783

I think this will do it (Per the tip from @indrakumara):

    /**
     * Gets all ValidationContext container values fpr a 
     * Object_property string valued key or a key that is an instance of 
     * IValidationContextIndex).
     * @returns An array of ValidationContext instances contained in the cache that corresponds to a specific Object_property key.
     */
    public static getValidationContextValues(key: any ): Array<ValidationContext> {
      if (typeof key === `string` ) {
        const vci = ValidationContainer.cache[<string>key];
        return Object.values(<any> vci);
      }
      else {
        return Object.values(<IValidationContextIndex> key);
      }
    }

score 1 · Answer 3 · answered Jun 28 '18 at 03:54

1

Interfaces in typescript doesn't transpile into any code in javascript. So in your code, "instanceof IValidationContextIndex", IValidationContextIndex doesn't exist in javascript.

Change your interface to class or have a class implementing the interface and then check if passed parameter is instanceof that class.

answered Jun 28 '18 at 03:54

Amit Kumar

620
4
17

It was a little easier to just check whether the instance is a string, and if not assume that is an instance of the interface ... So I cheated a little, but it's lightweight and works ...I think your way would be robust when the union type is composed of multiple interfaces ... – Ole Jun 28 '18 at 03:56

Does typescript allow interface and string union types?

3 Answers3