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
– Ole, Commented Jun 28, 2018 at 3:47
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
– ippi, Commented Jun 28, 2018 at 3:47
Yup I understand that part - basically the interface is gone, so Typescript is like "Dude I got nottin ..." — Ole
– Ole, Commented Jun 28, 2018 at 3:48
Maybe this will help then: stackoverflow.com/a/40718205/1497533 — ippi
– ippi, Commented Jun 28, 2018 at 3:50

Jaya · Accepted Answer · 2018-06-28 03:51:40Z

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, 2018 at 3:51

Jaya

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Comments

Ole · Accepted Answer · 2018-06-28 04:01:18Z

2

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);
      }
    }

edited Jun 28, 2018 at 4:01

answered Jun 28, 2018 at 3:53

Ole

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Comments

Amit Kumar · Accepted Answer · 2018-06-28 03:54:45Z

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, 2018 at 3:54

Amit Kumar

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1 Comment

Ole Over a year ago

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 ...

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