Adding property to function with TypeScript

Question

Say I have a function expression called run:

let run = function(){


};

I want to add a property called "default" to this run function, a reference to itself.

run.default = run;

this is to support multiple module export formats in JS.

I hear the best way to do this is with a namespace.

Here is my actual code:

namespace run {

  export interface Run {
    (paths: Array<string>, opts: run.IOpts, cb: Function): void
    default: Run;
  }

  export interface IOpts {
    babelExec?: string,
    all?: boolean
  }
}


  const run : run.Run = function (paths: Array<string>, opts: run.IOpts, cb: Function): void {

    //....

  }


run.default = run;
export = run;

but I get this warning:

Does anyone know why that error message occurs?

As you can see, TS thinks I don't have the property typings for the default property, but I am pretty sure I do with the namespace declaration...or not? So confused by this error - the error/warning seems incorrect.

cartant · Accepted Answer · 2017-04-02 22:20:38Z

2

The error is effected because the function you are assigning to default does not have a default property - it's just a function.

You've declared default as Run, so anything assigned to it will also have to have a default property.

You could either make default optional:

default?: Run;

Or could declare it as a function:

default: (paths: Array<string>, opts: run.IOpts, cb: Function) => void;

answered Apr 2, 2017 at 22:20

cartant

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3 Comments

Alexander Mills Over a year ago

hmm, thanks, but the default property in my code should point to the run function which has a default property, which points to a run function which has the default property. It's just a circular reference, not sure what's wrong with what I have but maybe this solution will work, thanks!

Alexander Mills Over a year ago

I think what you are saying is that the default property is not attached to the run function until after declaration...that must be the problem. This solution seems to work, thanks.

cartant Over a year ago

Yep, you've got it.

Roy Art · Accepted Answer · 2018-06-03 12:11:31Z

2

If you weren't using a reserved word (default) you could do it like:

export function run() {
  // ...
}

export namespace run {
  export const DEFAULT = run;
}

But for what you seemed to be trying to do I think the simplest would be:

export function run() {
  // ...
}

export default run;

answered Jun 3, 2018 at 12:11

Roy Art

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1 Comment

Xunnamius Over a year ago

Thank you from the future! I needed to augment the types of some exported functions and didn't want to convert them to inline constants :)

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