Use Typescript for API enforcement

Question

I have a Node.js module that just looks like this:

module.exports = function(data){

    return {

      limit: 4,
      values: {}

    }


};

Using TypeScript, that might look like:

interface ISomethingA {
    values: Object,
    limit?: number
}

export = function(data: ISomethingB){

    return {

      limit: 4,
      values: {}

    } as ISomethingA;


};

These modules have to adhere to a certain API - the object returned from the function needs both "values" and a "limit" properties.

What TypeScript constructs can I use here to give users feedback so that they know they are adhering to the API?

The "as" syntax so far hasn't been working for me as I would have expected. I am looking for a way to define the type for the object that is returned from the function.

Robby Cornelissen · Accepted Answer · 2017-04-04 05:33:16Z

3

Specify the return type in the function declaration:

export = function(data: ISomethingB): ISomethingA {
    return {
      limit: 4,
      values: {}
    };
};

answered Apr 4, 2017 at 5:33

Robby Cornelissen

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2 Comments

Alexander Mills Over a year ago

ah yes, thanks, I really like the idea of the "as" notation, but I cannot see any good use cases for it

unional Over a year ago

The as notation is telling the compiler that "hey, I know you think that object is of type X, but I want you to treat it as type Y". That's why you can use "as" for this purpose.

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