Runtime type checking of constructor options that get passed to super

Question

I have this:

export class XMLParser extends Transform {

  constructor(opts?: XMLParserOpts, to?: TransformOptions) {

    super(Object.assign({}, to || {}, {objectMode: true}));

  }

}

basically I always need objectMode to be true. However, what if the user passed in some weird value for to? How can I check it to make sure the type is defined an object?

I can let the Transform class do its validation, but I'd rather do that manually and give a custom error message. But the problem is, I can't do this:

   constructor(opts?: XMLParserOpts, to?: TransformOptions) {

        if(to && typeof to !== 'object'){
          throw new Error('no good bozo');
        }

        super(Object.assign({}, to || {}, {objectMode: true}));

    }

how can do what I am looking for? (TS won't compile if super is not the first call in the constructor).

What are the reasons for this 'problem' and why you can't do that? Because this is how it's usually done. — Estus Flask, May 09 '18 at 19:20
TS won't compile if super is not the first call in the constructor — Alexander Mills, May 09 '18 at 19:21
Of course not, this would make the life unbearable. It won't compile if you use this prior to super. — Estus Flask, May 09 '18 at 19:23

score 0 · Answer 1 · answered May 10 '18 at 00:50

I guess the best thing to do is something like:

  constructor(opts?: XMLParserOpts, to?: TransformOptions) {

        super((function(){

          if(to && typeof to !== 'object'){
              throw new Error('no good bozo');
           }

           return Object.assign({}, to || {}, {objectMode: true})

        })());

    }

use cases for IIFEs really do pop up don't they.

Runtime type checking of constructor options that get passed to super

1 Answers1