How to solve Type error in an ionic framework

well to me this confirms what I first said. you are indeed pointing to a child of an object within an observable (whether you know what observables are or not) (observables sometimes move all the way up the priority list and happen before hydration of any other var).

so here's the precautions I like to take to make sure things don't go badly: splitting things up.

Also, here's a "hack" I like to use on a general basis in order to ensure I can continue my dev cycle unhindered and have a better notion of where things are failing other than just "undefined":

this.category['slug'];

I never go this far but this is also allowed : this['category']['slug'];

see this is different because it will actually return the undefined instead of failing there at a JS level.

now to me it seems pretty obvious that you need to ascertain this.category.slug in the instant it's being evaluated isn't in a state in it's lifeline prior to it's initialization :

var myFunctionToLogSlug = () => { //here you could pass this.category.slug as an arg but
//that's the same as simply calling in within the body of this method because your
//context (this) is the same, ergo the pointer and timeframe are also the same.
  console.log("is this.category.slug empty ? : ", this.category.slug);
  return this.category.slug;
};

this.WooCommerce.getAsync(
   "products?filter[category]=" + myFunctionToLogSlug()).then(data =>{
       console.log(JSON.parse(data.body));
});

once you've ascertained this.category.slug is indeed undefined at the point it is called, you can either move this.WooCommerce.getAsync to a time-place you know this.category.slug to be defined, or move this.category.slug's hydration to the inside of the getAsync.

hope this helps ! :)