typeIntrospection.md

Type introspection

Inspecting an instance to find out kind of object it is.

Example: instanceof or isPrototypeOf

var Foo = { ... }

var Bar = Object.create(Foo);

var b1 = Object.create(Bar);

Foo.isPrototypeOf(Bar); // true
Object.getPrototypeOf(Bar) === Foo; // true

Foo.isPrototypeOf(b1); //true
Bar.isPrototypeOf(b1); //true

instanceof can be confusingly pretending to have something to do with classes.

Other type of type introspection is called duck typing.

if(a1.something){
  a1.something();
}

Here, we assume that the test for a1.something passing means a1 has the capability to call .something() (regardless of it being found directly on a1 or delegated to some other object).

However, duck typing is sometimes used for other assumptions - ES6 Promises.

The test is done to check if an object happens to have a then() function present on it. If you have any non-Promise object that happens to have a then() on it, keep away from the ES6 Promise mechanism.