"If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck"
Ha a wikipedian megfogalmazottakbol indulunk ki:
"With normal typing, suitability is assumed to be determined by an object's type only. In duck typing, an object's suitability is determined by the presence of certain methods and properties (with appropriate meaning), rather than the actual type of the object."
Duck typing-gal azt mondjuk, hogy futasidoben ez a fuggveny ilyen es olyan metodusokkal es/vagy propertykkel kell rendelkezzen. JavaScriptben gyakorlatilag ez az egyetlen lehetoseg arra, hogy egy fuggveny eldonthesse, hogy megfelelo objektumot kapta-e.