Dynamic Properties - language feature

class Person {}
const person = new Person()
person.age = 50

Example from Python, Cython:

class Person (object):
  def __init__(self, name):
   self.name = name

person = Person("John")
person.age = 50

Languages with Dynamic Properties include JavaScript, Python, TypeScript, Cython, Aardvark

Languages without Dynamic Properties include C3, Speedie

This question asks: Does all properties have to be declared or does the language support dynamic properties?

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