Python vs ECMAScript default params semantics

1-python-defaults.py

# Python
#
# Closures when they are used as default params, capture bindings
# from the environment one level above the class.
#

x = 1 # global env

def wrap():

  x = 2 # env of wrap

  class A(object):
    x = 3 # env of class
    def foo(self, x = 4, y = lambda: x): # no env of params (!)
      x = 5 # env of foo
      return y() # 2, from env of wrap


  print(A().foo()) # 2
  print(A.x, A().x) # 3, 3

wrap()

2-ecmascript-defaults.js

// JavaScript
//
// Closures capture bindings from the scope of parameters
// if such name exists in the parameters list.

var x = 1; // global env

function wrap() {

  var x = 2; // env of wrap

  class A {
    // x = 3, no yet env of class in ES2015
    foo(x = 4, y = () => x) { // env of params
      var x = 5; // env of foo
      return y(); // 4, from env of params
    }
  }

  console.log(new A().foo()); // 4
}

wrap();