# Python VS JavaScript ES6 syntax comparison

python-es6-comparison.md

Python VS ES6 syntax comparison

Python syntax here : 2.7 - online REPL

Javascript ES6 via Babel transpilation - online REPL

Imports

import math
print math.log(42)

from math import log
print log(42)

# not a good practice (pollutes local scope) :
from math import *
print log(42)

import math from 'math';
console.log(math.log(42));

import { log } from 'math';
console.log(log(42));

import * from 'math';
console.log(log(42));

Range

print range(5)
# 0, 1, 2, 3, 4

console.log(Array.from(new Array(5), (x,i) => i));
// 0, 1, 2, 3, 4

Generators

def foo():
    yield 1
    yield 2
    yield 3

function *foo() {
    yield 1;
    yield 2;
    yield 3;
}

Lambdas

lambda a: a * 2

a => a * 2

Destructuring

status, data = getResult()

var [status, data] = getResult();

Spread

search_db(**parameters)

searchDb(...parameters);

Iterators

def fibonacci():
    pre, cur = 0, 1
    while True:
        pre, cur = cur, pre + cur
        yield cur

for x in fibonacci():
    if (x > 1000):
        break
    print x,

var fibonacci = {
  [Symbol.iterator]: function*() {
    var pre = 0, cur = 1;
    for (;;) {
      var temp = pre;
      pre = cur;
      cur += temp;
      yield cur;
    }
  }
}
for (var n of fibonacci) {
  if (n > 1000)
    break;
  console.log(n);
}

Classes

(Python has builtin support for multiple inheritance)

class SpiderMan(Human, SuperHero):
    def __init__(self, age):
        super(SpiderMan, self).__init__(age)
        self.age = age
    def attack(self):
        print 'launch web'

class SpiderMan extends SuperHero {
    constructor(age) {
        super();
        this.age = age;
    }
    attack() {
        console.log('launch web')
    }
}

Comprehensions

names = [c.name for c in customers if c.admin]

(Experimental in Babel)

var names = [for (c of customers) if (c.admin) c.name];

Map

map(lambda: x*2, [1,2,3,4])

[1,2,3,4].map(x => x*2)

length

 len([])

[].length

What's better in Python

• help(anything) : get docstring for any module/method/function
• list comprehensions, class magic methods !
• very powerful OOP
• huge and coherent standard library, ex : string has 38 useful methods
• built-in strings and array slicing.

What's better in Javascript

• Builtin JSON support
• NPM packaging is a killer-feature : simple and fast, light-years ahead pip+virtualenv.
• Works in the browser :)

var is not part of ES6

Having a lot of pain switching from JS to Python. I'm trying to get used to. For Example: JSON in Js will be Object, Dict in Python and the Dict is not quite good. JSON is actually Javascript Object Notation, so it's Object, why made it Dict

Found a lot of good stuff here. Thanks, guys

Getters and setters

class Foo:
  @property
  def x(self):
    return self.__x
  @x.setter
  def x(self, x):
    self.__x = x

class Foo {
  get x() {
    return this.__x
  }
  set x(x) {
    this.__x = x
  }
}

Anonymous functions

def fn(outer)
  def anonymous(inner):
    print('Hello')
    return outer + inner
  return anonymous

function fn(outer) {
  return inner => {
    console.log('Hello')
    return outer + inner
  }
}

Iterators

iterator = get_iterator()
try:
  while True:
    next = next(iterator)
    print(next)
except StopIteration:
  pass

const iterator = getIterator()
let next = iterator.next()
while (!next.done) {
  console.log(next)
  next = iterator.next()
}

Slices

arr_slice = arr[start:end]
str_slice = str[start:end]

const arrSlice = arr.slice(start, end)
const strSlice = str.substring(start, end)

This gist is a LIFE-SAVER for those of us who are more inclined to JS thanks! awesome!

it's useful for me, thanks

Obrigado mano!

It will help me a lot!

Cheers!!

JS:
let up = (x, y = 12) => x + y
up('WebAlwaysWin') // WORK
Py:
up = lambda x: x + 2
up('ok') #OOOPS

[...Array(5)].map((i,v) => i)
// [0,1,2,3,4,5]

one of my favorite new syntax in JS: Array and Object deconstructing
a =1
b = 2
c = {a,b}
// {a:1, b:2}
d = {...c, e:4}
it also works as function arguments. quite cool. js catching up with python.

I'd argue that Python does a little better than before in terms of package management with stuff like pipenv. Basically local NPM-like management that is super easy.

I've put together something that goes over a more modern feature set of Python 3.7+ and ES2018+

iterator = get_iterator()
try:
  while True:
    next = next(iterator)
    print(next)
except StopIteration:
  pass

No. Instead:

iterator = get_iterator()
for x in iterator:
    print(x)

:)

What I like about JS is the functional part of it. With a lot of iteration methods being a part of a prototype chain, one can nicely chain any iterable processing and easily shuffle between complex types while doing it

let reversed = Object.entries({ a: 1, b: 2 })
    .map(([key, val]) => ({ [val]: key }))
    .reduce((acc, o) => ({ ...acc, ...o }), {})
console.log(reversed)

Also anonymous functions with fat arrow syntax makes it so much nicer to write functional code with, higher order functions and function composition is a joy.

In JS you can do things like bind functions to different contexts, borrow methods from other objects and so on.

I really miss object destructuring in Python. On the other hand, things like comprehensions make up for it a ton :)

Fun fact, both Javascript and Python leverage prototypal inheritance model under the covers, but JS is the one to expose it as public API.

@revolunet could you possibly add ternaries to the comparison? 😉 thanks!

@bhch It only leads to inconsistent code if you aren't using something like ESLint.

In your simple example, yes they all do the same thing, because there's only one parameter and it's only one line. Braces are for code blocks.

// fine and dandy
const foo = (x, y) => x * y;

// Returns undefined because we're in a block now
const foobar = (x, y) => { x * y }

// fine and dandy
const bar = (x, y) => { return x * y }

It's not really an inconsistency. Code inside braces is a sequence of statements.

If you are used to writing functional code in JS with either Ramda or Sanctuary etc, it looks like funcy is a good counterpart in Python. I haven't seen any support for higher kinded types or ADT's but it has some very useful utility functions for composition and partial application.

Great gist!
would add to the spread section a way for python to spread arrays:

Spread

[*array1, *array2]

search_db(**parameters)

[...array1, ...array2]

searchDb(...parameters);

# map

map of python version missed one arg in lambda expression.

Hi bro, does Python have something equivalent to:

let {
  name,
  age,
} = person

No syntax support yet. You can use some workaround, for example:

from operator import itemgetter
dct = dict(name='sam', age=88)
name, age = itemgetter('name', 'age')(dct)

Hi bro, does Python have something equivalent to:

let {
  name,
  age,
} = person

No syntax support yet. You can use some workaround, for example:

from operator import itemgetter
dct = dict(name='sam', age=88)
name, age = itemgetter('name', 'age')(dct)

I feel better using
name, age = (person['name'], person['age'])
as "workaround". Also, the first time I seen that JS syntax I got a lot confused (the thing has got many fields with only 2 extractions), didn't even knew how to search for it lmao

if someday itemgetter gets into builtins then maybe I would start using it, it's a bit more syntactic sugar but the import feels a bit nasty 'cuz it's not like you want to extract a dict like that many times, or at least I don't do it 😁

from operator import itemgetter

That's really a killer feature I miss the most in python. Destructuring data is so conveniant and useful.

I'd to add that also working with object is more handy with nodejs (ES6) than python. In Python you have

• To declare a dict and use quote/double quote to create key value
• You have to use .get method to access a value.. We can easily access key and value in ES6 object.value.
• Destructing object missing (of course)
• Convert json to dict and vice versa take more effort. It's fluent in ES6 and transparent.

Enum is also an overload of no need code. Like if you want to use a value of an enum, you must use the .value of the attribue. in TS we don't need that, just access the Enum.KEY and you have access to the value.

I stop here, there are a lot of stuff I miss a lot from ES6 and a few I like in python