How to write X in both Python 3 and JavaScript (ES2015)

Python and JavaScript are two of the most popular programming languages. If you're are Python programmer learning JavaScript, or a JavaScript programmer learning Python, this handy cheat sheet might help you.

This page is open sourced on GitHub. Pull requests are welcome!

About the Author

I'm Saya, a jewelry designer and a junior JavaScript/Python developer living in Tokyo. Find me on Twitter: @sayajewels

See also: Ilya Schurov has made a similar page - check it out!

Table of Contents

Math: Absolute Value

Python 3 / abs

# 100
print(abs(-100))
# 50
print(abs(50))

JavaScript (ES2015) / abs

// 100
console.log(Math.abs(-100))
// 50
console.log(Math.abs(50))

Math: Round Numbers

Python 3 / ceil, floor, round

import math

# 2
print(math.ceil(1.5))
# 1
print(math.floor(1.5))
# 2
print(round(1.5))

JavaScript (ES2015) / ceil, floor, round

// 2
console.log(Math.ceil(1.5))
// 1
console.log(Math.floor(1.5))
// 2
console.log(Math.round(1.5))

Math: Max and Min

Python 3 / max, min

# 100
print(max(100, 50))
# 40
print(min(80, 40))

JavaScript (ES2015) / max, min

// 100
console.log(Math.max(100, 50))

// 40
console.log(Math.min(80, 40))

Control Flow: If else

Python 3 / if

some_number = 3

# Number is 3
if some_number == 1:
    print("Number is 1")
elif some_number == 2:
    print("Number is 2")
elif some_number == 3:
    print("Number is 3")
else:
    print("?")

JavaScript (ES2015) / if

const someNumber = 3

// Number is 3
if (someNumber === 1) {
  console.log('Number is 1')
} else if (someNumber === 2) {
  console.log('Number is 2')
} else if (someNumber === 3) {
  console.log('Number is 3')
} else {
  console.log('?')
}

Control Flow: Ternary

Python 3

x = 2
y = 3

# even
print("even" if x % 2 == 0 else "odd")
# odd
print("even" if y % 2 == 0 else "odd")

JavaScript (ES2015) / Conditional Operator

const x = 2
const y = 3

// even
console.log(x % 2 === 0 ? 'even' : 'odd')
// odd
console.log(y % 2 === 0 ? 'even' : 'odd')

Control Flow: Boolean

Python 3

# yes
if "abc" == "abc":
    print("yes")
else:
    print("no")

# yes
if "abc" != "def":
    print("yes")
else:
    print("no")

# no
if True and False:
    print("yes")
else:
    print("no")

# yes
if True or False:
    print("yes")
else:
    print("no")

# yes
if not False:
    print("yes")
else:
    print("no")

# no
if 0:
    print("yes")
else:
    print("no")

# no
if "":
    print("yes")
else:
    print("no")

# no
if []:
    print("yes")
else:
    print("no")

# no
if None:
    print("yes")
else:
    print("no")

# yes
if not not not None:
    print("yes")
else:
    print("no")

JavaScript (ES2015) / Falsy Values

// yes
if ('abc' === 'abc') {
  console.log('yes')
} else {
  console.log('no')
}

// yes
if ('abc' !== 'def') {
  console.log('yes')
} else {
  console.log('no')
}

// no
if (true && false) {
  console.log('yes')
} else {
  console.log('no')
}

// yes
if (true || false) {
  console.log('yes')
} else {
  console.log('no')
}

// yes
if (!false) {
  console.log('yes')
} else {
  console.log('no')
}

// no
if (0) {
  console.log('yes')
} else {
  console.log('no')
}

// no
if ('') {
  console.log('yes')
} else {
  console.log('no')
}

// no
if (undefined) {
  console.log('yes')
} else {
  console.log('no')
}

// no
if (null) {
  console.log('yes')
} else {
  console.log('no')
}

// yes
if (!!!null) {
  console.log('yes')
} else {
  console.log('no')
}

Control Flow: Exceptions

Python 3 / Errors

# foo is not defined
try:
    foo()
except NameError:
    print("foo is not defined")

JavaScript (ES2015) / try...catch

// foo is not defined
try {
  foo()
} catch (error) {
  console.log('foo is not defined')
}

Control Flow: Continue / Break

Python 3 / break/continue

# 1
# 2
# Fizz
# 4
# Buzz
for number in range(1, 101):
    if number == 3:
        print("Fizz")
        continue
    if number == 5:
        print("Buzz")
        break
    print(number)

JavaScript (ES2015) / continue, break

// 1
// 2
// Fizz
// 4
// Buzz
for (let i = 1; i <= 100; i = i + 1) {
  if (i === 3) {
    console.log('Fizz')
    continue
  }

  if (i === 5) {
    console.log('Buzz')
    break
  }

  console.log(i)
}

String: Length

Python 3 / len

some_string = "abcd"

# 4
print(len(some_string))

JavaScript (ES2015) / length

const someString = 'abcd'

// 4
console.log(someString.length)

String: Interpolation

Python 3

x = "Hello"

# Hello World
print(f"{x} World")

JavaScript (ES2015)

const x = 'Hello'

// Hello World
console.log(`${x} World`)

String: Multiline

Python 3

x = """------
Line 1
Line 2
Line 3
------"""

# ------
# Line 1
# Line 2
# Line 3
# ------
print(x)

JavaScript (ES2015) / Multiline Strings

const x = `------
Line 1
Line 2
Line 3
------`

// ------
// Line 1
// Line 2
// Line 3
// ------
console.log(x)

String: String to Integer

Python 3 / int

string_1 = "1"
number_1 = int(string_1)

# 3
print(number_1 + 2)

JavaScript (ES2015) / parseInt

const string1 = '1'
const number1 = parseInt(string1)

// 3
console.log(number1 + 2)

String: Contains

Python 3 / in

# 2 is in the string
if "2" in "123":
    print("2 is in the string")

# 2 is not in the string
if "2" not in "456":
    print("2 is not in the string")

JavaScript (ES2015) / includes

// 2 is in the string
if ('123'.includes('2')) {
  console.log('2 is in the string')
}

// 2 is not in the string
if (!'456'.includes('2')) {
  console.log('2 is not in the string')
}

String: Substring

Python 3 / [i:j]

some_string = "0123456"

# 234
print(some_string[2:5])

JavaScript (ES2015) / substring

const someString = '0123456'

// 234
console.log(someString.substring(2, 5))

String: Join

Python 3 / join

some_list = ["a", "b", "c"]

# a,b,c
print(",".join(some_list))

JavaScript (ES2015) / join

const someList = ['a', 'b', 'c']

// a,b,c
console.log(someList.join(','))

String: Strip

Python 3 / strip

some_string = "   abc   "

# abc
print(some_string.strip())

JavaScript (ES2015) / trim

const someString = '   abc   '

// abc
console.log(someString.trim())

String: Split

Python 3 / split

some_string = "a,b,c"
some_string_split = some_string.split(",")

# a
print(some_string_split[0])
# b
print(some_string_split[1])
# c
print(some_string_split[2])

JavaScript (ES2015) / split

const someString = 'a,b,c'
const someStringSplit = someString.split(',')

// a
console.log(someStringSplit[0])
// b
console.log(someStringSplit[1])
// c
console.log(someStringSplit[2])

String: Replace

Python 3 / replace

some_string = "a b c d e"

# a,b,c,d,e
print(some_string.replace(" ", ","))

JavaScript (ES2015) / Regular Expressions, replace

const someString = 'a b c d e'

// Only changes the first space
// a,b c d e
// console.log(someString.replace(' ', ','))

// Use / /g instead of ' ' to change every space
console.log(someString.replace(/ /g, ','))

String: Search

Python 3 / search

import re

# Has a number
if re.search(r"\d", "iphone 8"):
    print("Has a number")

# Doesn't have a number
if not re.search(r"\d", "iphone x"):
    print("Doesn't have a number")

JavaScript (ES2015) / Regular Expressions, match

// Has a number
if ('iphone 8'.match(/\d/g)) {
  console.log('Has a number')
}

// Doesn't have a number
if (!'iphone x'.match(/\d/g)) {
  console.log("Doesn't have a number")
}

List/Array: Iteration

Python 3

some_list = [6, 3, 5]

# 6
# 3
# 5
for item in some_list:
    print(item)

JavaScript (ES2015) / forEach

const someList = [6, 3, 5]

// 6
// 3
// 5
someList.forEach(element => {
  console.log(element)
})

List/Array: Length

Python 3 / len

some_list = [1, 4, 9]

# 3
print(len(some_list))

JavaScript (ES2015) / length

const someList = [1, 4, 9]

// 3
console.log(someList.length)

List/Array: Enumerate

Python 3 / enumerate, list

some_list = [6, 3, 5]

# 0 6
# 1 3
# 2 5
for i, item in enumerate(some_list):
    print(f"{i} {item}")

# If you're not using this in a for loop, use list()
# list(enumerate(some_list)) # [(0, 6), (1, 3), (2, 5)]

JavaScript (ES2015) / forEach, map

const someList = [6, 3, 5]

// 0 6
// 1 3
// 2 5

someList.forEach((element, index) => {
  console.log(`${index} ${element}`)
})

List/Array: Contains

Python 3 / in

# 2 is in the list
if 2 in [1, 2, 3]:
    print("2 is in the list")

# 2 is not in the list
if 2 not in [4, 5, 6]:
    print("2 is not in the list")

JavaScript (ES2015) / includes

// 2 is in the list
if ([1, 2, 3].includes(2)) {
  console.log('2 is in the list')
}

// 2 is not in the list
if (![4, 5, 6].includes(2)) {
  console.log('2 is not in the list')
}

List/Array: Reverse

Python 3 / reversed, list

some_list = [1, 2, 3, 4]

# reversed(some_list) is just an iterable.
# To convert an iterable into a list, use list()
reversed_list = list(reversed(some_list))

# 4
# 3
# 2
# 1
for item in reversed_list:
    print(item)

# You can use an iterable instead of a list in a for loop
# for item in reversed(some_list):

JavaScript (ES2015) / reverse

const someList = [1, 2, 3, 4]

someList.reverse()

// 4
// 3
// 2
// 1
someList.forEach(element => {
  console.log(element)
})

List/Array: Range Iteration

Python 3 / range

# 0
# 1
# 2
# 3
for i in range(4):
    print(i)

# 4
# 5
# 6
# 7
for i in range(4, 8):
    print(i)

# 6
# 5
# 4
for i in range(6, 3, -1):
    print(i)

JavaScript (ES2015)

// 0
// 1
// 2
// 3
for (let i = 0; i < 4; i++) {
  console.log(i)
}

// 4
// 5
// 6
// 7
for (let i = 4; i < 8; i++) {
  console.log(i)
}

// 6
// 5
// 4
for (let i = 6; i > 3; i--) {
  console.log(i)
}

List/Array: Append with Modification

Python 3 / append

some_list = [1, 2]

some_list.append(3)

# 1
# 2
# 3
for x in some_list:
    print(x)

JavaScript (ES2015) / push

const someList = [1, 2]

someList.push(3)

// 1
// 2
// 3
someList.forEach(element => {
  console.log(element)
})

List/Array: Append without Modification

Python 3 / Unpacking

original_list = [1, 2]

# [original_list, 3] -> [[1, 2], 3]
# [*original_list, 3] -> [1, 2, 3]
new_list = [*original_list, 3]
original_list[0] = 5

# 1
# 2
# 3
for x in new_list:
    print(x)

JavaScript (ES2015) / Spread

const originalList = [1, 2]

// [originalList, 3] -> [[1, 2], 3]
// [...originalList, 3] -> [1, 2, 3]
const newList = [...originalList, 3]
originalList[0] = 5

// 1
// 2
// 3
newList.forEach(element => {
  console.log(element)
})

List/Array: Extend with Modification

Python 3 / extend

some_list = [1]

some_list.extend([2, 3])

# 1
# 2
# 3
for x in some_list:
    print(x)

JavaScript (ES2015) / Spread

const someList = [1]

someList.push(...[2, 3])

// 1
// 2
// 3
someList.forEach(element => {
  console.log(element)
})

List/Array: Extend without Modification

Python 3 / List addition

original_list = [1]
new_list = original_list + [2, 3]
original_list[0] = 5

# 1
# 2
# 3
for x in new_list:
    print(x)

JavaScript (ES2015) / concat

const originalList = [1]
const newList = originalList.concat([2, 3])
originalList[0] = 5

// 1
// 2
// 3
newList.forEach(element => {
  console.log(element)
})

List/Array: Prepend

Python 3 / insert

some_list = [4, 5]

some_list.insert(0, 3)

# 3
# 4
# 5
for x in some_list:
    print(x)

JavaScript (ES2015) / unshift

const someList = [4, 5]
someList.unshift(3)

// 3
// 4
// 5
someList.forEach(element => {
  console.log(element)
})

List/Array: Remove

Python 3 / del

some_list = ["a", "b", "c"]
del some_list[1]

# a
# c
for x in some_list:
    print(x)

JavaScript (ES2015) / splice

const someList = ['a', 'b', 'c']
someList.splice(1, 1)

// a
// c
someList.forEach(element => {
  console.log(element)
})

List/Array: Pop

Python 3 / pop

some_list = [1, 2, 3, 4]

# 4
print(some_list.pop())
# 1
print(some_list.pop(0))

# 2
# 3
for x in some_list:
    print(x)

JavaScript (ES2015) / pop, shift

const someList = [1, 2, 3, 4]

// 4
console.log(someList.pop())

// 1
console.log(someList.shift())

// 2
// 3
someList.forEach(element => {
  console.log(element)
})

List/Array: Index

Python 3 / index

some_list = ["a", "b", "c", "d", "e"]

# 2
print(some_list.index("c"))

JavaScript (ES2015) / indexOf

const someList = ['a', 'b', 'c', 'd', 'e']

// 2
console.log(someList.indexOf('c'))

List/Array: Copy

Python 3 / [i:j]

original_list = [1, 2, 3]
new_list = original_list[:]  # or original_list.copy()

original_list[2] = 4

# 1
# 2
# 3
for x in new_list:
    print(x)

JavaScript (ES2015) / Spread

const originalList = [1, 2, 3]
const newList = [...originalList]

originalList[2] = 4

// 1
// 2
// 3
newList.forEach(element => {
  console.log(element)
})

List/Array: Map

Python 3 / List Comprehensions

original_list = [1, 2, 3]
new_list = [x * 2 for x in original_list]

# 2
# 4
# 6
for x in new_list:
    print(x)

JavaScript (ES2015) / map

const originalList = [1, 2, 3]

// You can also do this:
// const newList = originalList.map(x => { return x * 2 })
const newList = originalList.map(x => x * 2)

// 2
// 4
// 6
newList.forEach(element => {
  console.log(element)
})

List/Array: Map (Nested)

Python 3 / List Comprehensions

first_list = [1, 3]
second_list = [3, 4]
combined_list = [[x + y for y in second_list] for x in first_list]

# 4
print(combined_list[0][0])
# 5
print(combined_list[0][1])
# 6
print(combined_list[1][0])
# 7
print(combined_list[1][1])

JavaScript (ES2015) / map

const firstList = [1, 3]
const secondList = [3, 4]

const conbinedList = firstList.map(x => {
  return secondList.map(y => {
    return x + y
  })
})

// 4
console.log(conbinedList[0][0])
// 5
console.log(conbinedList[0][1])
// 6
console.log(conbinedList[1][0])
// 7
console.log(conbinedList[1][1])

List/Array: Filter

Python 3 / List Comprehensions

original_list = [1, 2, 3, 4, 5, 6]
new_list = [x for x in original_list if x % 2 == 0]

# 2
# 4
# 6
for x in new_list:
    print(x)

JavaScript (ES2015) / filter

const originalList = [1, 2, 3, 4, 5, 6]
const newList = originalList.filter(x => x % 2 == 0)

// 2
// 4
// 6
newList.forEach(element => {
  console.log(element)
})

List/Array: Sum

Python 3 / sum

some_list = [1, 2, 3]

# 6
print(sum(some_list))

JavaScript (ES2015) / reduce

const someList = [1, 2, 3]
const reducer = (accumulator, currentValue) => accumulator + currentValue

// 6
console.log(someList.reduce(reducer))

List/Array: Zip

Python 3 / zip

list_1 = [1, 3, 5]
list_2 = [2, 4, 6]

# 1 2
# 3 4
# 5 6
for x, y in zip(list_1, list_2):
    print(f"{x} {y}")

JavaScript (ES2015) / map

const list1 = [1, 3, 5]
const list2 = [2, 4, 6]

// [[1, 2], [3, 4], [5, 6]]
const zippedList = list1.map((x, y) => {
  return [x, list2[y]]
})

zippedList.forEach(element => {
  console.log(`${element[0]} ${element[1]}`)
})

List/Array: Slice

Python 3 / [i:j]

original_list = ["a", "b", "c", "d", "e"]
new_list = original_list[1:3]
original_list[1] = "x"

# b
# c
for x in new_list:
    print(x)

JavaScript (ES2015) / slice

const originalList = ['a', 'b', 'c', 'd', 'e']
const newList = originalList.slice(1, 3)
originalList[1] = 'x'

// b
// c
newList.forEach(element => {
  console.log(element)
})

List/Array: Sort

Python 3 / sorted

some_list = [4, 2, 1, 3]

# 1
# 2
# 3
# 4
for item in sorted(some_list):
    print(item)

JavaScript (ES2015) / sort

const someList = [4, 2, 1, 3]

// 1
// 2
// 3
// 4
someList.sort().forEach(element => {
  console.log(element)
})

List/Array: Sort Custom

Python 3 / sorted

some_list = [["c", 2], ["b", 3], ["a", 1]]

# a 1
# c 2
# b 3
for item in sorted(some_list, key=lambda x: x[1]):
    print(f"{item[0]} {item[1]}")

JavaScript (ES2015) / sort

const someList = [['c', 2], ['b', 3], ['a', 1]]

// a 1
// c 2
// b 3
someList
  .sort((a, b) => {
    return a[1] - b[1]
  })
  .forEach(element => {
    console.log(`${element[0]} ${element[1]}`)
  })

Dictionary/Object: Iteration

Python 3 / items

some_dict = {"one": 1, "two": 2, "three": 3}

# one 1
# two 2
# three 3
# NOTE: If you're not using this in a for loop,
# convert it into a list: list(some_dict.items())
for key, value in some_dict.items():
    print(f"{key} {value}")

JavaScript (ES2015) / entries

const someDict = { one: 1, two: 2, three: 3 }

// one 1
// two 2
// three 3

Object.entries(someDict).forEach(element => {
  console.log(`${element[0]} ${element[1]}`)
})

Dictionary/Object: Contains

Python 3 / in

some_dict = {"one": 1, "two": 2, "three": 3}

# one is in the dict
if "one" in some_dict:
    print("one is in the dict")

# four is not in the dict
if "four" not in some_dict:
    print("four is not in the dict")

JavaScript (ES2015) / hasOwnProperty

const someDict = { one: 1, two: 2, three: 3 }

// one is in the dict
if (someDict.hasOwnProperty('one')) {
  console.log('one is in the dict')
}

// four is not in the dict
if (!someDict.hasOwnProperty('four')) {
  console.log('four is not in the dict')
}

Dictionary/Object: Add with Modification

Python 3

original_dict = {"one": 1, "two": 2}
original_dict["three"] = 3

# one 1
# two 2
# three 3
for key, value in original_dict.items():
    print(f"{key} {value}")

JavaScript (ES2015)

const originalDict = { one: 1, two: 2 }
originalDict.three = 3

// one 1
// two 2
// three 3
Object.entries(originalDict).forEach(element => {
  console.log(`${element[0]} ${element[1]}`)
})

Dictionary/Object: Add without Modification

Python 3 / Unpacking

original_dict = {"one": 1, "two": 2}
new_dict = {**original_dict, "three": 3}
original_dict["one"] = 100

# one 1
# two 2
# three 3
for key, value in new_dict.items():
    print(f"{key} {value}")

JavaScript (ES2015) / Spread

const originalDict = { one: 1, two: 2 }
const newDict = { ...originalDict, three: 3 }
originalDict.one = 100

// one 1
// two 2
// three 3
Object.entries(newDict).forEach(element => {
  console.log(`${element[0]} ${element[1]}`)
})

Dictionary/Object: Keys

Python 3 / keys

some_dict = {"one": 1, "two": 2, "three": 3}

# one
# two
# three
# NOTE: If you're not using this in a for loop,
# convert it into a list: list(some_dict.keys())
for x in some_dict.keys():
    print(x)

JavaScript (ES2015) / keys

const someDict = { one: 1, two: 2, three: 3 }

// one
// two
// three
Object.keys(someDict).forEach(element => {
  console.log(element)
})

Dictionary/Object: Values

Python 3 / values

some_dict = {"one": 1, "two": 2, "three": 3}


# 1
# 2
# 3
# NOTE: If you're not using this in a for loop,
# convert it into a list: list(some_dict.values())
for x in some_dict.values():
    print(x)

JavaScript (ES2015) / values

const someDict = { one: 1, two: 2, three: 3 }

// 1
// 2
// 3
Object.values(someDict).forEach(element => {
  console.log(element)
})

Dictionary/Object: Remove

Python 3 / del

some_dict = {"one": 1, "two": 2, "three": 3}

del some_dict["two"]

# one 1
# three 3
for key, value in some_dict.items():
    print(f"{key} {value}")

JavaScript (ES2015) / delete

const someDict = { one: 1, two: 2, three: 3 }

delete someDict.two

Object.entries(someDict).forEach(element => {
  console.log(`${element[0]} ${element[1]}`)
})

Dictionary/Object: Get

Python 3 / get

some_dict = {"one": 1, "two": 2, "three": 3}

# some_dict["four"] will be KeyError

# Doesn't exist
print(some_dict.get("five", "Doesn't exist"))

JavaScript (ES2015) / ||

const someDict = { one: 1, two: 2, three: 3 }

// Doesn't exist
console.log(someDict.five || "Doesn't exist")

Dictionary/Object: Map

Python 3 / Dictionary Comprehensions

original_list = {"one": 1, "two": 2}

# {"one": 1, "two": 4}
new_list = {key: value * value for key, value in original_list.items()}

# one 1
# two 4
for key, value in new_list.items():
    print(f"{key} {value}")

JavaScript (ES2015) / entries

const originalDict = { one: 1, two: 2 }
const newDict = {}

Object.entries(originalDict).forEach(element => {
  // newDict.element[0] doesn't work - for variable key use []
  newDict[element[0]] = element[1] * element[1]
})

// one 1
// two 4
Object.entries(newDict).forEach(element => {
  console.log(`${element[0]} ${element[1]}`)
})

Dictionary/Object: Copy

Python 3 / copy

original_dict = {"one": 1, "two": 2, "three": 3}
new_dict = original_dict.copy()
original_dict["one"] = 100

# one 1
# two 2
# three 3
for key, value in new_dict.items():
    print(f"{key} {value}")

JavaScript (ES2015) / Spread

const originalDict = { one: 1, two: 2, three: 3 }
const newDict = { ...originalDict }
originalDict.one = 100

// one 1
// two 2
// three 3
Object.entries(newDict).forEach(element => {
  console.log(`${element[0]} ${element[1]}`)
})

Dictionary/Object: Dictionary/Object to JSON string

Python 3 / json.dumps

import json

some_dict = {"one": 1, "two": 2, "three": 3}

# {
#   "one": 1,
#   "two": 2,
#   "three": 3
# }
print(json.dumps(some_dict, indent=2))

JavaScript (ES2015) / JSON.stringify

const someDict = { one: 1, two: 2, three: 3 }

// {
//   "one": 1,
//   "two": 2,
//   "three": 3
// }
console.log(JSON.stringify(someDict, null, 2))

Dictionary/Object: JSON string to Dictionary/Object

Python 3 / json.loads

import json

some_json = """{
  "one": 1,
  "two": 2,
  "three": 3
}"""

dict_from_json = json.loads(some_json)

# 2
print(dict_from_json["two"])

JavaScript (ES2015) / JSON.parse

const someJson = `{
  "one": 1,
  "two": 2,
  "three": 3
}`

const dictFromJson = JSON.parse(someJson)

// 2
console.log(dictFromJson.two)

Dictionary/Object: Variable Key

Python 3

some_variable = 2
some_dict = {(some_variable + 1): "three"}

# three
print(some_dict[3])

JavaScript (ES2015) / Computed Property Names

const someVariable = 2
const someDict = { [someVariable + 1]: 'three' }

// three
console.log(someDict[3])

Function: Definitions

Python 3

def add(x, y):
    print(f"Adding {x} and {y}")
    return x + y


# Adding 1 and 2
# 3
print(add(1, 2))

JavaScript (ES2015)

const add = (x, y) => {
  console.log(`Adding ${x} and ${y}`)
  return x + y
}
// Adding 1 and 2
// 3
console.log(add(1, 2))

Function: Keyword Arguments

Python 3

def add(a, b, c):
    return a + b + c


# 6
print(add(b=1, a=2, c=3))

JavaScript (ES2015)

const add = ({ a, b, c }) => {
  return a + b + c
}

// 6
console.log(
  add({
    b: 1,
    a: 2,
    c: 3
  })
)

Function: Default Arguments

Python 3

def greet(name, word="Hello"):
    print(f"{word} {name}")


# Hello World
greet("World")

# Goodbye World
greet("World", "Goodbye")

JavaScript (ES2015) / Default Parameters

const greet = (name, word = 'Hello') => {
  console.log(`${word} ${name}`)
}

// Hello World
greet('World')

// Goodbye World
greet('World', 'Goodbye')

Function: Default Keyword Arguments

Python 3

def greet(name="World", word="Hello"):
    print(f"{word} {name}")


# Goodbye World
greet(word="Goodbye")

# Hello Programming
greet(name="Programming")

JavaScript (ES2015) / Default Value

const greet = ({ name = 'World', word = 'Hello' }) => {
  console.log(`${word} ${name}`)
}

// Goodbye World
greet({ word: 'Goodbye' })

// Hello Programming
greet({ name: 'Programming' })

Function: Positional Arguments

Python 3 / Arbitrary Argument Lists

def positional_args(a, b, *args):
    print(a)
    print(b)
    for x in args:
        print(x)


# 1
# 2
# 3
# 4
positional_args(1, 2, 3, 4)

JavaScript (ES2015) / Rest Parameters

const positionalArgs = (a, b, ...args) => {
  console.log(a)
  console.log(b)
  args.forEach(element => {
    console.log(element)
  })
}

// 1
// 2
// 3
// 4
positionalArgs(1, 2, 3, 4)

Function: Variable Keyword Arguments

Python 3 / Keyword Arguments

def func_1(**kwargs):
    for key, value in kwargs.items():
        print(f"{key} {value}")


def func_2(x, *args, **kwargs):
    print(x)
    for arg in args:
        print(arg)
    for key, value in kwargs.items():
        print(f"{key} {value}")


# one 1
# two 2
func_1(one=1, two=2)

# 1
# 2
# 3
# a 4
# b 5
func_2(1, 2, 3, a=4, b=5)

JavaScript (ES2015) / Rest Parameters

const func1 = kwargs => {
  Object.entries(kwargs).forEach(element => {
    console.log(`${element[0]} ${element[1]}`)
  })
}

//  ...args must be the last argument
const func2 = (x, kwargs, ...args) => {
  console.log(x)
  args.forEach(element => {
    console.log(element)
  })
  Object.entries(kwargs).forEach(element => {
    console.log(`${element[0]} ${element[1]}`)
  })
}

// one 1
// two 2
func1({ one: 1, two: 2 })

// 1
// 2
// 3
// a 4
// b 5
func2(1, { a: 4, b: 5 }, 2, 3)

Class: Basics

Python 3 / Classes

class Duck:
    def __init__(self, name):
        self.name = name

    def fly(self):
        print(f"{self.name} can fly")

    # not @classmethod: call a method on an instance
    # duck = Duck(...)
    # duck.create(...)
    #
    # @classmethod: call a method on a class
    # Duck.create(...)
    @classmethod
    def create(cls, name, kind):
        if kind == "mallard":
            return MallardDuck(name)
        elif kind == "rubber":
            return RubberDuck(name)
        else:
            # cls = Duck
            return cls(name)


class MallardDuck(Duck):
    # @property:
    # use duck.color instead of duck.color()
    @property
    def color(self):
        return "green"


class RubberDuck(Duck):
    def __init__(self, name, eye_color="black"):
        super().__init__(name)
        self.eye_color = eye_color

    def fly(self):
        super().fly()
        print(f"Just kidding, {self.name} cannot fly")

    @property
    def color(self):
        return "yellow"


regularDuck = Duck("reggie")
# reggie can fly
regularDuck.fly()

mallardDuck = Duck.create("mal", "mallard")
# mal
print(mallardDuck.name)
# green
print(mallardDuck.color)

rubberDuck = RubberDuck("vic", "blue")
# vic can fly
# Just kidding, vic cannot fly
rubberDuck.fly()
# yellow
print(rubberDuck.color)
# blue
print(rubberDuck.eye_color)

JavaScript (ES2015) / Classes, Method Definitions, static, get, super

class Duck {
  constructor(name) {
    this.name = name
  }

  fly() {
    console.log(`${this.name} can fly`)
  }

  // not static: call a method on an instance
  // const duck = new Duck(...)
  // duck.create(...)
  //
  // static: call a method on a class
  // Duck.create(...)
  static create(name, kind) {
    if (kind === 'mallard') {
      return new MallardDuck(name)
    } else if (kind === 'rubber') {
      return new RubberDuck(name)
    } else {
      // this = Duck
      return new this(name)
    }
  }
}

class MallardDuck extends Duck {
  // get:
  // use duck.color instead of duck.color()
  get color() {
    return 'green'
  }
}

class RubberDuck extends Duck {
  constructor(name, eyeColor = 'black') {
    super(name)
    this.eyeColor = eyeColor
  }

  fly() {
    super.fly()
    console.log(`Just kidding, ${this.name} cannot fly`)
  }

  get color() {
    return 'yellow'
  }
}

const regularDuck = new Duck('reggie')
// reggie can fly
regularDuck.fly()

const mallardDuck = Duck.create('mal', 'mallard')
// mal
console.log(mallardDuck.name)
// green
console.log(mallardDuck.color)

rubberDuck = new RubberDuck('vic', 'blue')
// vic can fly
// Just kidding, vic cannot fly
rubberDuck.fly()

// yellow
console.log(rubberDuck.color)

// blue
console.log(rubberDuck.eyeColor)