Syntax Cheatsheet for Javascript/Python developers

7/12/2023, 9:40:33 AM

I enjoy watching tutorials on other languages before going to bed, for exposure. One of the things that I've spent my recent nights watching was Al Sweigart teaching how to Automate The Boring Stuff With Python on Udemy. While I don't see myself switching my working language to Python in the near future, I thought it would be interesting to keep an easy-to-reference cheatsheet of the syntax differences between both languages, in case me or other people need one.

Naming Conventions
Declaring variables
Data types
Data Type Conversions
Expressions
Mathematical Operators
Bitwise Operators
Utilities
Strings
Arrays/Lists
Objects/Dictionaries
Classes
Loops

Naming Conventions

	Javascript	Python	Notes
Variables/Functions	camelCase	snake_case
Classes	PascalCase	PascalCase
Constants	SCREAMING_SNAKE_CASE	SCREAMING_SNAKE_CASE	Python uses this as a convention for constants that shouldn't be reassigned. Because Javascript has 'const' assignments, this is usually reserved for naming variables that would be otherwise magic values, like PI.

Declaring Variables

	Javascript	Python	Notes
Global variable	var count = 0	global count = 0
Scoped variable	let count = 0	count = 0
Immutable variable	const count = 0		Python doesn't have const, all variables are mutable and reassignable. Python offers some data types that have immutable values, like Tuples and Frozensets.
Multiple assignment	let a = 5, b = a, c = b	a = b = c = 5

Data Types

	Javascript	Python	Notes
Boolean	true / false	True / False	Python capitalizes both boolean values.
Strings	'' / "" / ``	'' / "" / r'' / f''	Python can also use triple single-quotes or triple double-quotes to create a multiline string.
Integers		13
Floats		12.4
Complex		complex(1j)
Numbers	13 / 12.4		Javascript doesn't have separate type for integers or floats, they are all just Number.
BigInt	BigInt(bigNumberString)
Array/List	[]	[]	In Javascript, Arrays are technically, a subtype of Object.
Set	new Set(['a'])	set(['a'])	Like Arrays/Lists, but cannot contain duplicate values.
Frozenset		frozenset(('a'))	Immutable version of set().
Range		range(6)
Object/Dictionary	{key: 'value'}	{key: 'value'}	In Python, putting values inside curly braces without providing a key will create a set() instead.
Date	new Date()		Technically, a subtype of Object.
Null/None	null	None
Undefined	undefined		The closest equivalent in Python would be a Nonetype for whenever you assign a variable to the result of a function that doesn't return any value.
Symbol	Symbol('a')
Bytes		b'hi'
Bytearray		bytearray(4)
Memoryview		memoryview(bytes('a'))

Converting Data Types

	Javascript	Python	Notes
Boolean	!!'word'	bool('word')
String (1)	String(10)	str(10)
String (2)	10.toString()
Number (1)	Number('10')	int('10')
Number (2)	parseInt('10')
Number (3)	+'10'		For this to work in Javascript, there must be no spaces between the plus symbol (Unary Operator) and the value/variable.
Number (4)	parseFloat('10')
Number (5)	'10' * 1		Javascript will attempt convert a String to a Number when it interacts with any mathematical operator other than the plus symbol (+).
Number (6)	~~'10'
Float	parseFloat('1.5')	float('1.5')	Javascript doesn't have floats or ints, it's all just Number.
Range		range(8)	Javascript doesn't have Range data types.

Expressions

	Javascript	Python	Notes
Weak equality	'1' == 1		Python doesn't have weak equality checks.
Strict equality	10 === 10	10 == 0
Weak inequality	true != false		Python doesn't have weak inequality checks.
Strict inequality	true !== false	True != False
Less than	10 < 20	10 < 20
Less than or equal to	15 <= 20	15 <= 20
Greater than	30 > 15	30 > 15
Greater than or equal to	50 >= 25	50 >= 25
Both sides of expression must be true	10 > 5 && isTrue === true	10 > 5 and isTrue == True
Either side of expression must be true	20 > 30 \|\| isFalse === false	20 > 30 or isFalse == False
Item present in Array/List	[1, 2, 3].includes(1)	1 in [1, 2, 3]
Text present in String	"Hello world!".includes("Hello")	"Hello" in "Hello World!"
Item missing from Array/List	![1, 2, 3].includes(1)	1 not in [1, 2, 3]	Same for strings in both languages.
Checking for unassigned variable	typeof a === 'undefined'		Python throws an error if you try to access a variable that wasn't previously created. If a Python variable is assigned to the result of a function that didn't return anything, it will be 'NoneType'd.

Mathematical Operators

	Javascript	Python	Notes
Add	5 + 8	5 + 8
Shorthand addition	num += 5	num += 5
Increment	i++		Python doesn't have pre/post increment operators.
Subtract	13 - 5	13 - 5
Shorthand subtraction	num -= 5	num -= 5
Decrement	i--		Python doesn't have pre/post decrement operators.
Multiply	3 * 5	3 * 5
Shorthand multiplication	num *= 5	num *= 5
Exponent	num ** 2	num ** 2
Matrix multiplication		num @ 2	No Javascript equivalent
Matrix shorthand		num @= 2	No Javascript equivalent
Divide	3 / 5	3 / 5	After a division, all numbers in Python are converted to floats, even if the result is an integer.
Shorthand division	num /= 5	num /= 5
Floored divide		3 // 5	Floored division will return the closest integer rounded down. No 1:1 Javascript equivalent, but you can wrap the expression within a 'Math.floor()' function to achieve essentially the same.
Shorthand division		num //= 5	No Javascript equivalent.
Modulo	num % 2	num % 2
Shorthand modulo	num %= 2	num %= 2
Divmod		a, b = divmod(23, 5)	No Javascript equivalent.

Bitwise Operators (a.k.a. Binary Black Magic)

	Javascript	Python
AND	&	&
	&=	&=
OR	\|	\|
	\|=	\|=
XOR	^	^
	^=	^=
NOT	~	~
Right shift	>>	>>
	>>=	>>=
Left shift	<<	<<
	<<=	<<=

Functions

	Javascript	Python	Notes
Functions	function multiplyByTwo(x) { return x * 2 }	def multiplyByTwo(x): return x * 2	Python defines scope through indentation, Javascript usually does it through the use of curly braces
Default arguments	function logX(x = 'hi') { console.log(x) }	def printX(x='hi'): print(x)
Fat Arrow Functions/Lambdas	const add = (a, b) => a + b	add = lambda a, b: a + b
Generators	function* timer(counter) { while (counter > 0) { yield counter; counter--; } }	def timer(counter): while counter > 0: yield counter counter -= 1

Utilities

	Javascript	Python	Notes
Write to console	console.log('hi')	print('hi')
If	if (condition === true) {	if condition == True:
Else if/Elif	} else if (condition === true) {	elif condition == True:
Else	} else {	else:
Switch/Match	switch (arg){ case 'word': doThing(); break; default: doOtherThing(); }	match arg: case 'word': doThing() case _: doOtherThing()	Python doesn't have break statements.
Ternary operator	happy === true ? "smile" : "frown"	"smile" if happy == True else "frown"
Try + Catch/Except + Finally	try { badFunction(); } catch (error) { if (error instanceof ThrownException) { console.log('Sorry! ' + error.message); } } finally { console.log('Finishing'); }	try: badFunction() except ThrownException as error: print('Sorry! {}'.format(error)) finally: print('Finishing')
Single line comments	// comment	# comment
Multi line comments	/* This is a multiline comment */		Python doesn't exactly have multi line comments, but you can use a multi line string to do basically the same thing.
Destructuring/Unpacking	const [a, b, ...rest] = [1, 2, 3, 4]	a, b, *rest = [1, 2, 3, 4]	In Javascript, the '...rest' argument must be the last one. Python doesn't have this restriction.
Spread	oneFunction(...args)	oneFunction(**args)
Type checking (1)	typeof 10 === 'number'	type(10) is int
Type checking (2)		isinstance(10, int)
Parse JSON	JSON.parse(jasonToParse)	import json json.loads(jason_to_parse)	Built in for Javascript, requires a library import for Python.
Convert to JSON	JSON.stringify(objToJason)	import json json.dumps(dict_to_jason)	Built in for Javascript, requires a library import for Python.
Replace match with Regex	const phonePattern = /($?\d{3}$?[- ]?\d{3}-?\d{4})/; "Please call me at (555) 444-9876".replace(phonePattern, "REDACTED PHONE NUMBER");	import re phonePattern = re.compile(r'($?\d{3}$?[- ]?\d{3}-?\d{4})') phonePattern.sub(phonePattern, "REDACTED PHONE NUMBER", "Please call me at (555) 444-9876")	Built in for Javascript, requires a library import for Python.

Strings

	Javascript	Python	Notes
Single quotes string	const styling = 'css'	styling = 'css'
Double quotes string	const markup = "html"	markup = "html"
Length	"text".length	len("text")
Concatenation	"two " + "strings"	"two " + "strings"
Escaping characters	'Don\'t repeat yourself'	'Don\'t repeat yourself'
Raw strings		r'\back\slash'	Javascript doesn't have raw strings.
Multiline strings	` multiline text here `	""" multiline text here """	Python can use double or single quotes for multilines, as long as what you use at the start, you also use at the end. Javascript exclusively uses backticks.
Slicing (set range)	word.substring(1, 4)	word[1:4]
Slicing (start until set end)	word.substring(0, 3)	word[:3]
Slicing (set start until end)	word.slice(-3)	word[-3:]
String interpolation (1)		'My name is %s.' % (name)
String interpolation (2)	`My name is ${name}.`	f'My name is {name}.'	Python's f-strings require Python 3.6 or higher.
Text present in String	"Hello world!".includes("Hello")	"Hello" in "Hello World!"	Same for Arrays/Lists in both languages.
Join Array/List elements as single string	['Hello', 'world!'].join(' ')	hello = ['Hello', 'world!'] ' '.join(hello)
Split a string into an Array/List	'Hello world!'.split(' ')	'Hello world!'.split()	Python will split on whitespace by default, if a separator is not provided. With Javascript, not providing a separator will cause every single character to become an Array element.
Trim	' word '.trim()	' word '.strip()	Python also allows you to pass parameters to strip in order to remove something else other than spaces. Javascript doesn't have that flexibility.
Trim leading spaces only	' word'.trimStart()	' word '.lstrip()
Trim trailing spaces only	'word '.trimEnd()	'word '.rstrip()
Uppercase entire string	'word'.toUpperCase()	'word'.upper()
Lowercase entire string	'SCREAMING'.toLowerCase()	'SCREAMING'.lower()
Uppercase first letter of first word, lowercase everything else		'two WORDS'.capitalize()	No Javascript equivalent.
Uppercase first letter of every word		'breaking news'.title()	No Javascript equivalent.

Arrays/Lists

	Javascript	Python	Notes
New array/list (1)	const a = new Array()	a = list()	In Javascript, it's bad practice to initialize arrays like this.
New array/list (2)	const a = [1, 2]	a = [1, 2]
Accessing element by index	a[0]	a[0]
Modifying element at index	a[0] = 4	a[0] = 4
Adding element at final position	a.push(8)	a.append(8)	Python can use 'append' to place an item anywhere in the List, but Javascript's 'push' will only add the element after the final element.
Inserting at index	a.splice(1, 0, 'item added')	a.insert(1, 'item added')	The second argument for Javascript's '.splice()' is for how many elements you want to remove starting on the position of the first argument.
Removing last element	a.pop()	a.pop()
Removing first element with value		a.remove(4)	Javascript doesn't offer an easy way to do remove the first match and stop. You can do a 'for loop' to remove one and break after the first match or use '.findIndex()' and then '.splice()' on that element.
Join Array/List elements as single string	['Hello', 'world!'].join(' ')	hello = ['Hello', 'world!'] ' '.join(hello)
Slicing (set range)	a.slice(1, 4)	a[1:4]
Slicing (start until set end)	a.slice(3)	a.[:3]	In Javascript, providing only one POSITIVE argument implies that is the end point.
Slicing (set start until end)	a.slice(-3)	a[-3:]	In Javascript, providing only one NEGATIVE argument implies that is the start point.
Sorting	a.sort()	a.sort()
Sorting descending	a.sort((a, b) => b - a)	a.sort(reverse=True)
Counting occurrences of element		a.count(4)	No Javascript equivalent.
Array/List length	[1, 2, 3].length	len([1, 2, 3])
New Set (1)	const uniqueSet = new Set([1, 2, 3])	uniqueSet = set((1, 2, 3))
New Set (2)		uniqueSet = {1, 2, 3}
Modifying (map) every element in an Array/List	[1, 2, 3].map(n => n * 2)	list(map(lambda n: n * 2, [1, 2, 3]))
Reduce an Array/List to single result	[1, 2, 3].reduce((total, curr) => total + curr)	reduce(lambda total, curr: total + curr, [1, 2, 3])
Count Set items	uniqueSet.size	len(uniqueSet)
Tuples		x = (1, 2, 3)	Javascript doesn't support Tuples yet, but there is a TC39 proposal to add them. Python's Tuples are also not modifiable. Once set, forever defined.

Objects/Dictionaries

	Javascript	Python	Notes
New object/dict (1)	const o = {}	o = {}
New object/dict (2)	const o = new Object()	o = dict()
New object/dict (3)	const o = Object.create(null)		No Python equivalent.
Adding property (1)	obj['keyName'] = 'value'	dict['key_name'] = 'value'
Adding property (2)	obj.keyName = 'value'	setattr(dict, key_name, value)
Accessing property value (1)	obj['keyName']	dict['key_name']
Accessing property value (2)	obj.keyName	getattr(dict, key_name)
Length		len(dict)	Can't use obj.length in Javascript, but you can convert the Object to an Array using Object.entries(obj)/Object.keys(obj)/Object.values(obj) and then get the length of that.
Check if key exists	'key_name' in dict	'keyName' in obj
Comparing two objects/dictionaries	const a = {name: 'yuri'} const b = {name: 'yuri'} a == b // false	a = {name: 'yuri'} b = {name: 'yuri'} a == b # True	Python makes a deep equality check by default, Javascript instead compares if the reference in memory are the same.
Deleting a key	delete obj['key']	del obj['key']
Merging objects/dictionaries	{...objA, ...objB}	{dictA, dictB}

Classes

	Javascript	Python	Notes
New Class	class Person { constructor(name) { this.name = name; } }	class Person: def __init__(self, name): self.name = name
Inheritance	class Student extends Person { constructor(name, age) { super(name); this.age = age } }	class Student(Person): def __init__(self, name, age): super().__init__(name) self.age = age	Python has 'pass' for whenever you want to create/inherit a class, but not initialize properties.
Printing a class to console		class Person: def __str__(self): return f"Person {self.name} printed."	Javascript logs the object version of a class to the console, while Python outputs human-unreadable code if '__str__()' is not provided.

Loops

	Javascript	Python	Notes
Looping through obj/dict keys	for (const key of Object.keys(obj)) { console.log(key) }	for key in dict.keys(): print(key)
Looping through obj/dict values	for (const value of Object.values(obj)) { console.log(value) }	for value in dict.values(): print(value)
Looping through obj/dict values	for (const val in obj) { console.log(val) }	for val in dict: print(val)
Looping through obj/dict keys + values	for (const [k, v] of Object.entries(obj)) { console.log(k, ':', v) }	for k, v in dict.items(): print(k, ':', v)
Looping a set number of times	for (let i = 0; i < 10; i++) { console.log(i) }	for i in range(10): print(i)
Looping Array/List (1)	for (let item of loopingArray) { console.log(item) }	for item in loopingList: print(item)
Looping Array/List (2)	for (let i = 0; i < loopingArray.length; i++) { console.log(loopingArray[i]); }
Looping Array/List (3)	loopingArray.forEach(function (item) { console.log(item); }) }
While loop	while (condition) { console.log('condition is true') }	while condition: print('condition is true')
Do While loop	do { i++; console.log(i); } while (i < 5);		No Python equivalent.