Topic	Python	both	JavaScript
comments	# """ """		// /* */
declaration	my_var = 5		var myVar = 5 let myVar = 5 const myVar = 5
operators	//	+ - * ** / %
assignment		= += -= *= **= /= %=
deletion	del(my_var)		delete(myVar)
increment decrement			++ --
equivalence	is	== !=	=== !==
equivalence rules	[]==[] returns True		[]==[] returns false (because not referring to the same instance)
casting	str() int() float() bool()		String() Number() Boolean()
special types	without imports, float('inf') float(nan)		NaN undefined null Infinity
type checking	type()		typeof() isNaN() isFinite()
array data structure	[] is called a 'list' but is implemented as an array	[1,2,3]	[] is aptly referred to as an array
indexing	[1,2][0][0] returns TypeError	[1,2][0] returns 1	[1,2][0][0] returns undefined
immutable arrays	(1,2,3) called a tuple		(2,3) is syntactically valid but not useful (returns last element only (3))
inserting array element	a=[] a[0]=10 returns SyntaxError		var a = [] a[0]=10 a[100]=20 returns length 101 array ([10, <99 empty items>, 20])
removing array element	arr=['h','i','!'] del arr[1] returns ['h','!']		var arr=['h','i','!']; delete arr[1] returns ['h','!']
array length	len([1,2,3])		[1,2,3].length
access last array element	[1,2,3][-1]		arr=[1,2,3]; arr[arr.length-1];
slicing	[1,2,3][:1] [1,2,3][1:]		[1,2,3].slice(null,2) [1,2,3].slice(1)
clear array	arr.clear()		arr.length = 0;
array methods	arr.append(5) arr.extend([5,6]) .insert() .remove() .index()	.reverse() .sort() .pop()	arr.push(5) arr.push(5,6) .filter() .map() .reduce() .every() .some() .keys() .shift() .unshift()
in	in refers to values	in	in refers to indexes
range array	x=list(range(10)) or as a list comprehension x=[i for i in range(10)] or in Python3 with unpacking operator x=[*range(10)]		x=[]; for (i=0;i<10;i++) {x[i]=i};
string element indexing		'hello'[1] returns 'e'
string overloaded ops	*	+	'2'*'50' returns type number 100 so does '2'*50
string methods	capitalize() find() upper() lower() isupper() islower() split() count()		.length (no parens) .indexOf() .lastIndexOf() .slice(,) .replace(,) .toUpperCase() .toLowerCase() .concat(,) .charAt() .split()
array-of-string concatenation	['hello'] + [' world']		['hello'].concat([' world'])
string casting	list('hi') returns ['h','i'] tuple('hi') returns ('h','i')		var arr = new Array('4') returns ['4'] this next one is not sting casting, but var arr = new Array(4) returns length 4 array [,,,,] (note that JS ignores the last comment when there is no data between it and the bracket)
regular expressions	not built in, import re		var a = myStr.search(/mySearchStr/i)
JSON-like syntax	i,j='k',1 b={i:j} returns hash table (dictionary) {'k':1}		i='k';j=1; b={i:j} returns object {i:'k',j:1}
printing	print()		console.log()
datetime	import datetime as dt d=dt.datetime.now().timestamp() gets seconds with millisecond precision since 1970		var d = new Date() d.getTime(); gets milliseconds since 1970
control flow keywords	pass	continue break
if	if (conditions): elif (conditions): else:		if(conditions) { } else if (conditions) { } else { };
while	while(conditions):		while(conditions) { };
do-while	not built in while True: ....#code ....if (condition): ........break		do {] while(conditions) { };
for	for i in range(5): for items in list:		for ( var i=0; i<5; i++) { };
switch	not built in		switch(myCase) { case 1: break; ....default: };
try-catch	try: except: finally:		try { throw "myErr";} catch(err) { } finally { };
doctests	def sum(a,b): ....""" >>> sum(2,2) ....4 """		not built in npm install doctest
object properties (attributes and methods)	class MyClass(object): ....def __init__(self): ........self.attrib=5 ....def myMethod(self): ........pass myInstance = MyClass()		there is a lot more to properly comparing OOP between JS and Python - I will update soon. myInst={myAttrib:5,myMethod:function(){},};
promises	not built in		var promise1 = new Promise(function(resolve, reject) { ....setTimeout(function() { ........resolve('foo'); }, 300); }); promise1.then(function(value) { ....console.log(value); }); console.log(promise1);
lambdas	def addTen(myNum): ....return myNum + 10 can be written as addTen = lambda myNum: myNum + 10		function addTen(myNum) { return myNum + 10 }; can be written as const addTen = (myNum) => myNum + 10;
map example	def double(x): ....return x*2 y=map(double,[1,2,3]) can be written as y= list(map(lambda x: x*2, [1,2,3])) or as a list comprehension (not a map) y=[i*2 for i in [1,2,3]]		const y=[1,2,3].map(x => x*2);