Python3 vs ES2016 cheatsheet

Feature comparison for learning & reference

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Task	Python 3	ES2016	Notes
Assignment
Variables	y = 1	var y = 1;
Block scope		let y = 1;
Constants	Y = 1	const Y = 1;	py: only a naming convention ES2015: can't reassign, but mutable if object is
Many at once	x = 1; y = 2	var x = 1, y = 2;
unpacking / destructuring	x, y = 2, 1	var [x, y] = [1, 2];
Many to same value	x = y = 1
local existance	'x' in locals()
global existance	'x' in globals()
Unpacking / spread
Swap	x, y = y, x	[x, y] = [y, x];
First & Rest	head, *rest = xs	[head, ...rest] = xs
First, Last, Middle	head, *middles, tail = xs
Comparison
chained comparisons	1 < x <= 9	1 < x && x <= 9	left associative in JS
ternary	x if pred else y	pred ? x : y;	for some predicate
truthiness	if x	if (x)	JS: bool, int, float, str only
Math
exponents	x **= y ** z	x **= y ** z;	ES2016
int division	x // y	Math.floor(x / y);
increments	x += 1; x -= 1	x ++; x--;
abs	abs(-1)	Math.abs(-1)
arbitrarily large ints	by default	use external library	JS: https://github.com/jtobey/javascript-bignum, Pyinfo
Sets
init	u = {}	var u = new Set();	py: also set()
from array	u = {*xs}	var u = new Set(xs);	py: also set(xs)
to array	xs = [*u]	var xs = [...u];	py: also list(u)
add to	u.add(x)	u.add(x)	multi-typed, py: also `u
element of	x in u	u.has(x)
remove from	u.remove(x)	u.delete(x)	py: also u -= {x}
size	len(u)	u.length
union in place	`u	= v`
union new object	`u	v`
intersection in place	u &= v	``
intersection new	u & v	v.filter(x => u.has(x))	`O
difference in place	u -= v	``	`O
difference new	u - v	u.filter(x => !v.has(x))	`O
Arrays / Lists
access by index	xs[i]	xs[i]	py: allows negative indices
length	len(xs)	xs.length
slicing	xs[start:end:step]	xs.slice(start, [end]);
adding to end	xs.append(x)	xs.push(x)
concat in place	xs.extend(ys)	xs.concat(ys, zs)	py: xs += ys fails for nonlocal, can't chain to functional call
any / some	any(map(pred, xs))	xs.some(pred)	for some predicate
all / every	all(map(pred, xs))	xs.every(pred)	sim
existance	x in xs	xs.includes(x)	ES2016
zip	zip(xs, ys)		eg: for x, y in zip(xs, ys): f(x, y)
Loops
for each	for x in xs:	for (x in xs){}
enumerate	for i, x in enumerate(xs):	xs.forEach((x, i) => ...)
IO & Strings		c
print	print(x)	console.log(x);
split (by space)	s.split()	s.split(' ')	py: space default, js reqs arg
join (by comma)	','.join(xs)	xs.join()	js: comma default
interpolation	f'my age is {age + 1}' \	`my age is ${age + 1}
aka template strings	'years old'	years old`
Functions
declaration	def f(x):	function f(x){}	js: hoisted to top
global var access			by default
global var mutate	global x; x = 2		js: by default, py: global hoisted to start of function
global var dec inside	global x; x = 2
same name local var	x = 2	let/const x = 2;
precedence		local vars / params, global vars
Objects
has attribute	hasattr(obj, 'attr_name')
Functional
unary lambdas	lambda x: x + 1	x => x + 1;
nullary	lambda : 1	() => 1;
multi-airy	lambda x, y: x + y	(x, y) => x + y;
bind to var	f = lambda x: x + 1	var f = x => x + 1;
map	sqrs = map(square, xs)	sqrs = xs.map(square);
filter	odds = filter(is_odd, xs)	odds = xs.filter(is_odd);
reduce	sum = reduce(add, xs)	sum = xs.reduce(add);	py: import from functools
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