Cheat sheet: JavaScript Array methods

Credits: Axel Rauschmayer

Adding or removing an element at either end of an Array: (return value: item or new array length)

array before	method	return value	array after
["🟦","🟡","🔺"]	.push("🟩")	4	["🟦","🟡","🔺","🟩"]
["🟦","🟡","🔺"]	.pop()	"🔺"	["🟦","🟡"]
["🟦","🟡","🔺"]	.unshift("🟩")	4	["🟩","🟦","🟡","🔺"]
["🟦","🟡","🔺"]	.shift()	"🟦"	["🟡","🔺"]
["🟦","🟡","🔺"]	.unshift(arr.pop())	3	["🔺","🟦","🟡"]

Changing all of an Array (the input Array is modified and returned):

array before	method	return value
["🟦","🟡","🔺","🟩"]	.fill("🟡")	["🟡","🟡","🟡","🟡"]
Array(4)	.fill("🔺")	["🔺","🔺","🔺","🔺"]
Array(4)	.fill("🔺").map( (val,idx) => idx )	[ 0, 1, 2, 3 ]
["🟦","🟡","🔺","🟩"]	.reverse()	["🟩","🔺","🟡","🟦"]
["c","a","d","b"]	.sort()	["a","b","c","d"]
["🟦","🟡","🔺","🟩"]	.sort()	["🔺","🟡","🟦","🟩"]
["🟦","🟡","🔺","🟩" ]	.copyWithin(1,2,3)	["🟦",🔺","🔺","🟩" ]

Finding Array elements:

array	method	return value
["🟦","🟡","🔺"]	.includes( "🟦" )	true
["🟦","🟡","🔺"]	.indexOf( "🟦" )	0
["🟦","🟡","🟦"]	.lastIndexOf( "🟦" )	2
["🟦","🟡","🔺"]	.find( x => x==="🟦" )	"🟦"
["🟦","🟡","🔺"]	.findIndex( x => x==="🟦" )	0

Creating a new Array from an existing Array:

array before	method (links to MDN)	return value	array after
["🟦","🟡","🔺"]	.slice(1, 3)	["🟡","🔺"]	["🟦","🟡","🔺"]
["🟦","🟡","🔺"]	.splice(1, 2)	["🟡","🔺"]	["🟦"]
["🟦","🟡","🟦"]	.filter( x => x==="🟦")	["🟦","🟦"]	["🟦","🟡","🟦"]
["🟦","🟡"]	.map( x => x+x )	["🟦🟦", "🟡🟡"]	["🟦","🟡"]
["🟦","🟡"]	.map( x => [x+x] )	[["🟦🟦"], ["🟡🟡"]]	["🟦","🟡"]
["🟦","🟡"]	.flatMap( x => [x,x] )	["🟦","🟦","🟡","🟡"]	["🟦","🟡"]
["🟦","🟡","🔺"]	.concat( ["🟩","🔴"] )	["🟦","🟡","🔺","🟩","🔴"]	["🟦","🟡","🔺"]

Computing a summary of an Array:

array	method	return value
["🟦","🟡","🔺"]	.some( x => x==="🟡" )	true
["🟦","🟡","🔺"]	.every( x => x==="🟡" )	false
["🟦","🟡","🔺"]	.join( "🟩" )	"🟦🟩🟡🟩🔺"
[ 2, 3, 4 ]	.reduce( (result,x) => result+x, 10 )	19 10+2+3+4
["🟦","🟡","🔺"]	.reduce( (result,x) => result+x,"🟩")	"🟩🟦🟡🔺"
["🟦","🟡","🔺"]	.reduceRight( (result,x) => result+x,"🟩")	"🟩🔺🟡🟦"

Listing elements:

array	method	return value (iterators)
["🟦","🟡","🔺"]	.keys()	[0,1,2]
["🟦","🟡","🔺"]	.values()	["🟦","🟡","🔺"]
["🟦","🟡","🔺"]	.entries()	[ [0,"🟦"], [1,"🟡"], [2,"🔺"] ]

spreading ... required because the above methods return iterators	return value
[ ...["🟦","🟡","🔺","🟩"].entries() ] .map( ([key,val]) => val.repeat(key) )	["","🟡","🔺🔺","🟩🟩🟩"]