Arrays

See Array cheatsheet on ShortCutFoo website.

Read

myArray = ['a', 'b', 'c', 'd']

Get length.

> myArray.length
4

Get element - returns one element.

> myArray[index]

> myArray[1]
'b'

Slice - returns an array of elements.

> myArray.slice(start, end)
[ ... ]
> myArray.slice(1, 3)
[ 'b', 'c' ]

Leave out end.

> myArray.slice(start)

> myArray.slice(1)
[ 'b', 'c', 'd' ]

Get last element

Use hard bracket slicing

> myArray = ['a', 'b', 'c', 'd']
> myArray[myArray.length - 1]
'd'

> myArray = ['a']
> myArray[myArray.length - 1]
'a'

In Python you simply use a negative index my_list[-1], but in JS that gives a undefined.

Use slice

You can use a negative index here.

> myArray = ['a', 'b', 'c', 'd']
> myArray.slice(-1)
[ 'd' ]

Warning - this returns an array, not a string, so do this.

> myArray.slice(-1)[0]
'd'

Update

Mutate element.

> myArray[index] = value

See also Pop, Push, Shift and Unshift Array Methods in JavaScript

Modify end

Remove last element.

> myArray.pop();

Insert element at the end. i.e. append.

> myArray.push(obj)

Modify at the start

Remove from the beginning. Like Python’s list.pop(0). Returns new length.

> myArray.shift()

Insert at the start. Like Python’s list.insert(0, obj). Returns new length.

> myArray.shift(obj)

Change order

Reverse.

> myArray.reverse()

Sort. See Sort for more info, as this will not sort numbers properly.

> myArray.sort()

Join arrays

Concatenate arrays.

> myArrayA.concat(myArrayB)

Join elements using a separator e.g. ', '. This will return a single string.

> myArray.join(sep)

Map, reduce, filter

Return a new object after applying transformation. The old object is no affected, unless you reassign over the same name.

Examples:

myArray.map((x) => x*2)

myArray.reduce((x, y) => x + y)

myArray.filter((x) => x > 2)