array

Declare an array:.

arr = [1,2,3,4,1]

Add items into the array (directly modify)

// Add to the end of the array
arr.push(5,6) // Number: new arr.length
// Add to the beginning of the array
arr.unshift(-1,0)

• modify directly and return new length

Remove an element from the array

// Remove at the end
arr.pop() // Removed item
// Remove at beginning
arr.shift()

Slice (get array from start to end index), similar to Python [:]

arr.slice(1,3) // Array [1,2]
arr.slice(-1) // Get the last item as arr[-1] won't work
arr.slice() // Make a shallow copy of the array

• does not modify the array
• start is inclusive, end is not

Sorting

arr.sort() // Array: sorted array; sorted alphabetically
arr.sort((first, second) => first - second)
result > 0 // a after b
result < 0 // b after a
result == 0 // keep original order

• sort the array or with a function in place and return the sorted array

[2,4,3]
a=2, b=4; a-b=-2; -2<0 // b after a [a,b]
b=4, c=3; b-c=1; 1>0 // b after c [a,c,b]

Reverse an array

arr.reverse() // New Array [3,2,1]

• will modify the array in place

Find the first item in an array

arr.find(el => el === 2) // Array Item: 2

• takes a function and find the first one that matches
• to find all items in an array and return a new array use filter
• if none is found → undefined

Find the first index of item in an array

arr.findIndex(el => el === 2) // Number: 1

• if not found → -1

Alternative, using indexOf and pass the value in directly

startIndex=0
arr.indexOf(2. startIndex) // Number: 1

• startIndex start the find only after that index

Find from end to beginning

arr.lastIndexOf(1, startIndex) // Number: 4

Whether an array has some that matches

arr.some(el => el === 2) // Bool: true

Alternative: use include to check if an array has a value, similar to Python x in mylist

arr.includes(2) // Bool: true

Check if every item in array matches

arr.every(i=>i.isInteger) // Bool: true

Merge 2 arrays or array with another item

arr.concat(value, anotherArray, moreValue)
// Array: [1,2,3,value,another,array,moreValue]

• it takes multiple arguments and concat all togethers, array or objects

Fill the array with some element at some index

arr.fill(value, startIndex, endIndex) // New Array

• modify the existing array and have it all be the same value
• start index is inclusive, end index is not

Join array into string, similar to "".join(arr) in Python

arr.join("|") // String 1|2|3

Splicing

arr.splice(startIndex, deleteCount, itemToAdd,,) // Array: deleted items

• start from the index, and delete n numbers from that position
• at that index add item(s)

Array from string

mystr = "123"
Array.from(mystr, el=>parseInt(el)) // New Array

• can take a map function, e.g. convert the string into an integer

Enumerate array, similar to enumerate(mylist)

arr.entries() // Array Iterator Object
[...arr.entries()] // [[0,1],[1,2],[2,3]]

• return the index and value

Flatten a multidimensional array

[1,2,[3,4,5]].flat(1) // New Array [1,2,3,4,5]

• create a new array, specify how deep it goes