12 Mar 2019

Essential JavaScript code for lists

edA-qa (@mortoray) has recently published a couple of great articles about list manipulation for coding interviews. The first article outlines the kinds of things to practice, and the second one shows code samples in Python. I thought it would be useful to show these examples in JavaScript too!

I tried to stay as close as I could to the Python examples in edA-qa’s article. However, it’s worth keeping in mind that Python’s standard library is significantly richer than JavaScript’s. For these list manipulation exercises, Python’s generator expressions and rich iteration api come in especially handy. I’ve kept things as simple and concise as I could, but in several places I did have to write utility functions to make up for this.

I’d like to thank edA-qa for consulting about this JavaScript version of the article and helping me to clarify and simplify some examples. I take responsibility for any mistakes or issues though.

These examples are written using modern JavaScript, including arrow functions, spread and rest syntax, as well as generator functions.


// Create a list
let items = []

// Add element to the front or back
let newLength = items.unshift("abc")
let newLength = items.push("def")

// Pop element off front or back
let start = items.shift()
let end = items.pop()

// Forward iterate over elements
for (const item of items)

// Also works

// Get the length of a list
let length = items.length

// Test if empty. May need extra logic for special cases,
// such as null, undefined, or not iterable
if (items.length == 0)
  console.log("list is empty")

// Also works
console.log(items.length == 0 ? "list is empty" : "list is not empty")

Location Based Operations

// Get item at location
let thirdItem = items[2]

// Insert an item at location
items.splice(3, 0, "abc")

// Remove an item from a location
let removedItems = items.splice(2, 1)

// Replace/Assign item at location
items[1] = "def"

Sorting and Searching

// Find an item
let index = items.indexOf(item) // -1 if not found

// Find and remove an item
let index = items.indexOf(item)
if (index > -1)
  items.splice(index, 1)

// Find last matching item
let index = items.lastIndexOf(item) // -1 if not found

// Sort by natural order
// in-place sort

// Sort with custom comparator
let items = [['a', 10], ['b', 7], ['c', 13], ['d', 1]]
items.sort((a, b) => a[1]-b[1])
// produces [ [ 'd', 1 ], [ 'b', 7 ], [ 'a', 10 ], [ 'c', 13 ] ]

Segment Manipulation

// Split the list at arbitrary location
let left = items.slice(0, 4)
let right = items.slice(4, items.length)

// Multiple splits based on a match
let items = ['a', 'b', 'c', 'd', 'b', 'e']
let splits = groupBy(items, item=>item=='b').filter(group=>group.indexOf('b')==-1)
// produces [['a'], ['c','d'], ['e']]

// groupBy is not built in. Groups items by whether or not they 
// match the `isSeparator` function. The result is an array of arrays
const groupBy = (items, isSeparator) => {
  const groups = []

  let rest = items
  while (rest.length > 0) {
    const {group, leftover} = nextGroup(rest,  isSeparator)
    rest = leftover

  return groups

const nextGroup = (items, isSeparator) => {
  const foundIndex = items.findIndex(isSeparator(items[0]) != isSeparator(item))
  const endIndex = foundIndex > -1 ? foundIndex : items.length

  return {group: items.slice(0, endIndex), leftover: items.slice(endIndex)}

// Clear the list (in place)
items.length = 0

// Remove segment
// delete 2 items starting with the second item (index 1)
let removedItems = items.splice(1, 2)

// Concatenate lists
let combined = items.concat(moreItems)

// Insert list at location
items.splice(index, 0, ...moreItems) //not a good idea for large lists... watch the stack!

//Get a sublist
//sublist starting at position 1 up to, but excluding, position 3
let slice = items.slice(1, 3)

More Iteration

// Backward
for (const item of items.reverse())

// Partial segment iteration
for (const item of items.slice(1, 4))

// Skipping elements
// step from element at index 1 to 6 (exclusive) by 2
items.slice(1,6).filter((item, index)=>index % 2 == 0).map(item=>console.log(item))


// Create from a static list of items
let items = [ 'abc', 'def', 'ghi']

// Create a range of numbers
// a list of numbers from 10..20 (exclusive)
let numbers = [...Array(10).keys()].map(n=>n+10)

// Also works
let numbers = []
for (let i=10; i<20; i++)

Data Manipulation

// Mapping
let numbersTimes10 = numbers.map(n=>n*10)

// Filtering
let evenNumbers = numbers.filter(n=>n%2==0)

// Fold / Reduce
// If a default value is not supplied to reduce,
// then reducing an empty list will produce a TypeError at runtime
let total = numbers.reduce((acc,n)=>acc+n, 0)

// Joining string representations of items
let joined = items.reduce((left,right)=>left + '/' + right, '')

// Zip
let results = zip([[1,2,3], ['a','b','c'], ['x','y','z']]) 
// produces [[1,'a','x'],[2,'b','y'],['3','c','z']]

// zip is not built in
const zip = rows=>rows[0].map((val,colIndex)=>rows.map(row=>row[colIndex]))

// alternative version of zip (takes each array as separate parameter)
let results = zip([1,2,3], ['a','b','c'], ['x','y','z'])

const zip = (...rows)=>rows[0].map((val,colIndex)=>rows.map(row=>row[colIndex]))

// to alternate items into one list use reduce
zip([0,2,4], [1,3,5]).reduce((acc, item)=>acc.concat(item)) //[0,1,2,3,4,5]


// Swap elements at two locations
[items[3], items[5]] = [items[5], items[3]]

// Reserve capacity
// JavaScript arrays do not expose capacity. In fact, 
// they are not arrays in the usual sense, since items 
// are not stored contiguously in memory. 
// See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Typed_arrays
// if you need something closer to the metal

// Replace content in a list in place
replace(mylist, anotherList)

const replace = (target, source) => {
  target.length = 0
  return target.splice(0,0,...source) // not a good idea for large lists... watch the stack!

// Compare two arrays
JSON.stringify(items1) === JSON.stringify(items2)  //simple-minded approach

// Search a sorted list
const foundIndex = findFirst(sortedNumbers, 4)
if (foundIndex !== -1)
  console.log(`Found at ${foundIndex}`)

const findFirst = (sortedItems, target) => {
  const index = bisectLeft(sortedItems, target)
  return index < sortedItems.length && sortedItems[index] === target ? index : -1

// bisectLeft is not built in. Performs a binary search 
// for the left-most matching item. If the item isn't found, 
// returns the index where it should go.
// Comparison only works properly if `valueOf` has been implemented.
const bisectLeft = (sortedItems, target) => {
  let start = 0
  let end = sortedItems.length

  while (start < end) {
    let mid = Math.trunc((start + end) / 2)
    if (sortedItems[mid] < target) 
      start = mid + 1
      end = mid

  return start

// Multiple iterators at the same time
let itemsA = [1,2,3]
let itemsB = ['a','b','c','d','e','f']

let iterA = itemsA[Symbol.iterator]()
let iterB = itemsB[Symbol.iterator]()

while(true) {
  let a = iterA.next()
  let b = iterB.next()

  if (a.done || b.done)

  console.log(a.value + ', ' + b.value)

Appendix A: Iteration

The following examples show how to use generator functions to iterate over part of an array or in reverse order without having to modify or copy the original array.

// Backward
for (const item of reverseGenerator(items))

const reverseGenerator = function* (items) {
  for (let i = items.length-1; i>=0; i--)
    yield items[i]

// Partial segment iteration
for (const item of sliceGenerator(items, 1, 4))

// Skipping elements
// step from element at index 1 to 6 (exclusive) by 2
for (const item of sliceGenerator(items,1,6,2))

const sliceGenerator = function* (items, start=0, end=items.length, step=1) {
  for (let i=start; i<end; i+=step)
    yield items[i]

Appendix B: Mutability

Some of these examples modify the array in place whereas others return a copy of the array with the desired changes. The following operations change the array in place:

  • push, pop, unshift, shift, splice, sort, reverse
  • assign new value to array at index, e.g. items[index] = newValue
  • reset array to empty, e.g. items.length = 0
  • swap, e.g. [items[3], items[5]] = [items[5],items[3]]

These operations don’t modify the original array:

map, filter, and reduce could theoretically be used to modify the contents of the original array, but this is a bad practice and should be avoided.


Thanks for reading if you’ve got all the way to the end. I think it would be great to expand this cheatsheet to more and more languages. If you found this interesting, I urge you to give it a try in a programming language of your choice!