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Print a literal string on standard output
document.write("Hello World!");
Alternative implementation:
console.log('Hello World');
Loop to execute some code a constant number of times
for (let i = 0; i < 10; i++) {
Alternative implementation:
let count = 0;
while (count < 10) {
Alternative implementation:
[...Array(10)].forEach(() => console.log('Hello'))
Alternative implementation:
console.log( 'Hello\n'.repeat(10) )
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
var bli = function() { 
	console.log('Hello World!!!');
Alternative implementation:
function bli() { 
	console.log('Hello World!!!');
Alternative implementation:
let dog = 'Poodle';
const dogAlert = () => alert(dog);
Alternative implementation:
const greet = who => console.log(`Hi ${who}!`)
Create a function which returns the square of an integer
function square(x) { 
	return x * x;
Alternative implementation:
const square = (x) => x * x;
Alternative implementation:
const square = n => n**2
Alternative implementation:
const square = (number) => Math.pow(number, 2);
Declare a container type for two floating-point numbers x and y
var p = { x: 1.122, y: 7.45 };
Alternative implementation:
const point = { x: 1, y: 2 };
Do something with each item x of the list (or array) items, regardless indexes.
items.forEach((x) => {
Alternative implementation:
for (x of items) {
Alternative implementation:
Alternative implementation:
for (var i = 0; i<items.length; i++) {
  var x = items[i];
Alternative implementation:
Print each index i with its value x from an array-like collection items
items.forEach((val, idx) => {
  console.log("index=" + idx + ", value=" + val);
Alternative implementation:
for (var i in items) {
   console.log("index=" + i + ", value=" + items[i]);
Create a new map object x, and provide some (key, value) pairs as initial content.
const x = {one: 1, two:2}
Alternative implementation:
const x = new Map();
x.set("one", 1);
x.set("two", 2);
Alternative implementation:
const x = new Map([["one",1],["two",2]]);
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent.
class Node {
  constructor (data) { = data
    this.left = null
    this.right = null
Generate a random permutation of the elements of list x
for (var i = x.length - 1; i > 0; i--) {
    var j = Math.floor(Math.random() * (i + 1));
    var temp = x[j];
    x[j] = x[i];
    x[i] = temp;
The list x must be non-empty.
x[Math.floor(Math.random() * x.length)]
Alternative implementation:
x[~~(Math.random() * x.length)];
Check if the list contains the value x.
list is an iterable finite container.
return list.indexOf(x) !== -1;
Alternative implementation:
return list.includes(x);
Access each key k with its value x from an associative array mymap, and print them.
Object.entries(mymap).forEach(([key, value]) => {
	console.log('key:', key, 'value:', value);
Alternative implementation:
for (const key in mymap) {
    console.log('key:', key, 'value:', mymap[key]);
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
a + (b-a) * Math.random();
Alternative implementation:
a + Math.random() * (b - a)
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
function pick(a, b) {
    return a + Math.floor(Math.random() * (b - a + 1));
Call a function f on every node of binary tree bt, in depth-first infix order
function dfs(bt) {
	if (bt === undefined) return;
The structure must be recursive. A node may have zero or more children. A node has access to its children nodes, but not to its parent.
class Node {
  constructor (value, children = []) {
    this.value = value
    this.children = children
Call a function f on every node of a tree, in depth-first prefix order
function DFS(f, root) {
	if (root.children) {
		root.children.forEach(child => DFS(f, child))
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering.
x = x.reverse();
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time.
function search(m, x) {
    for (var i = 0; i < m.length; i++) {
        for (var j = 0; j < m[i].length; j++) {
            if (m[i][j] == x) {
                return [i, j];
    return false;
Swap the values of the variables a and b
var tmp = a;
a = b;
b = tmp;
Alternative implementation:
[a, b] = [b, a];
Extract the integer value i from its string representation s (in radix 10)
let i = parseInt(s, 10)
Alternative implementation:
const i = Number(s);
Alternative implementation:
const i = +s
Given a real number x, create its string representation s with 2 decimal digits following the dot.
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
s = "ネコ";
Share the string value "Alan" with an existing running process which will then display "Hello, Alan"
  // in file worker.js
  onmessage = ({data}) => {
    console.log (`Hello, ${data}`)
  // in file main.js
  const worker = new Worker ('worker.js')
  worker.postMessage ('Alan')
Alternative implementation:
if (isMainThread) {
  const worker = new Worker(new URL(import.meta.url));
} else {
  parentPort.once('message', (message) => {
    console.log(`Hello, ${message}`);
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
var x = [];
for (var i = 0; i < m; i++) {
  x[i] = [];
Alternative implementation:
const x = new Array(m).fill(new Array(n).fill(Math.random()));
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
const x = new Array(m).fill(
  new Array(n).fill(
    new Array(p).fill(Math.random())
Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items.
items.sort(function(a,b) {
  return compareFieldP(a.p, b.p);
Remove i-th item from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
Note that in most languages, the smallest valid value for i is 0.
let new_list = items.filter(function(val,idx,ary) { return idx != i });
Alternative implementation:
Launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
for (let i = 1; i <= 1000; i++) setTimeout(() => f(i), 0);
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
function f(i) {
   return i<2 ? 1 : i * f(i-1);
Alternative implementation:
const f = i => i === 0 ? 1 : i * f(i-1)
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
function exp(x, n) {
   if (n===0) return 1;
   if (n===1) return x;
   return n%2 ? x * exp(x*x, (n-1)/2)
              : exp(x*x, n/2);
Alternative implementation:
const exp = Math.pow;
Assign to the variable x the new value f(x), making sure that no other thread may modify x between the read and the write.
let x = f(x)
Declare and initialize a set x containing unique objects of type T.
let x = new Set();
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns the composition function g ∘ f
function compose(f,g){
    return function(x){
        return g(f(x));
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter.
function compose(f,g){
    return function(x){
        return g(f(x));
Alternative implementation:
const compose = f => g => x => g(f(x));
Transform a function that takes multiple arguments into a function for which some of the arguments are preset.
function curry (fn, scope) {
    scope = scope || window;
    // omit curry function first arguments fn and scope
    var args =, 2);
    return function() {
	var trueArgs = args.concat(, 0));
        fn.apply(scope, trueArgs);
Alternative implementation:
const curry = (fn, ...initialArgs) => (...args) => fn(...initialArgs, ...args);

const add = (a, b) => a + b;

const add5 = curry(add, 5);

const result = add5(1) // 6
Find substring t consisting in characters i (included) to j (excluded) of string s.
Character indices start at 0 unless specified otherwise.
Make sure that multibyte characters are properly handled.
let t = s.substring(i, j);
Alternative implementation:
let t = s.slice(i, j);
Set the boolean ok to true if the string word is contained in string s as a substring, or to false otherwise.
var ok = s.indexOf(word) !== -1;
Alternative implementation:
var ok = s.includes(word);
Create string t containing the same characters as string s, in reverse order.
Original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory.
var t = s.split("").reverse().join("");
Print each item v of list a which is not contained in list b.
For this, write an outer loop to iterate on a and an inner loop to iterate on b.
for (var i in a) {
   for (var j in b) {
      if (a[i] === b[j]) {
         continue OUTER;
   console.log(a[i] + " not in the list");
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
for (var i in m) {
   for (var j in m[i]) {
      if (m[i][j] < 0) {
         console.log("Negative value found: "+m[i][j]);
         break OUTER;
Insert the element x at position i in the list s. Further elements must be shifted to the right.
s.splice(i, 0, x);
Sleep for 5 seconds in current thread, before proceeding with the next instructions.
	// Instructions after delay
Alternative implementation:
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));

await sleep(5000);
Alternative implementation:
await new Promise(r => setTimeout(r, 5000));
Create the string t consisting of the 5 first characters of the string s.
Make sure that multibyte characters are properly handled.
let t = s.substring(0,5);
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
var t = s.slice(-5);
Assign to variable s a string literal consisting in several lines of text, including newlines.
let s = "This is a very long string which needs \n" +
        "to wrap across multiple lines because \n" +
        "otherwise my code is unreadable.";
Alternative implementation:
let s = "This is a very long string which needs \
to wrap across multiple lines because \
otherwise my code is unreadable.";
Alternative implementation:
let s = `This is a very long string which needs 
to wrap across multiple lines because 
otherwise my code is unreadable.`;
Build list chunks consisting in substrings of the string s, separated by one or more space characters.
let chunks = s.split(/ +/);
Write a loop that has no end clause.
while (true) {
	// to infinity
Alternative implementation:
for(;;) {
Determine whether the map m contains an entry for the key k
k in m
Alternative implementation:
Alternative implementation:
Determine whether the map m contains an entry with the value v, for some key.
Alternative implementation:
Concatenate elements of string list x joined by the separator ", " to create a single string y.
y = x.join(", ");
Calculate the sum s of the integer list or array x.
var s = x.reduce((a, b) => a + b, 0);
Alternative implementation:
var s = x.reduce((a, b) => a + b)
Create the string representation s (in radix 10) of the integer value i.
var s = i.toString();
Alternative implementation:
var s = i + "";
Fork-join : launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
Wait for the completion of the 1000 tasks and then print "Finished".
  // in file worker.js
  onmessage = f
  // in file main.js
  for (let i = 0; i < 1000; i++) {
    new Worker ('worker.js')
      .postMessage (i)
Alternative implementation:
const tasks = [];
for (let i = 0; i < 1000; i++) {

await Promise.all(tasks);
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place.
y = x.filter(p);
Create the string lines from the content of the file with filename f.
fs.readFile(f, (err, lines) => {
    if (err) {
        // Handle error...

    // Work with `lines` here.
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
console.error(util.format("%d is negative", x));
Alternative implementation:
console.error(x, "is negative");
Alternative implementation:
console.error(`${x} is negative`);
Assign to x the string value of the first command line parameter, after the program name.
const x = process.argv[2]
Assign to the variable d the current date/time value, in the most standard type.
var d =;
Alternative implementation:
var d = new Date();
Set i to the first position of string y inside string x, if exists.

Specify if i should be regarded as a character index or as a byte index.

Explain the behavior when y is not contained in x.
let i = x.indexOf(y);
Assign to x2 the value of string x with all occurrences of y replaced by z.
Assume occurrences of y are not overlapping.
var x2 = x.replace(new RegExp(y, 'g'), z);
Alternative implementation:
const x2 = x.replaceAll(y, z);
Assign to x the value 3^247
let x = 3n ** 247n;
From the real value x in [0,1], create its percentage string representation s with one digit after decimal point. E.g. 0.15625 -> "15.6%"
const s = Math.round (x * 1000) / 10 + '%'
Alternative implementation:
const percentFormatter = new Intl.NumberFormat('en-US', {
  style: 'percent',
  maximumSignificantDigits: 3

const s = percentFormatter.format(x);
Calculate the result z of x power n, where x is a big integer and n is a positive integer.
let z = x**n
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers.
const fac = x => x ? x * fac (x - 1) : x + 1
const binom = (n, k) => fac (n) / fac (k) / fac (n - k >= 0 ? n - k : NaN)
Create an object x to store n bits (n being potentially large).
let x = new Buffer (Math.ceil (n / 8))
Use seed s to initialize a random generator.

If s is constant, the generator output will be the same each time the program runs. If s is based on the current value of the system clock, the generator output will be different each time.
seed (s)
Get the current datetime and provide it as a seed to a random generator. The generator sequence will be different at each run.
Math.random ()
Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline.
The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout.
console.log(process.argv.slice(2).join(" "));
Create a factory named fact for any sub class of Parent and taking exactly one string str as constructor parameter.
class Parent {
    constructor(str) {}
    fact(new_class, str) {
        if (new_class.prototype instanceof Parent) {
            return new new_class(str)

class Child extends Parent {}
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers.
const gcd = (a, b) => b === 0 ? a : gcd (b, a % b)
Compute the least common multiple x of big integers a and b. Use an integer type able to handle huge numbers.
const gcd = (a, b) => b === 0 ? a : gcd (b, a % b)
let x = (a * b) / gcd(a, b)
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
let s = x.toString(2);
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i.
var x = math.complex(-2, 3);
x = math.multiply(x, math.i);
Execute a block once, then execute it again as long as boolean condition c is true.
do {
} while (c);
Declare the floating point number y and initialize it with the value of the integer x .
let y = x + .0
Declare integer y and initialize it with the value of floating point number x . Ignore non-integer digits of x .
Make sure to truncate towards zero: a negative x must yield the closest greater integer (not lesser).
let y = BigInt (x | 0)
Declare the integer y and initialize it with the rounded value of the floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity).
var y = Math.round(x);
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted.
let n = 0 // the number of occurences
let acc = s
let i
while ((i = acc.indexOf (t)) + 1) {
  acc = acc.slice (i + 1)
Declare regular expression r matching strings "http", "htttp", "httttp", etc.
const r = /htt+p/
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2
const c = i.toString(2).replace(/[^1]/g, '').length
Write boolean function addingWillOverflow which takes two integers x, y and return true if (x+y) overflows.

An overflow may be above the max positive value, or below the min negative value.
function addingWillOverflow(x, y) {
  return x > 0 && y > 0 && x > Number.MAX_SAFE_INTEGER - y
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
Create a new bytes buffer buf of size 1,000,000.
let buf = new Buffer (1e6)
You've detected that the integer value of argument x passed to the current function is invalid. Write the idiomatic way to abort the function execution and signal the problem.
throw new Error ('x is invalid')
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo.
const Foo = function Counter () {
  let n = 0
  Object.defineProperty (this, 'value', {get: () => n++})
  const counter = new Foo ()
  counter.value // 0
  counter.value // 1
Alternative implementation:
class Foo {
  #x = 123;
  get x() {
    return this.#x;
Read from the file data.json and write its content into the object x.
Assume the JSON data is suitable for the type of x.
const x = JSON.parse(fs.readFileSync('./data.json'));
Alternative implementation:
const x = require('./data.json');
Write the contents of the object x into the file data.json.
fs.writeFileSync('data.json', JSON.stringify(x));
Implement the procedure control which receives one parameter f, and runs f.
function control(f){
Print the name of the type of x. Explain if it is a static type or dynamic type.

This may not make sense in all languages.
console.log(typeof x);
Alternative implementation:
console.log (x == null ? x + '' :;
Assign to variable x the length (number of bytes) of the local file at path.
let x = read(path).length
Set the boolean b to true if string s starts with prefix prefix, false otherwise.
var b = s.startsWith(prefix);
Alternative implementation:
var b = (s.lastIndexOf(prefix, 0) === 0);
Set boolean b to true if string s ends with string suffix, false otherwise.
var b = s.endsWith(suffix);
Convert a timestamp ts (number of seconds in epoch-time) to a date with time d. E.g. 0 -> 1970-01-01 00:00:00
new Date (ts * 1000)
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
let x = d.toISOString().slice(0, 10)
Sort elements of array-like collection items, using a comparator c.
Make an HTTP request with method GET to the URL u, then store the body of the response in the string s.
$.get(u, function(data){
  s = data;
Alternative implementation:
var xmlHttp = new XMLHttpRequest();
xmlHttp.onreadystatechange = function() { 
	if (xmlHttp.readyState == 4 && xmlHttp.status == 200)
		s = xmlHttp.responseText;
}"GET", u, true);
Alternative implementation:
  .then(res => res.text())
  .then(text => s = text)
Alternative implementation:
const res = await fetch(u)
s = await res.text()
Assign to the string s the name of the currently executing program (but not its full path).
var s = process.argv0;
Assign to string dir the path of the working directory.
(This is not necessarily the folder containing the executable itself)
let dir = process.cwd ()
Assign to string dir the path of the folder containing the currently running executable.
(This is not necessarily the working directory, though.)
const dir = path.resolve();
Alternative implementation:
const dir = __dirname;
Print the value of variable x, but only if x has been declared in this program.
This makes sense in some languages, not all of them. (Null values are not the point, rather the very existence of the variable.)
if (typeof x !== 'undefined') {
Alternative implementation:
try {
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
const blank = s == null || s.trim() === ''
From current process, run program x with command-line parameters "a", "b".
exec(`${x} a b`);
Print each key k with its value x from an associative array mymap, in ascending order of k.
[...mymap.entries()].sort().map(([_, x]) => console.log(x))
Print each key k with its value x from an associative array mymap, in ascending order of x.
Multiple entries may exist for the same value x.
  .sort((a, b) => a[1] - b[1])
  .forEach(([key, value]) => {
    console.log('key:', key, 'value:', value);
Set boolean b to true if objects x and y contain the same values, recursively comparing all referenced elements in x and y.
Tell if the code correctly handles recursive types.
const b = JSON.stringify(x) === JSON.stringify(y);
Alternative implementation:
const arrayDeepEqual = (a, b) => a.length === b.length && a.every((x, i) => deepEqual(x, b[i]))

const deepEqual = (a, b) =>
  Array.isArray(a) && Array.isArray(b)
    ? arrayDeepEqual(a, b)
    : typeof a == 'object' && a && typeof b == 'object' && b
    ? arrayDeepEqual(Object.entries(a), Object.entries(b))
    : Number.isNaN(a) && Number.isNaN(b) || a === b

const b = deepEqual(x, y)
Alternative implementation:
const b = isDeepStrictEqual(x, y)
Alternative implementation:
const b = _.isEqual(x, y);
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise.
let b = d1 < d2
Remove all occurrences of string w from string s1, and store the result in s2.
var regex = RegExp(w, 'g');
var s2 = s1.replace(regex, '');
Alternative implementation:
const s2 = s1.split(w).join('')
Set n to the number of elements of the list x.
var n = x.length;
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
var y = new Set(x);
Remove duplicates from the list x.
Explain if the original order is preserved.
x = [ Set(x)];
Alternative implementation:
x = Array.from(new Set(x));
Alternative implementation:
const seen = new Set();
x = x.filter( v => {
    return false;
  return true;
Read an integer value from the standard input into the variable n
const {createInterface} = require('readline')

const rl = createInterface ({
  input: process.stdin,
  output: process.stdout

rl.question('Input an integer: ', response => {
  let n = parseInt (response)
  // stuff to be done with n goes here

Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS.
const spades = 0
const hearts = 1
const diamonds = 2
const clubs = 3
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not.
Write the function binarySearch which returns the index of an element having the value x in the sorted array a, or -1 if no such element exists.
function binarySearch(a, x, i = 0) {
  if (a.length === 0) return -1
  const half = (a.length / 2) | 0
  return (a[half] === x) ?
    i + half :
    (a[half] > x) ?
    binarySearch(a.slice(0, half), x, i) :
    binarySearch(a.slice(half + 1), x, half + i + 1)
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration.
Write a function foo that returns a string and a boolean value.
const foo = () => ({string: 'string', bool: true})
Alternative implementation:
const foo = () => ['string', true];
Import the source code for the function foo body from a file "foobody.txt".
const foo = new Function(await readFile('foobody.txt'));
Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true.
Don't evaluate a condition when a previous condition was true.
c1 ? f1 : c2 ? f2 : f3
Alternative implementation:
switch (true) {
  case c1:
  case c2:
  case c3:
Alternative implementation:
if (c1) {
} else if (c2) {
} else if (c3) {
Run the procedure f, and return the duration of the execution of f.
function clock(f) {
  var start = new Date().getTime();
  var end = new Date().getTime();
  return end - start;
Alternative implementation:
function clock(f) {
  var start =;
  var end =;
  return end - start;
Set boolean ok to true if string word is contained in string s as a substring, even if the case doesn't match, or to false otherwise.
var lowerS = s.toLowerCase();
var lowerWord = word.toLowerCase();
var ok = lowerS.indexOf(lowerWord) !== -1;
Declare and initialize a new list items, containing 3 elements a, b, c.
const items = [a, b, c];
Alternative implementation:
const items = new Array(a, b, c);
Remove at most 1 item from list items, having the value x.
This will alter the original list or return a new list, depending on which is more idiomatic.
If there are several occurrences of x in items, remove only one of them. If x is absent, keep items unchanged.
const idx = items.indexOf(x)
if (idx !== -1) items.splice(idx, 1)
Remove all occurrences of the value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
const newlist = items.filter(y => x !== y)
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
var b = /^[0-9]+$/.test(s);
Create a new temporary file on the filesystem.
const tempFile = tempy.file()
Alternative implementation:
const resultOfCallback = tempy.file.task(tempFile => {
 // do something with tempFile
Create a new temporary folder on filesystem, for writing.
const tempDir = await Deno.makeTempDir();
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m.
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element.
for (let item of items1) console.log(item)
for (let item of items2) console.log(item)
Alternative implementation:
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
const s = x.toString(16)
Iterate alternatively over the elements of the lists items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size.
const shorter = _items1.length > _items2.length ? _items2 : _items1;
const longer = _items1.length <= _items2.length ? _items2 : _items1;, i) => {
Alternative implementation:
const iterator1 = items1[Symbol.iterator]()
const iterator2 = items2[Symbol.iterator]()

let result1 =
let result2 =

while(!(result1.done && result2.done)) {
  if (!result1.done) {
    result1 =
  if (!result2.done) {
    result2 =
Set boolean b to true if file at path fp exists on filesystem; false otherwise.

Beware that you should never do this and then in the next instruction assume the result is still valid, this is a race condition on any multitasking OS.
const b = fs.existsSync(fp);
Alternative implementation:
let b = true;
try {
	await access(fp);
} catch {
	b = false;
Alternative implementation:
try {
} catch(_e) {console.error("File does not exist.")}
Print message msg, prepended by current date and time.

Explain what behavior is idiomatic: to stdout or stderr, and what the date format is.
console.log(Date(), msg);
Alternative implementation:
console.error(Date(), msg);
Extract floating point value f from its string representation s
let f = +s
Create string t from string s, keeping only ASCII characters
const t = [...s].filter(c => c.charCodeAt(0) <= 0x7f).join('')
Alternative implementation:
const t = s.replace(/[^\x00-\x7f]/gu, '')
Read a list of integer numbers from the standard input, until EOF.
process.stdin.on('data', processLine)

function processLine (line) {
  const string = line + ''
Alternative implementation:
const ints = await new Promise(resolve => {
  let string = ''
    .on('data', data => string += data.toString())
    .on('close', () => resolve(string.split('\n').map(line => Number.parseInt(line))))
As an exception, this content is not under license CC BY-SA 3.0 like the rest of this website.
Remove the last character from the string p, if this character is a forward slash /
const slashscrape = p => (
  p.slice (-1) === '/' ?
    p.slice (0, -1) :
Remove last character from string p, if this character is the file path separator of current platform.

Note that this also transforms unix root path "/" into the empty string!
p = p.endsWith(path.sep) ? p.slice(0, -path.sep.length) : p
Create string s containing only the character c.
let s = c
Create the string t as the concatenation of the string s and the integer i.
let t = s + i;
Alternative implementation:
let t = `${s}${i}`
Find color c, the average between colors c1, c2.

c, c1, c2 are strings of hex color codes: 7 chars, beginning with a number sign # .
Assume linear computations, ignore gamma corrections.
var c = "#";
for(var i = 0; i<3; i++) {
  var sub1 = c1.substring(1+2*i, 3+2*i);
  var sub2 = c2.substring(1+2*i, 3+2*i);
  var v1 = parseInt(sub1, 16);
  var v2 = parseInt(sub2, 16);
  var v = Math.floor((v1 + v2) / 2);
  var sub = v.toString(16).toUpperCase();
  var padsub = ('0'+sub).slice(-2);
  c += padsub;
Alternative implementation:
c = "#" + (() => {
  const [p1, p2] = [c1, c2].map((color) => parseInt(color.slice(1), 16)),
    a = [];

  for (let i = 0; i <= 2; i += 1) {
    a.push(Math.floor(((p1 >> (i * 8) & 0xff) + (p2 >> (i * 8) & 0xff)) / 2));

  return a.reverse().map((num) => num.toString(16).padStart(2, "0")).join("");
Delete from filesystem the file having path filepath.
try {
} catch (err) {
Alternative implementation:
await unlink(filepath)
Alternative implementation:
Deno.remove(filepath, { recursive: true }).catch((err) => console.error(err));
Assign to the string s the value of the integer i in 3 decimal digits. Pad with zeros if i < 100. Keep all digits if i1000.
let s = i.toString();
  s = ('00'+i).slice(-3);
Initialize a constant planet with string value "Earth".
const planet = 'Earth'
Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not.
const idx =, i) => i);
while (y.length < k) {
  const i = parseInt(Math.random() * idx.length, 10);
  idx.splice(i, 1);
Execute f32() if platform is 32-bit, or f64() if platform is 64-bit.
This can be either a compile-time condition (depending on target) or a runtime detection.
const is64Bit = os.arch() === 'x64' || process.env.hasOwnProperty('PROCESSOR_ARCHITEW6432');
is64Bit ? _f64() : _f32();
Multiply all the elements of the list elements by a constant c
elements = => x * c)
execute bat if b is a program option and fox if f is a program option.
const args = process.argv.slice(2)
if (args.includes('b')) bat()
else if (args.includes('f')) fox()
Print all the list elements, two by two, assuming list length is even.
for (let index = 0; index < list.length; index = index + 2) {
  console.log(list[index], list[index + 1])
Assign to the variable x the last element of the list items.
const x = items[items.length - 1];
Alternative implementation:
const x =;
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
var ab = a.concat(b);
Create the string t consisting of the string s with its prefix p removed (if s starts with p).
let t = s.startsWith(p) ? s.substring(p.length) : s;
Create string t consisting of string s with its suffix w removed (if s ends with w).
const t = s.endsWith(w) ? s.slice(0, -w.length) : s
Assign to the integer n the number of characters of the string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s.
const n = s.length;
Alternative implementation:
const n = [...s].length;
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity.
const n = mymap.size
Append the element x to the list s.
Alternative implementation:
s = [...s, x];
Insert value v for key k in map m.
m.set(k, v);
Alternative implementation:
m[k] = v;
Number will be formatted with a comma separator between every group of thousands.
new Intl.NumberFormat().format(1000);
Make a HTTP request with method POST to the URL u
fetch(u, {
        method: "POST",
	body: JSON.stringify(data)
From array a of n bytes, build the equivalent hex string s of 2n digits.
Each byte (256 possible values) is encoded as two hexadecimal characters (16 possible values per digit).
const s = Buffer.from(a).toString('hex')
Alternative implementation:
const s = => n.toString(16).padStart(2, '0')).join('')
From hex string s of 2n digits, build the equivalent array a of n bytes.
Each pair of hexadecimal characters (16 possible values per digit) is decoded into one byte (256 possible values).
  .map((el, ix, arr) => ix % 2 ? null : el + arr[ix + 1])
  .filter(el => el !== null)
  .map(x => parseInt(x, 16))
Set boolean b to true if if the point with coordinates (x,y) is inside the rectangle with coordinates (x1,y1,x2,y2) , or to false otherwise.
Describe if the edges are considered to be inside the rectangle.
const pointInRect = ({x1, y1, x2, y2}, {x, y}) => (
  (x > x1 && x < x2) && (y > y1 && y < y2)
Return the center c of the rectangle with coördinates(x1,y1,x2,y2)
const center = ({x1, y1, x2, y2}) => ({x: (x1 + x2) / 2, y: (y1 + y2) / 2})
Alternative implementation:
class Point {
  constructor (x, y) {
    this.x = x
    this.y = y
const center = ({x1, y1, x2, y2}) => new Point ((x1 + x2) / 2, (y1 + y2) / 2)
Create the list x containing the contents of the directory d.

x may contain files and subfolders.
No recursive subfolder listing.
const x = fs.readdirSync(d)
Output the source of the program.
Make a HTTP request with method PUT to the URL u
fetch(u, {
        method: "PUT",
	body: JSON.stringify(data)
Assign to variable t a string representing the day, month and year of the day after the current date.
var nextDate = new Date(new Date().getTime() + 24 * 60 * 60 * 1000);
var day = nextDate.getDate()
var month = nextDate.getMonth() + 1
var year = nextDate.getFullYear()
var t = `${day}/${month}/${year}`;
Alternative implementation:
var tomorrow = new Date();
tomorrow.setDate(tomorrow.getDate() + 1);
Alternative implementation:
var now = new Date()
var year = now.getFullYear()
var month = now.getMonth()
var day = now.getDate()

var tomorrow = new Date(0)
tomorrow.setFullYear(year, month, day + 1)
tomorrow.setHours(0, 0, 0, 0)

var shortDateFormat = Intl.DateTimeFormat(undefined, { dateStyle: "short" })
var t = shortDateFormat.format(tomorrow)
Schedule the execution of f(42) in 30 seconds.
let id = setTimeout(f, 30000, 42);
Exit a program cleanly indicating no error to OS
Produce a new list y containing the result of the function T applied to all elements e of the list x that match the predicate P.
y = x.filter(e => P(e)).map(e => T(e))
Given a one-dimensional array a, check if any value is larger than x, and execute the procedure f if that is the case
if(a.some(item => item > x)){
Declare two two-dimensional arrays a and b of dimension n*m and m*n, respectively. Assign to b the transpose of a (i.e. the value with index interchange).
const a = [[1, 2, 3], [4, 5, 6]]
const m = a[0].length
const b = Array.from({ length: m }, (_, n) => => row[n]))
Pass an array a of real numbers to the procedure (resp. function) foo. Output the size of the array, and the sum of all its elements when each element is multiplied with the array indices i and j (assuming they start from one).
 * @param {Array<Array>} arry
 * @return {Array<Array>}
function foo(arry) {
  let len = 0;
  let sum = 0;

  arry.forEach(function(base, i) {
    len += base.length;

    base.forEach(function(a, j) {
      sum += a * (i + 1) * (j + 1);

  console.log('Array size:', arry.length, ',', len);

  return sum;

Retrieve the contents of file at path into a list of strings lines, in which each element is a line of the file.
Abort program execution with error condition x (where x is an integer value)
Returns the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y.
var h = Math.sqrt(x*x + y*y);
Alternative implementation:
const h = Math.hypot(x, y);
Calculate n, the Euclidean norm of data (an array or list of floating point values).
const n = Math.hypot(
Alternative implementation:
var n = Math.hypot.apply(null, data)
Calculate the sum of squares s of data, an array of floating point values.
s = data.reduce((a, c) => a + c ** 2, 0)
Given a real number a, print the fractional part and the exponent of the internal representation of that number. For 3.14, this should print (approximately)

0.785 2
function frexp(a) {
    exponent = ( Math.floor(Math.log(a, 2)) + 1 )
    mantissa = ( a * Math.pow(2, -a) )

    return [ mantissa, exponent ]
Read an environment variable with the name "FOO" and assign it to the string variable foo. If it does not exist or if the system does not support environment variables, assign a value of "none".
const foo = process.env["FOO"] || "none";
Alternative implementation:
const foo = process.env.FOO ?? 'none'
Execute different procedures foo, bar, baz and barfl if the string str contains the name of the respective procedure. Do it in a way natural to the language.
switch (str) {
  case "foo":
  case "bar":
  case "baz":
  case "barfl":
Given the arrays a,b,c,d of equal length and the scalar e, calculate a = e*(a+b*c+cos(d)).
Store the results in a.
a.forEach((aa, i) => a[i] = e * (aa + b[i] * c[i] + Math.cos(d[i])))
Assign, at runtime, the compiler version and the options the program was compilerd with to variables version and options, respectively, and print them. For interpreted languages, substitute the version of the interpreter.

Example output:

GCC version 10.0.0 20190914 (experimental)
-mtune=generic -march=x86-64
const { version } = process;
Create the folder at path on the filesystem
await mkdir(path);
Append extra character c at the end of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
s = s.padEnd(m, c);
Prepend extra character c at the beginning of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
s = s.padStart(m, c);
Create the list c containing all unique elements that are contained in both lists a and b.
c should not contain any duplicates, even if a and b do.
The order of c doesn't matter.
const c = [ Set(a)].filter(e => b.includes(e));
Create the string t from the value of string s with each sequence of spaces replaced by a single space.

Explain if only the space characters will be replaced, or the other whitespaces as well: tabs, newlines.
let t = s.replace(/\s+/g, ' ');
Create t consisting of 3 values having different types.

Explain if the elements of t are strongly typed or not.
let t = [2.5, "hello", -1];
Create string t from string s, keeping only digit characters 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
t = s.replace(/[^\d]/gm,"");
Set i to the first index in list items at which the element x can be found, or -1 if items does not contain x.
let i = items.indexOf(x);
Loop through list items checking a condition. Do something else if no matches are found.

A typical use case is looping through a series of containers looking for one that matches a condition. If found, an item is inserted; otherwise, a new container is created.

These are mostly used as an inner nested loop, and in a location where refactoring inner logic into a separate function reduces clarity.
const found = items.some(condition);

if (!found) doSomethingElse();
Insert the element x at the beginning of the list items.
Alternative implementation:
items = [x, ...items];
Declare an optional integer argument x to procedure f, printing out "Present" and its value if it is present, "Not present" otherwise
function f(x) {
	console.log(x ? `Present: ${x}` : 'Not present');
Remove the last element from the list items.
Create the new list y containing the same elements as the list x.

Subsequent modifications of y must not affect x (except for the contents referenced by the elements themselves if they contain pointers).
let y = x.slice();
Copy the file at path src to dst.
copyFileSync(src, dst);
Print "verbose is true" if the flag -v was passed to the program command line, "verbose is false" otherwise.
const verbose = process.argv.includes('-v');
console.log('verbose is', verbose);
Assign to the string s the standard base64 encoding of the byte array data, as specified by RFC 4648.
let s = btoa(data);
Assign to byte array data the bytes represented by the base64 string s, as specified by RFC 4648.
var data = atob(s)
Assign to c the result of (a xor b)
const c = a ^ b;
Write in a new byte array c the xor result of byte arrays a and b.

a and b have the same size.
const c = Uint8Array.from(a, (v, i) => v ^ b[i])
Assign to string x the first word of string s consisting of exactly 3 digits, or the empty string if no such match exists.

A word containing more digits, or 3 digits as a substring fragment, must not match.
const matches = s.match(/\b\d{3}\b/);
const x = matches ? matches[0] : '';
Call a function f on each element e of a set x.
for (const e of x) {
Alternative implementation:
let v = x.values();
let result =;
while (!result.done) {
  result =;
Alternative implementation:
Print the contents of the list or array a on the standard output.
Alternative implementation:
Print the contents of the map m to the standard output: keys and values.
Alternative implementation:
Print the value of object x having custom type T, for log or debug.
Set c to the number of distinct elements in the list items.
const c = new Set(items).size;
Remove all the elements from list x that don't satisfy the predicate p, without allocating a new list.
Keep all the elements that do satisfy p.

For languages that don't have mutable lists, refer to idiom #57 instead.
for (const [key, value] of x.entries()) {
	if (!p(value)) x.splice(key, 1);
Alternative implementation:
x = x.filter((e) => p(e));
Define variables a, b and c in a concise way.
Explain if they need to have the same type.
const [a, b, c] = [42, "hello", 5.0];
Choose a value x from map m.
m must not be empty. Ignore the keys.
// Objects
const values = Object.values(m);

// Maps
const values = [...m.values()];

const x = values[~~(Math.random() * values.length)]
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13
const i = parseInt(s, 2)
Assign to the variable x the string value "a" if calling the function condition returns true, or the value "b" otherwise.
const x = condition() ? 'a' : 'b';
Print the stack frames of the current execution thread of the program.
Replace all exact occurrences of "foo" with "bar" in the string list x
x = => e === 'foo' ? 'bar' : e);
Print the values of the set x to the standard output.
The order of the elements is irrelevant and is not required to remain the same next time.
Print the numbers 5, 4, ..., 0 (included), one line per number.
for (let i = 5; i >= 0; i--) {
Print each index i and value x from the list items, from the last down to the first.
[...items].reverse().forEach((item, index) => 
console.log(Math.abs(index -= items.length), item));
Convert the string values from list a into a list of integers b.
let b =
Build the list parts consisting of substrings of the input string s, separated by any of the characters ',' (comma), '-' (dash), '_' (underscore).
var parts = s.split(/[-_,]/)
Declare a new list items of string elements, containing zero elements
let items = [];
Assign to the string x the value of fields (hours, minutes, seconds) of the date d, in format HH:MM:SS.
const d = new Date();

let hr = d.getHours();
let min = d.getMinutes();
let sec = d.getSeconds();

if ( hr.toString().length === 1 ) {
  hr = '0' + hr;

if ( min.toString().length === 1 ) {
  min = '0' + min;

if ( sec.toString().length === 1 ) {
  sec = '0' + sec;

const x = '' + hr + ':' + min + ':' + sec;
Calculate the parity p of the integer variable i : 0 if it contains an even number of bits set, 1 if it contains an odd number of bits set.
let i = 42
  .reduce((parity, bit) => parity ^ bit, 0)
Assign to the string s the value of the string v repeated n times, and write it out.

E.g. v="abc", n=5 ⇒ s="abcabcabcabcabc"
const s = v.repeat(n)
Declare an argument x to a procedure foo that can be of any type. If the type of the argument is a string, print it, otherwise print "Nothing."

Test by passing "Hello, world!" and 42 to the procedure.
function foo(x) {
  console.log(typeof x == 'string' ? x : 'Nothing.')

foo('Hello, world!')
Fizz buzz is a children's counting game, and a trivial programming task used to affirm that a programmer knows the basics of a language: loops, conditions and I/O.

The typical fizz buzz game is to count from 1 to 100, saying each number in turn. When the number is divisible by 3, instead say "Fizz". When the number is divisible by 5, instead say "Buzz". When the number is divisible by both 3 and 5, say "FizzBuzz"
for (let i = 1; i <= 100; i++) {
    let out = "";
    if (!(i % 3)) out += "Fizz";
    if (!(i % 5)) out += "Buzz";
    if (!out) out = i;
Create the string t from the string s, removing all the spaces, newlines, tabulations, etc.
let t = s.replace(/\s/g,'');
Insert an element e into the set x.
Remove the element e from the set x.

Explains what happens if e was already absent from x.
let letters = new Set(["a","b","c"]);
console.log([...letters].join(', '));
Build the list parts consisting of substrings of input string s, separated by the string sep.
const parts = s.split(sep);
Create a new list a (or array, or slice) of size n, where all elements are integers initialized with the value 0.
const a = new Array(n).fill(0);
Print a line "Char i is c" for each character c of the string s, where i is the character index of c in s (not the byte index).

Make sure that multi-byte characters are properly handled, and count for a single character.
for (const [i, c] of [...s].entries()) {
	console.log(`Char ${i} is ${c}`);
Set the boolean b to true if the set x contains the element e, false otherwise.
let b = x.has(e);
Create the string s by concatenating the strings a and b.
const s = a + b;
Create a new stack s, push an element x, then pop the element into the variable y.
const s = [1, 2, 3];
const y = s.pop();
Write a line of comments.

This line will not be compiled or executed.
// This is a comment
Given the integer x = 8, assign to the string s the value "Our sun has 8 planets", where the number 8 was evaluated from x.
let s = `Our sun has ${x} planets`;
Create the array of bytes data by encoding the string s in UTF-8.
const data = new TextEncoder().encode(s);
Create the string representation s of the integer value n in base b.

18 in base 3 -> "200"
26 in base 5 -> "101"
121 in base 12 -> "a1"

let s = n.toString(b);
Create the new object y by cloning the all the contents of x, recursively.
let y = structuredClone(x);
Determine the number c of elements in the list x that satisfy the predicate p.
let c = x.filter(p).length
Create a string s of n characters having uniform random values out of the 62 alphanumeric values A-Z, a-z, 0-9
const s = ((n) => {
    const alphanum = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
    let s = "";
    for (let i = 0; i < n; i += 1) {
        s += alphanum[~~(Math.random() * alphanum.length)];
    return s;
Assign to t the value of the string s, with all letters mapped to their lower case.
let t = s.toLowerCase();
Assign to t the value of the string s, with all letters mapped to their upper case.
let t = s.toUpperCase();
Assign to v the value stored in the map m for the key k.

Explain what happens if there is no entry for k in m.
v = m.get(k);
Remove all entries from the map m.

Explain if other references to the same map now see an empty map as well.
Create the new map c containing all of the (key, value) entries of the two maps a and b.

Explain what happens for keys existing in both a and b.
let c = {...a, ...b}