Develop Reference

Finding the sum of even-valued terms in a Fibonacci sequence where the sum of those terms does not exceed 5 million

the code below is set to perform the sum of even valued terms in a Fibonacci sequence. The first part creates the terms so the first 2 ones and the addition from that. The second part creates an array that stores all these terms. The third part seeks out only the even terms and adds them and a condition is added to only do such for those less than 5 million.
//The below creates Fibonacci sequence as we know it
var memo = [0, 1];
function fib (n) {
if(memo.length-1 < n) {
memo[n] = fib(n-1)+fib(n-2);
}
return memo[n];
}
// creates array with elements from function that takes the index as argument while a given condition holds
function takeWhile (fromFunc, cond, arr=[]) {
var n = arr.length;
var val = fromFunc(n);
if(cond(val)) {
arr.push(val);
return takeWhile(fromFunc, cond, arr);
}
return arr;
}
//adds all the numbers in the sequence below 5 million
var sum =
takeWhile(fib, n => n < 5000000)
//the below line makes sure that only the even numbers in the sequence are added
.filter(n => n%2==0)
.reduce((acc, c) => acc+c, 0);
console.log(sum);

You need to take the index, not the value for filtering.
.filter((_, i) => i % 2 == 0)
function fib(n) {
if (memo.length - 1 < n) memo[n] = fib(n - 1) + fib(n - 2);
return memo[n];
}
function takeWhile(fromFunc, cond, arr = []) {
var val = fromFunc(arr.length);
if (!cond(val)) return arr;
arr.push(val);
return takeWhile(fromFunc, cond, arr);
}
var memo = [0, 1],
sum = takeWhile(fib, n => n < 5)
.filter((_, i) => i % 2 == 0)
.reduce((acc, c) => acc + c, 0);
console.log(sum);
console.log(...memo);

How to display value from object in a random only once [duplicate]

I have an array like this:
var arr1 = ["a", "b", "c", "d"];
How can I randomize / shuffle it?

The de-facto unbiased shuffle algorithm is the Fisher-Yates (aka Knuth) Shuffle.
You can see a great visualization here (and the original post linked to this)
function shuffle(array) {
let currentIndex = array.length, randomIndex;
// While there remain elements to shuffle.
while (currentIndex != 0) {
// Pick a remaining element.
randomIndex = Math.floor(Math.random() * currentIndex);
currentIndex--;
// And swap it with the current element.
[array[currentIndex], array[randomIndex]] = [
array[randomIndex], array[currentIndex]];
}
return array;
}
// Used like so
var arr = [2, 11, 37, 42];
shuffle(arr);
console.log(arr);
Some more info about the algorithm used.

Here's a JavaScript implementation of the Durstenfeld shuffle, an optimized version of Fisher-Yates:
/* Randomize array in-place using Durstenfeld shuffle algorithm */
function shuffleArray(array) {
for (var i = array.length - 1; i > 0; i--) {
var j = Math.floor(Math.random() * (i + 1));
var temp = array[i];
array[i] = array[j];
array[j] = temp;
}
}
It picks a random element for each original array element, and excludes it from the next draw, like picking randomly from a deck of cards.
This clever exclusion swaps the picked element with the current one, then picks the next random element from the remainder, looping backwards for optimal efficiency, ensuring the random pick is simplified (it can always start at 0), and thereby skipping the final element.
Algorithm runtime is O(n). Note that the shuffle is done in-place so if you don't want to modify the original array, first make a copy of it with .slice(0).
EDIT: Updating to ES6 / ECMAScript 2015
The new ES6 allows us to assign two variables at once. This is especially handy when we want to swap the values of two variables, as we can do it in one line of code. Here is a shorter form of the same function, using this feature.
function shuffleArray(array) {
for (let i = array.length - 1; i > 0; i--) {
const j = Math.floor(Math.random() * (i + 1));
[array[i], array[j]] = [array[j], array[i]];
}
}

You can do it easily with map and sort:
let unshuffled = ['hello', 'a', 't', 'q', 1, 2, 3, {cats: true}]
let shuffled = unshuffled
.map(value => ({ value, sort: Math.random() }))
.sort((a, b) => a.sort - b.sort)
.map(({ value }) => value)
console.log(shuffled)
We put each element in the array in an object, and give it a random sort key
We sort using the random key
We unmap to get the original objects
You can shuffle polymorphic arrays, and the sort is as random as Math.random, which is good enough for most purposes.
Since the elements are sorted against consistent keys that are not regenerated each iteration, and each comparison pulls from the same distribution, any non-randomness in the distribution of Math.random is canceled out.
Speed
Time complexity is O(N log N), same as quick sort. Space complexity is O(N). This is not as efficient as a Fischer Yates shuffle but, in my opinion, the code is significantly shorter and more functional. If you have a large array you should certainly use Fischer Yates. If you have a small array with a few hundred items, you might do this.

Warning!
The use of this algorithm is not recommended, because it is inefficient and strongly biased; see comments. It is being left here for future reference, because the idea is not that rare.
[1,2,3,4,5,6].sort( () => .5 - Math.random() );
This https://javascript.info/array-methods#shuffle-an-array tutorial explains the differences straightforwardly.

Use the underscore.js library. The method _.shuffle() is nice for this case.
Here is an example with the method:
var _ = require("underscore");
var arr = [1,2,3,4,5,6];
// Testing _.shuffle
var testShuffle = function () {
var indexOne = 0;
var stObj = {
'0': 0,
'1': 1,
'2': 2,
'3': 3,
'4': 4,
'5': 5
};
for (var i = 0; i < 1000; i++) {
arr = _.shuffle(arr);
indexOne = _.indexOf(arr, 1);
stObj[indexOne] ++;
}
console.log(stObj);
};
testShuffle();

One could (but should NOT) use it as a protoype from Array:
From ChristopheD:
Array.prototype.shuffle = function() {
var i = this.length, j, temp;
if ( i == 0 ) return this;
while ( --i ) {
j = Math.floor( Math.random() * ( i + 1 ) );
temp = this[i];
this[i] = this[j];
this[j] = temp;
}
return this;
}

NEW!
Shorter & probably *faster Fisher-Yates shuffle algorithm
it uses while---
bitwise to floor (numbers up to 10 decimal digits (32bit))
removed unecessary closures & other stuff
function fy(a,b,c,d){//array,placeholder,placeholder,placeholder
c=a.length;while(c)b=Math.random()*(--c+1)|0,d=a[c],a[c]=a[b],a[b]=d
}
script size (with fy as function name): 90bytes
DEMO
http://jsfiddle.net/vvpoma8w/
*faster probably on all browsers except chrome.
If you have any questions just ask.
EDIT
yes it is faster
PERFORMANCE: http://jsperf.com/fyshuffle
using the top voted functions.
EDIT
There was a calculation in excess (don't need --c+1) and noone noticed
shorter(4bytes)&faster(test it!).
function fy(a,b,c,d){//array,placeholder,placeholder,placeholder
c=a.length;while(c)b=Math.random()*c--|0,d=a[c],a[c]=a[b],a[b]=d
}
Caching somewhere else var rnd=Math.random and then use rnd() would also increase slightly the performance on big arrays.
http://jsfiddle.net/vvpoma8w/2/
Readable version (use the original version. this is slower, vars are useless, like the closures & ";", the code itself is also shorter ... maybe read this How to 'minify' Javascript code , btw you are not able to compress the following code in a javascript minifiers like the above one.)
function fisherYates( array ){
var count = array.length,
randomnumber,
temp;
while( count ){
randomnumber = Math.random() * count-- | 0;
temp = array[count];
array[count] = array[randomnumber];
array[randomnumber] = temp
}
}

Shuffle Array In place
function shuffleArr (array){
for (var i = array.length - 1; i > 0; i--) {
var rand = Math.floor(Math.random() * (i + 1));
[array[i], array[rand]] = [array[rand], array[i]]
}
}
ES6 Pure, Iterative
const getShuffledArr = arr => {
const newArr = arr.slice()
for (let i = newArr.length - 1; i > 0; i--) {
const rand = Math.floor(Math.random() * (i + 1));
[newArr[i], newArr[rand]] = [newArr[rand], newArr[i]];
}
return newArr
};
Reliability and Performance Test
Some solutions on this page aren't reliable (they only partially randomise the array). Other solutions are significantly less efficient. With testShuffleArrayFun (see below) we can test array shuffling functions for reliability and performance.
function testShuffleArrayFun(getShuffledArrayFun){
const arr = [0,1,2,3,4,5,6,7,8,9]
var countArr = arr.map(el=>{
return arr.map(
el=> 0
)
}) // For each possible position in the shuffledArr and for
// each possible value, we'll create a counter.
const t0 = performance.now()
const n = 1000000
for (var i=0 ; i<n ; i++){
// We'll call getShuffledArrayFun n times.
// And for each iteration, we'll increment the counter.
var shuffledArr = getShuffledArrayFun(arr)
shuffledArr.forEach(
(value,key)=>{countArr[key][value]++}
)
}
const t1 = performance.now()
console.log(`Count Values in position`)
console.table(countArr)
const frequencyArr = countArr.map( positionArr => (
positionArr.map(
count => count/n
)
))
console.log("Frequency of value in position")
console.table(frequencyArr)
console.log(`total time: ${t1-t0}`)
}
Other Solutions
Other solutions just for fun.
ES6 Pure, Recursive
const getShuffledArr = arr => {
if (arr.length === 1) {return arr};
const rand = Math.floor(Math.random() * arr.length);
return [arr[rand], ...getShuffledArr(arr.filter((_, i) => i != rand))];
};
ES6 Pure using array.map
function getShuffledArr (arr){
return [...arr].map( (_, i, arrCopy) => {
var rand = i + ( Math.floor( Math.random() * (arrCopy.length - i) ) );
[arrCopy[rand], arrCopy[i]] = [arrCopy[i], arrCopy[rand]]
return arrCopy[i]
})
}
ES6 Pure using array.reduce
function getShuffledArr (arr){
return arr.reduce(
(newArr, _, i) => {
var rand = i + ( Math.floor( Math.random() * (newArr.length - i) ) );
[newArr[rand], newArr[i]] = [newArr[i], newArr[rand]]
return newArr
}, [...arr]
)
}

Edit: This answer is incorrect
See comments and https://stackoverflow.com/a/18650169/28234. It is being left here for reference because the idea isn't rare.
A very simple way for small arrays is simply this:
const someArray = [1, 2, 3, 4, 5];
someArray.sort(() => Math.random() - 0.5);
It's probably not very efficient, but for small arrays this works just fine. Here's an example so you can see how random (or not) it is, and whether it fits your usecase or not.
const resultsEl = document.querySelector('#results');
const buttonEl = document.querySelector('#trigger');
const generateArrayAndRandomize = () => {
const someArray = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
someArray.sort(() => Math.random() - 0.5);
return someArray;
};
const renderResultsToDom = (results, el) => {
el.innerHTML = results.join(' ');
};
buttonEl.addEventListener('click', () => renderResultsToDom(generateArrayAndRandomize(), resultsEl));
<h1>Randomize!</h1>
<button id="trigger">Generate</button>
<p id="results">0 1 2 3 4 5 6 7 8 9</p>

Adding to #Laurens Holsts answer. This is 50% compressed.
function shuffleArray(d) {
for (var c = d.length - 1; c > 0; c--) {
var b = Math.floor(Math.random() * (c + 1));
var a = d[c];
d[c] = d[b];
d[b] = a;
}
return d
};

I found this variant hanging out in the "deleted by author" answers on a duplicate of this question. Unlike some of the other answers that have many upvotes already, this is:
Actually random
Not in-place (hence the shuffled name rather than shuffle)
Not already present here with multiple variants
Here's a jsfiddle showing it in use.
Array.prototype.shuffled = function() {
return this.map(function(n){ return [Math.random(), n] })
.sort().map(function(n){ return n[1] });
}

With ES2015 you can use this one:
Array.prototype.shuffle = function() {
let m = this.length, i;
while (m) {
i = (Math.random() * m--) >>> 0;
[this[m], this[i]] = [this[i], this[m]]
}
return this;
}
Usage:
[1, 2, 3, 4, 5, 6, 7].shuffle();

Warning!
Using this answer for randomizing large arrays, cryptography, or any other application requiring true randomness is not recommended, due to its bias and inefficiency. Elements position is only semi-randomized, and they will tend to stay closer to their original position. See https://stackoverflow.com/a/18650169/28234.
You can arbitrarily decide whether to return 1 : -1 by using Math.random:
[1, 2, 3, 4].sort(() => (Math.random() > 0.5) ? 1 : -1)
Try running the following example:
const array = [1, 2, 3, 4];
// Based on the value returned by Math.Random,
// the decision is arbitrarily made whether to return 1 : -1
const shuffeled = array.sort(() => {
const randomTrueOrFalse = Math.random() > 0.5;
return randomTrueOrFalse ? 1 : -1
});
console.log(shuffeled);

//one line solution
shuffle = (array) => array.sort(() => Math.random() - 0.5);
//Demo
let arr = [1, 2, 3];
shuffle(arr);
alert(arr);
https://javascript.info/task/shuffle
Math.random() - 0.5 is a random number that may be positive or
negative, so the sorting function reorders elements randomly.

benchmarks
Let's first see the results then we'll look at each implementation of shuffle below -
splice is slow
Any solution using splice or shift in a loop is going to be very slow. Which is especially noticeable when we increase the size of the array. In a naive algorithm we -
get a rand position, i, in the input array, t
add t[i] to the output
splice position i from array t
To exaggerate the slow effect, we'll demonstrate this on an array of one million elements. The following script almost 30 seconds -
const shuffle = t =>
Array.from(sample(t, t.length))
function* sample(t, n)
{ let r = Array.from(t)
while (n > 0 && r.length)
{ const i = rand(r.length) // 1
yield r[i] // 2
r.splice(i, 1) // 3
n = n - 1
}
}
const rand = n =>
0 | Math.random() * n
function swap (t, i, j)
{ let q = t[i]
t[i] = t[j]
t[j] = q
return t
}
const size = 1e6
const bigarray = Array.from(Array(size), (_,i) => i)
console.time("shuffle via splice")
const result = shuffle(bigarray)
console.timeEnd("shuffle via splice")
document.body.textContent = JSON.stringify(result, null, 2)
body::before {
content: "1 million elements via splice";
font-weight: bold;
display: block;
}
pop is fast
The trick is not to splice and instead use the super efficient pop. To do this, in place of the typical splice call, you -
select the position to splice, i
swap t[i] with the last element, t[t.length - 1]
add t.pop() to the result
Now we can shuffle one million elements in less than 100 milliseconds -
const shuffle = t =>
Array.from(sample(t, t.length))
function* sample(t, n)
{ let r = Array.from(t)
while (n > 0 && r.length)
{ const i = rand(r.length) // 1
swap(r, i, r.length - 1) // 2
yield r.pop() // 3
n = n - 1
}
}
const rand = n =>
0 | Math.random() * n
function swap (t, i, j)
{ let q = t[i]
t[i] = t[j]
t[j] = q
return t
}
const size = 1e6
const bigarray = Array.from(Array(size), (_,i) => i)
console.time("shuffle via pop")
const result = shuffle(bigarray)
console.timeEnd("shuffle via pop")
document.body.textContent = JSON.stringify(result, null, 2)
body::before {
content: "1 million elements via pop";
font-weight: bold;
display: block;
}
even faster
The two implementations of shuffle above produce a new output array. The input array is not modified. This is my preferred way of working however you can increase the speed even more by shuffling in place.
Below shuffle one million elements in less than 10 milliseconds -
function shuffle (t)
{ let last = t.length
let n
while (last > 0)
{ n = rand(last)
swap(t, n, --last)
}
}
const rand = n =>
0 | Math.random() * n
function swap (t, i, j)
{ let q = t[i]
t[i] = t[j]
t[j] = q
return t
}
const size = 1e6
const bigarray = Array.from(Array(size), (_,i) => i)
console.time("shuffle in place")
shuffle(bigarray)
console.timeEnd("shuffle in place")
document.body.textContent = JSON.stringify(bigarray, null, 2)
body::before {
content: "1 million elements in place";
font-weight: bold;
display: block;
}

var shuffle = function(array) {
temp = [];
originalLength = array.length;
for (var i = 0; i < originalLength; i++) {
temp.push(array.splice(Math.floor(Math.random()*array.length),1));
}
return temp;
};

A recursive solution:
function shuffle(a,b){
return a.length==0?b:function(c){
return shuffle(a,(b||[]).concat(c));
}(a.splice(Math.floor(Math.random()*a.length),1));
};

Fisher-Yates shuffle in javascript. I'm posting this here because the use of two utility functions (swap and randInt) clarifies the algorithm compared to the other answers here.
function swap(arr, i, j) {
// swaps two elements of an array in place
var temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
function randInt(max) {
// returns random integer between 0 and max-1 inclusive.
return Math.floor(Math.random()*max);
}
function shuffle(arr) {
// For each slot in the array (starting at the end),
// pick an element randomly from the unplaced elements and
// place it in the slot, exchanging places with the
// element in the slot.
for(var slot = arr.length - 1; slot > 0; slot--){
var element = randInt(slot+1);
swap(arr, element, slot);
}
}

Modern short inline solution using ES6 features:
['a','b','c','d'].map(x => [Math.random(), x]).sort(([a], [b]) => a - b).map(([_, x]) => x);
(for educational purposes)

Using Fisher-Yates shuffle algorithm and ES6:
// Original array
let array = ['a', 'b', 'c', 'd'];
// Create a copy of the original array to be randomized
let shuffle = [...array];
// Defining function returning random value from i to N
const getRandomValue = (i, N) => Math.floor(Math.random() * (N - i) + i);
// Shuffle a pair of two elements at random position j
shuffle.forEach( (elem, i, arr, j = getRandomValue(i, arr.length)) => [arr[i], arr[j]] = [arr[j], arr[i]] );
console.log(shuffle);
// ['d', 'a', 'b', 'c']

First of all, have a look here for a great visual comparison of different sorting methods in javascript.
Secondly, if you have a quick look at the link above you'll find that the random order sort seems to perform relatively well compared to the other methods, while being extremely easy and fast to implement as shown below:
function shuffle(array) {
var random = array.map(Math.random);
array.sort(function(a, b) {
return random[array.indexOf(a)] - random[array.indexOf(b)];
});
}
Edit: as pointed out by #gregers, the compare function is called with values rather than indices, which is why you need to use indexOf. Note that this change makes the code less suitable for larger arrays as indexOf runs in O(n) time.

All the other answers are based on Math.random() which is fast but not suitable for cryptgraphic level randomization.
The below code is using the well known Fisher-Yates algorithm while utilizing Web Cryptography API for cryptographic level of randomization.
var d = [1,2,3,4,5,6,7,8,9,10];
function shuffle(a) {
var x, t, r = new Uint32Array(1);
for (var i = 0, c = a.length - 1, m = a.length; i < c; i++, m--) {
crypto.getRandomValues(r);
x = Math.floor(r / 65536 / 65536 * m) + i;
t = a [i], a [i] = a [x], a [x] = t;
}
return a;
}
console.log(shuffle(d));

a shuffle function that doesn't change the source array
Update: Here I'm suggesting a relatively simple (not from complexity perspective) and short algorithm that will do just fine with small sized arrays, but it's definitely going to cost a lot more than the classic Durstenfeld algorithm when you deal with huge arrays. You can find the Durstenfeld in one of the top replies to this question.
Original answer:
If you don't wish your shuffle function to mutate the source array, you can copy it to a local variable, then do the rest with a simple shuffling logic.
function shuffle(array) {
var result = [], source = array.concat([]);
while (source.length) {
let index = Math.floor(Math.random() * source.length);
result.push(source[index]);
source.splice(index, 1);
}
return result;
}
Shuffling logic: pick up a random index, then add the corresponding element to the result array and delete it from the source array copy. Repeat this action until the source array gets empty.
And if you really want it short, here's how far I could get:
function shuffle(array) {
var result = [], source = array.concat([]);
while (source.length) {
let index = Math.floor(Math.random() * source.length);
result.push(source.splice(index, 1)[0]);
}
return result;
}

Here is the EASIEST one,
function shuffle(array) {
return array.sort(() => Math.random() - 0.5);
}
for further example, you can check it here

We're still shuffling arrays in 2019, so here goes my approach, which seems to be neat and fast to me:
const src = [...'abcdefg'];
const shuffle = arr =>
[...arr].reduceRight((res,_,__,s) =>
(res.push(s.splice(0|Math.random()*s.length,1)[0]), res),[]);
console.log(shuffle(src));
.as-console-wrapper {min-height: 100%}

You can do it easily with:
// array
var fruits = ["Banana", "Orange", "Apple", "Mango"];
// random
fruits.sort(function(a, b){return 0.5 - Math.random()});
// out
console.log(fruits);
Please reference at JavaScript Sorting Arrays

A simple modification of CoolAJ86's answer that does not modify the original array:
/**
* Returns a new array whose contents are a shuffled copy of the original array.
* #param {Array} The items to shuffle.
* https://stackoverflow.com/a/2450976/1673761
* https://stackoverflow.com/a/44071316/1673761
*/
const shuffle = (array) => {
let currentIndex = array.length;
let temporaryValue;
let randomIndex;
const newArray = array.slice();
// While there remains elements to shuffle...
while (currentIndex) {
randomIndex = Math.floor(Math.random() * currentIndex);
currentIndex -= 1;
// Swap it with the current element.
temporaryValue = newArray[currentIndex];
newArray[currentIndex] = newArray[randomIndex];
newArray[randomIndex] = temporaryValue;
}
return newArray;
};

For those of us who are not very gifted but have access to the wonders of lodash, there is such a thing as lodash.shuffle.

I found this useful:
const shuffle = (array: any[]) => {
return array.slice().sort(() => Math.random() - 0.5);
}
console.log(shuffle([1,2,3,4,5,6,7,8,9,10]));
// Output: [4, 3, 8, 10, 1, 7, 9, 2, 6, 5]

yet another implementation of Fisher-Yates, using strict mode:
function shuffleArray(a) {
"use strict";
var i, t, j;
for (i = a.length - 1; i > 0; i -= 1) {
t = a[i];
j = Math.floor(Math.random() * (i + 1));
a[i] = a[j];
a[j] = t;
}
return a;
}

Distributing array elements randomly to new arrays

I have an array of numbers from 1 to 60
var originalArray = [1, 2, 3, 4 .... 58, 59, 60] // etc
I want to - depending on another number between 2 and 4 - split those numbers randomly into the number of arrays specified, and for the result to be unique each and every time.
For example:
distributeArray(2) should result in two arrays, each with 30 numbers randomly selected from the original array.
distributeArray(3) should result in three arrays, each with 20 numbers randomly selected from original array.
I assume this is a reasonably common case so any pointers would be appreciated. Thanks in advance.

You could do something like this, first shuffle and then split array into n parts.
var arr = [...Array(61).keys()].slice(1)
function splitRandom(data, n) {
var seen = [];
var counter = 0;
var shuffle = data.reduce(function(r, e) {
function random() {
var rand = parseInt(Math.random(0, arr.length) * arr.length);
if (seen.indexOf(rand) != -1) {
return random()
} else {
seen.push(rand)
return rand;
}
}
r[random()] = e;
return r;
}, [])
var split = shuffle.reduce(function(r, e) {
var c = counter++;
r[c] = r[c].concat(e)
counter = counter % n;
return r;
}, Array(n).fill([]))
return split;
}
console.log(JSON.stringify(splitRandom(arr, 3)))
console.log(JSON.stringify(splitRandom(arr, 10)))
console.log(JSON.stringify(splitRandom(arr, 50)))

You can create a function which creates an array of n .length, and an array of x .length. Use do..while loop Array.prototype.splice() to remove a random index from originalArray, .push() the element to one of x random arrays, until originalArray.length evaluates to false, return array of arrays containing values.
const randomArrays = (n, x) => {
let [originalArray, result, len] = [
Array.from({length: n}, (_, key) => key)
, Array.from({length: x}, () => [])
, Math.ceil(n / x)
];
do {
let [curr, index] = [
originalArray
.splice(Math.floor(Math.random() * originalArray.length), 1)
.pop()
, Math.floor(Math.random() * result.length)
];
if (result[index].length < len)
result[index].push(curr);
else
for (let i = 0; i < result.length; i++) {
if (result[i].length < len) {
result[i].push(curr);
break;
}
}
} while (originalArray.length);
return result
}
console.log(
randomArrays(60, 3)
, randomArrays(21, 7)
, randomArrays(5, 3)
, randomArrays(27, 5)
);

Factorializing a number with .reduce()

I am trying to write a function that will produce the factorial of a provided integer and then reduce the factorial array (by multiplying each array element).
For example:
factor(5) >>> [1, 2, 3, 4, 5] >>> 1 * 2 * 3 * 4 * 5 >>> 120
var array = [ ];
function factor(num) {
for (i = 1; i <= num; i++) {
array.push(i);
}
array.reduce(function(a, b) {
return a * b;
})
};
factor(5);
However, it keeps returning undefined.
I think it has to do with the formatting of the function, but I'm not sure.

Try to pass the initial value for reduce,
array.reduce(function(a, b) {
return a * b;
},1);
Also return the reduced value of the array from the function,
function factor(num) {
for (i = 1; i <= num; i++) {
array.push(i);
}
return array.reduce(function(a, b) {
return a * b;
},1)
};
console.log(factor(5));

Your problem that your function does not actually return any value, and the "return" value of such a function is undefined. Let's look at why this is happening:
function factor(num) {
for (i = 1; i <= num; i++) {
array.push(i);
}
array.reduce(function(a, b) {
return a * b; // <-- Here is probably your misunderstanding
});
};
That return that I marked returns from the function that you pass to reduce(), but there is no return from factor().

.reduce() is not necessary to return expected results. Try utilizing single for loop
var array = [];
function factor(num) {
for (var i = 1, res = 1; i < num; array[i - 1] = i, res *= ++i);
return res
};
console.log(
factor(3)
, factor(4)
, factor(5)
, factor(6)
, array
)

using Array indexes and reduce if anybody doesnt want to fill their arrays with ordered numbers.
num = 10;
Array(num).fill(null).reduce((acc, val, i) => {
if (i == 0) {
return 1;
}
return acc * (i + 1);
}, 1)

function factor(num) {
return Array(num)
.fill(null)
.map((item, index) => index + 1)
.reduce((pv, cv) => pv * cv);
}
console.log(factor(3), factor(4), factor(5), factor(6));

Randomize, no repeat [duplicate]

I have an array like this:
var arr1 = ["a", "b", "c", "d"];
How can I randomize / shuffle it?

The de-facto unbiased shuffle algorithm is the Fisher-Yates (aka Knuth) Shuffle.
You can see a great visualization here (and the original post linked to this)
function shuffle(array) {
let currentIndex = array.length, randomIndex;
// While there remain elements to shuffle.
while (currentIndex != 0) {
// Pick a remaining element.
randomIndex = Math.floor(Math.random() * currentIndex);
currentIndex--;
// And swap it with the current element.
[array[currentIndex], array[randomIndex]] = [
array[randomIndex], array[currentIndex]];
}
return array;
}
// Used like so
var arr = [2, 11, 37, 42];
shuffle(arr);
console.log(arr);
Some more info about the algorithm used.

Here's a JavaScript implementation of the Durstenfeld shuffle, an optimized version of Fisher-Yates:
/* Randomize array in-place using Durstenfeld shuffle algorithm */
function shuffleArray(array) {
for (var i = array.length - 1; i > 0; i--) {
var j = Math.floor(Math.random() * (i + 1));
var temp = array[i];
array[i] = array[j];
array[j] = temp;
}
}
It picks a random element for each original array element, and excludes it from the next draw, like picking randomly from a deck of cards.
This clever exclusion swaps the picked element with the current one, then picks the next random element from the remainder, looping backwards for optimal efficiency, ensuring the random pick is simplified (it can always start at 0), and thereby skipping the final element.
Algorithm runtime is O(n). Note that the shuffle is done in-place so if you don't want to modify the original array, first make a copy of it with .slice(0).
EDIT: Updating to ES6 / ECMAScript 2015
The new ES6 allows us to assign two variables at once. This is especially handy when we want to swap the values of two variables, as we can do it in one line of code. Here is a shorter form of the same function, using this feature.
function shuffleArray(array) {
for (let i = array.length - 1; i > 0; i--) {
const j = Math.floor(Math.random() * (i + 1));
[array[i], array[j]] = [array[j], array[i]];
}
}

You can do it easily with map and sort:
let unshuffled = ['hello', 'a', 't', 'q', 1, 2, 3, {cats: true}]
let shuffled = unshuffled
.map(value => ({ value, sort: Math.random() }))
.sort((a, b) => a.sort - b.sort)
.map(({ value }) => value)
console.log(shuffled)
We put each element in the array in an object, and give it a random sort key
We sort using the random key
We unmap to get the original objects
You can shuffle polymorphic arrays, and the sort is as random as Math.random, which is good enough for most purposes.
Since the elements are sorted against consistent keys that are not regenerated each iteration, and each comparison pulls from the same distribution, any non-randomness in the distribution of Math.random is canceled out.
Speed
Time complexity is O(N log N), same as quick sort. Space complexity is O(N). This is not as efficient as a Fischer Yates shuffle but, in my opinion, the code is significantly shorter and more functional. If you have a large array you should certainly use Fischer Yates. If you have a small array with a few hundred items, you might do this.

Warning!
The use of this algorithm is not recommended, because it is inefficient and strongly biased; see comments. It is being left here for future reference, because the idea is not that rare.
[1,2,3,4,5,6].sort( () => .5 - Math.random() );
This https://javascript.info/array-methods#shuffle-an-array tutorial explains the differences straightforwardly.

Use the underscore.js library. The method _.shuffle() is nice for this case.
Here is an example with the method:
var _ = require("underscore");
var arr = [1,2,3,4,5,6];
// Testing _.shuffle
var testShuffle = function () {
var indexOne = 0;
var stObj = {
'0': 0,
'1': 1,
'2': 2,
'3': 3,
'4': 4,
'5': 5
};
for (var i = 0; i < 1000; i++) {
arr = _.shuffle(arr);
indexOne = _.indexOf(arr, 1);
stObj[indexOne] ++;
}
console.log(stObj);
};
testShuffle();

One could (but should NOT) use it as a protoype from Array:
From ChristopheD:
Array.prototype.shuffle = function() {
var i = this.length, j, temp;
if ( i == 0 ) return this;
while ( --i ) {
j = Math.floor( Math.random() * ( i + 1 ) );
temp = this[i];
this[i] = this[j];
this[j] = temp;
}
return this;
}

NEW!
Shorter & probably *faster Fisher-Yates shuffle algorithm
it uses while---
bitwise to floor (numbers up to 10 decimal digits (32bit))
removed unecessary closures & other stuff
function fy(a,b,c,d){//array,placeholder,placeholder,placeholder
c=a.length;while(c)b=Math.random()*(--c+1)|0,d=a[c],a[c]=a[b],a[b]=d
}
script size (with fy as function name): 90bytes
DEMO
http://jsfiddle.net/vvpoma8w/
*faster probably on all browsers except chrome.
If you have any questions just ask.
EDIT
yes it is faster
PERFORMANCE: http://jsperf.com/fyshuffle
using the top voted functions.
EDIT
There was a calculation in excess (don't need --c+1) and noone noticed
shorter(4bytes)&faster(test it!).
function fy(a,b,c,d){//array,placeholder,placeholder,placeholder
c=a.length;while(c)b=Math.random()*c--|0,d=a[c],a[c]=a[b],a[b]=d
}
Caching somewhere else var rnd=Math.random and then use rnd() would also increase slightly the performance on big arrays.
http://jsfiddle.net/vvpoma8w/2/
Readable version (use the original version. this is slower, vars are useless, like the closures & ";", the code itself is also shorter ... maybe read this How to 'minify' Javascript code , btw you are not able to compress the following code in a javascript minifiers like the above one.)
function fisherYates( array ){
var count = array.length,
randomnumber,
temp;
while( count ){
randomnumber = Math.random() * count-- | 0;
temp = array[count];
array[count] = array[randomnumber];
array[randomnumber] = temp
}
}

Shuffle Array In place
function shuffleArr (array){
for (var i = array.length - 1; i > 0; i--) {
var rand = Math.floor(Math.random() * (i + 1));
[array[i], array[rand]] = [array[rand], array[i]]
}
}
ES6 Pure, Iterative
const getShuffledArr = arr => {
const newArr = arr.slice()
for (let i = newArr.length - 1; i > 0; i--) {
const rand = Math.floor(Math.random() * (i + 1));
[newArr[i], newArr[rand]] = [newArr[rand], newArr[i]];
}
return newArr
};
Reliability and Performance Test
Some solutions on this page aren't reliable (they only partially randomise the array). Other solutions are significantly less efficient. With testShuffleArrayFun (see below) we can test array shuffling functions for reliability and performance.
function testShuffleArrayFun(getShuffledArrayFun){
const arr = [0,1,2,3,4,5,6,7,8,9]
var countArr = arr.map(el=>{
return arr.map(
el=> 0
)
}) // For each possible position in the shuffledArr and for
// each possible value, we'll create a counter.
const t0 = performance.now()
const n = 1000000
for (var i=0 ; i<n ; i++){
// We'll call getShuffledArrayFun n times.
// And for each iteration, we'll increment the counter.
var shuffledArr = getShuffledArrayFun(arr)
shuffledArr.forEach(
(value,key)=>{countArr[key][value]++}
)
}
const t1 = performance.now()
console.log(`Count Values in position`)
console.table(countArr)
const frequencyArr = countArr.map( positionArr => (
positionArr.map(
count => count/n
)
))
console.log("Frequency of value in position")
console.table(frequencyArr)
console.log(`total time: ${t1-t0}`)
}
Other Solutions
Other solutions just for fun.
ES6 Pure, Recursive
const getShuffledArr = arr => {
if (arr.length === 1) {return arr};
const rand = Math.floor(Math.random() * arr.length);
return [arr[rand], ...getShuffledArr(arr.filter((_, i) => i != rand))];
};
ES6 Pure using array.map
function getShuffledArr (arr){
return [...arr].map( (_, i, arrCopy) => {
var rand = i + ( Math.floor( Math.random() * (arrCopy.length - i) ) );
[arrCopy[rand], arrCopy[i]] = [arrCopy[i], arrCopy[rand]]
return arrCopy[i]
})
}
ES6 Pure using array.reduce
function getShuffledArr (arr){
return arr.reduce(
(newArr, _, i) => {
var rand = i + ( Math.floor( Math.random() * (newArr.length - i) ) );
[newArr[rand], newArr[i]] = [newArr[i], newArr[rand]]
return newArr
}, [...arr]
)
}

Edit: This answer is incorrect
See comments and https://stackoverflow.com/a/18650169/28234. It is being left here for reference because the idea isn't rare.
A very simple way for small arrays is simply this:
const someArray = [1, 2, 3, 4, 5];
someArray.sort(() => Math.random() - 0.5);
It's probably not very efficient, but for small arrays this works just fine. Here's an example so you can see how random (or not) it is, and whether it fits your usecase or not.
const resultsEl = document.querySelector('#results');
const buttonEl = document.querySelector('#trigger');
const generateArrayAndRandomize = () => {
const someArray = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
someArray.sort(() => Math.random() - 0.5);
return someArray;
};
const renderResultsToDom = (results, el) => {
el.innerHTML = results.join(' ');
};
buttonEl.addEventListener('click', () => renderResultsToDom(generateArrayAndRandomize(), resultsEl));
<h1>Randomize!</h1>
<button id="trigger">Generate</button>
<p id="results">0 1 2 3 4 5 6 7 8 9</p>

Adding to #Laurens Holsts answer. This is 50% compressed.
function shuffleArray(d) {
for (var c = d.length - 1; c > 0; c--) {
var b = Math.floor(Math.random() * (c + 1));
var a = d[c];
d[c] = d[b];
d[b] = a;
}
return d
};

I found this variant hanging out in the "deleted by author" answers on a duplicate of this question. Unlike some of the other answers that have many upvotes already, this is:
Actually random
Not in-place (hence the shuffled name rather than shuffle)
Not already present here with multiple variants
Here's a jsfiddle showing it in use.
Array.prototype.shuffled = function() {
return this.map(function(n){ return [Math.random(), n] })
.sort().map(function(n){ return n[1] });
}

With ES2015 you can use this one:
Array.prototype.shuffle = function() {
let m = this.length, i;
while (m) {
i = (Math.random() * m--) >>> 0;
[this[m], this[i]] = [this[i], this[m]]
}
return this;
}
Usage:
[1, 2, 3, 4, 5, 6, 7].shuffle();

Warning!
Using this answer for randomizing large arrays, cryptography, or any other application requiring true randomness is not recommended, due to its bias and inefficiency. Elements position is only semi-randomized, and they will tend to stay closer to their original position. See https://stackoverflow.com/a/18650169/28234.
You can arbitrarily decide whether to return 1 : -1 by using Math.random:
[1, 2, 3, 4].sort(() => (Math.random() > 0.5) ? 1 : -1)
Try running the following example:
const array = [1, 2, 3, 4];
// Based on the value returned by Math.Random,
// the decision is arbitrarily made whether to return 1 : -1
const shuffeled = array.sort(() => {
const randomTrueOrFalse = Math.random() > 0.5;
return randomTrueOrFalse ? 1 : -1
});
console.log(shuffeled);

//one line solution
shuffle = (array) => array.sort(() => Math.random() - 0.5);
//Demo
let arr = [1, 2, 3];
shuffle(arr);
alert(arr);
https://javascript.info/task/shuffle
Math.random() - 0.5 is a random number that may be positive or
negative, so the sorting function reorders elements randomly.

benchmarks
Let's first see the results then we'll look at each implementation of shuffle below -
splice is slow
Any solution using splice or shift in a loop is going to be very slow. Which is especially noticeable when we increase the size of the array. In a naive algorithm we -
get a rand position, i, in the input array, t
add t[i] to the output
splice position i from array t
To exaggerate the slow effect, we'll demonstrate this on an array of one million elements. The following script almost 30 seconds -
const shuffle = t =>
Array.from(sample(t, t.length))
function* sample(t, n)
{ let r = Array.from(t)
while (n > 0 && r.length)
{ const i = rand(r.length) // 1
yield r[i] // 2
r.splice(i, 1) // 3
n = n - 1
}
}
const rand = n =>
0 | Math.random() * n
function swap (t, i, j)
{ let q = t[i]
t[i] = t[j]
t[j] = q
return t
}
const size = 1e6
const bigarray = Array.from(Array(size), (_,i) => i)
console.time("shuffle via splice")
const result = shuffle(bigarray)
console.timeEnd("shuffle via splice")
document.body.textContent = JSON.stringify(result, null, 2)
body::before {
content: "1 million elements via splice";
font-weight: bold;
display: block;
}
pop is fast
The trick is not to splice and instead use the super efficient pop. To do this, in place of the typical splice call, you -
select the position to splice, i
swap t[i] with the last element, t[t.length - 1]
add t.pop() to the result
Now we can shuffle one million elements in less than 100 milliseconds -
const shuffle = t =>
Array.from(sample(t, t.length))
function* sample(t, n)
{ let r = Array.from(t)
while (n > 0 && r.length)
{ const i = rand(r.length) // 1
swap(r, i, r.length - 1) // 2
yield r.pop() // 3
n = n - 1
}
}
const rand = n =>
0 | Math.random() * n
function swap (t, i, j)
{ let q = t[i]
t[i] = t[j]
t[j] = q
return t
}
const size = 1e6
const bigarray = Array.from(Array(size), (_,i) => i)
console.time("shuffle via pop")
const result = shuffle(bigarray)
console.timeEnd("shuffle via pop")
document.body.textContent = JSON.stringify(result, null, 2)
body::before {
content: "1 million elements via pop";
font-weight: bold;
display: block;
}
even faster
The two implementations of shuffle above produce a new output array. The input array is not modified. This is my preferred way of working however you can increase the speed even more by shuffling in place.
Below shuffle one million elements in less than 10 milliseconds -
function shuffle (t)
{ let last = t.length
let n
while (last > 0)
{ n = rand(last)
swap(t, n, --last)
}
}
const rand = n =>
0 | Math.random() * n
function swap (t, i, j)
{ let q = t[i]
t[i] = t[j]
t[j] = q
return t
}
const size = 1e6
const bigarray = Array.from(Array(size), (_,i) => i)
console.time("shuffle in place")
shuffle(bigarray)
console.timeEnd("shuffle in place")
document.body.textContent = JSON.stringify(bigarray, null, 2)
body::before {
content: "1 million elements in place";
font-weight: bold;
display: block;
}

var shuffle = function(array) {
temp = [];
originalLength = array.length;
for (var i = 0; i < originalLength; i++) {
temp.push(array.splice(Math.floor(Math.random()*array.length),1));
}
return temp;
};

A recursive solution:
function shuffle(a,b){
return a.length==0?b:function(c){
return shuffle(a,(b||[]).concat(c));
}(a.splice(Math.floor(Math.random()*a.length),1));
};

Fisher-Yates shuffle in javascript. I'm posting this here because the use of two utility functions (swap and randInt) clarifies the algorithm compared to the other answers here.
function swap(arr, i, j) {
// swaps two elements of an array in place
var temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
function randInt(max) {
// returns random integer between 0 and max-1 inclusive.
return Math.floor(Math.random()*max);
}
function shuffle(arr) {
// For each slot in the array (starting at the end),
// pick an element randomly from the unplaced elements and
// place it in the slot, exchanging places with the
// element in the slot.
for(var slot = arr.length - 1; slot > 0; slot--){
var element = randInt(slot+1);
swap(arr, element, slot);
}
}

Modern short inline solution using ES6 features:
['a','b','c','d'].map(x => [Math.random(), x]).sort(([a], [b]) => a - b).map(([_, x]) => x);
(for educational purposes)

Using Fisher-Yates shuffle algorithm and ES6:
// Original array
let array = ['a', 'b', 'c', 'd'];
// Create a copy of the original array to be randomized
let shuffle = [...array];
// Defining function returning random value from i to N
const getRandomValue = (i, N) => Math.floor(Math.random() * (N - i) + i);
// Shuffle a pair of two elements at random position j
shuffle.forEach( (elem, i, arr, j = getRandomValue(i, arr.length)) => [arr[i], arr[j]] = [arr[j], arr[i]] );
console.log(shuffle);
// ['d', 'a', 'b', 'c']

First of all, have a look here for a great visual comparison of different sorting methods in javascript.
Secondly, if you have a quick look at the link above you'll find that the random order sort seems to perform relatively well compared to the other methods, while being extremely easy and fast to implement as shown below:
function shuffle(array) {
var random = array.map(Math.random);
array.sort(function(a, b) {
return random[array.indexOf(a)] - random[array.indexOf(b)];
});
}
Edit: as pointed out by #gregers, the compare function is called with values rather than indices, which is why you need to use indexOf. Note that this change makes the code less suitable for larger arrays as indexOf runs in O(n) time.

All the other answers are based on Math.random() which is fast but not suitable for cryptgraphic level randomization.
The below code is using the well known Fisher-Yates algorithm while utilizing Web Cryptography API for cryptographic level of randomization.
var d = [1,2,3,4,5,6,7,8,9,10];
function shuffle(a) {
var x, t, r = new Uint32Array(1);
for (var i = 0, c = a.length - 1, m = a.length; i < c; i++, m--) {
crypto.getRandomValues(r);
x = Math.floor(r / 65536 / 65536 * m) + i;
t = a [i], a [i] = a [x], a [x] = t;
}
return a;
}
console.log(shuffle(d));

a shuffle function that doesn't change the source array
Update: Here I'm suggesting a relatively simple (not from complexity perspective) and short algorithm that will do just fine with small sized arrays, but it's definitely going to cost a lot more than the classic Durstenfeld algorithm when you deal with huge arrays. You can find the Durstenfeld in one of the top replies to this question.
Original answer:
If you don't wish your shuffle function to mutate the source array, you can copy it to a local variable, then do the rest with a simple shuffling logic.
function shuffle(array) {
var result = [], source = array.concat([]);
while (source.length) {
let index = Math.floor(Math.random() * source.length);
result.push(source[index]);
source.splice(index, 1);
}
return result;
}
Shuffling logic: pick up a random index, then add the corresponding element to the result array and delete it from the source array copy. Repeat this action until the source array gets empty.
And if you really want it short, here's how far I could get:
function shuffle(array) {
var result = [], source = array.concat([]);
while (source.length) {
let index = Math.floor(Math.random() * source.length);
result.push(source.splice(index, 1)[0]);
}
return result;
}

Here is the EASIEST one,
function shuffle(array) {
return array.sort(() => Math.random() - 0.5);
}
for further example, you can check it here

We're still shuffling arrays in 2019, so here goes my approach, which seems to be neat and fast to me:
const src = [...'abcdefg'];
const shuffle = arr =>
[...arr].reduceRight((res,_,__,s) =>
(res.push(s.splice(0|Math.random()*s.length,1)[0]), res),[]);
console.log(shuffle(src));
.as-console-wrapper {min-height: 100%}

You can do it easily with:
// array
var fruits = ["Banana", "Orange", "Apple", "Mango"];
// random
fruits.sort(function(a, b){return 0.5 - Math.random()});
// out
console.log(fruits);
Please reference at JavaScript Sorting Arrays

A simple modification of CoolAJ86's answer that does not modify the original array:
/**
* Returns a new array whose contents are a shuffled copy of the original array.
* #param {Array} The items to shuffle.
* https://stackoverflow.com/a/2450976/1673761
* https://stackoverflow.com/a/44071316/1673761
*/
const shuffle = (array) => {
let currentIndex = array.length;
let temporaryValue;
let randomIndex;
const newArray = array.slice();
// While there remains elements to shuffle...
while (currentIndex) {
randomIndex = Math.floor(Math.random() * currentIndex);
currentIndex -= 1;
// Swap it with the current element.
temporaryValue = newArray[currentIndex];
newArray[currentIndex] = newArray[randomIndex];
newArray[randomIndex] = temporaryValue;
}
return newArray;
};

For those of us who are not very gifted but have access to the wonders of lodash, there is such a thing as lodash.shuffle.

I found this useful:
const shuffle = (array: any[]) => {
return array.slice().sort(() => Math.random() - 0.5);
}
console.log(shuffle([1,2,3,4,5,6,7,8,9,10]));
// Output: [4, 3, 8, 10, 1, 7, 9, 2, 6, 5]

yet another implementation of Fisher-Yates, using strict mode:
function shuffleArray(a) {
"use strict";
var i, t, j;
for (i = a.length - 1; i > 0; i -= 1) {
t = a[i];
j = Math.floor(Math.random() * (i + 1));
a[i] = a[j];
a[j] = t;
}
return a;
}