How to Generate a random number of fixed length using JavaScript

Question

I m trying to generate a random number that must have a fixed length of exactly 6 digits   I don t know if JavaScript has given below would ever create a number less than 6 digits   Math floor  Math random   1000000  1     I found this question and answer on StackOverflow here   But  it s unclear   EDIT    I ran the above code a bunch of times  and Yes  it frequently creates numbers less than 6 digits  Is there a quick fast way to make sure it s always exactly 6 digits

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Here is my function I use  n - string length you want to generate   x000D   x000D  function generateRandomNumber n    x000D    return Math floor Math random      9   Math pow 10  n - 1      Math pow 10  n - 1   x000D    x000D   x000D   x000D

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const generate   n   gt  String Math ceil Math random     10  n   padStart n   0      n being the length of the random number   Use a parseInt   or Number   on the result if you want an integer  If you don t want the first integer to be a 0 then you could use padEnd   instead of padStart

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This code provides nearly full randomness    x000D   x000D  function generator     x000D      const ran        gt   1  2  3  4  5  6  7  8  9  0  sort  x  z    gt    x000D          ren   Math random    x000D          if  ren    0 5  return 0  x000D          return ren  gt  0 5   1   -1 x000D         x000D      return Array 6  fill null  map x   gt  ran    Math random     9  toFixed     join     x000D    x000D   x000D  console log generator    x000D   x000D   x000D    This code provides complete randomness    x000D   x000D  function generator     x000D   x000D      const ran1        gt   1  2  3  4  5  6  7  8  9  0  sort  x  z    gt    x000D          ren   Math random    x000D          if  ren    0 5  return 0  x000D          return ren  gt  0 5   1   -1 x000D         x000D      const ran2        gt  ran1   sort  x  z    gt    x000D          ren   Math random    x000D          if  ren    0 5  return 0  x000D          return ren  gt  0 5   1   -1 x000D         x000D   x000D      return Array 6  fill null  map x   gt  ran2    Math random     9  toFixed     join     x000D    x000D   x000D  console log generator    x000D   x000D   x000D

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I created the below function to generate random number of fix length   function getRandomNum length        var randomNum             Math pow 10 length  toString   slice length-1             Math floor  Math random   Math pow 10 length   1  toString    slice -length       return randomNum      This will basically add 0 s at the beginning to make the length of the number as required

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x000D   x000D    var number   Math floor Math random     9000000000    1000000000  x000D      console log number   x000D   x000D   x000D    This can be simplest way and reliable one

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More generally  generating a random integer with fixed length can be done using Math pow    var randomFixedInteger   function  length        return Math floor Math pow 10  length-1    Math random      Math pow 10  length  - Math pow 10  length-1  - 1        To answer the question  randomFixedInteger 6

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I use randojs to make the randomness simpler and more readable  you can pick a random int between 100000 and 999999 like this with randojs    x000D   x000D  console log rando 100000  999999    x000D   lt script src  https   randojs com 1 0 0 js  gt  lt  script gt  x000D   x000D   x000D

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You can use this module https   www npmjs com package uid  it generates variable length unique id  uid 10    gt   hbswt489ts   uid     gt   rhvtfnt  Defaults to 7   Or you can have a look at this module https   www npmjs com package shortid  const shortid   require  shortid     console log shortid generate        PPBqWA9   Hope it works for you

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I would go with this solution   Math floor Math random     899999   100000

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For the length of 6  recursiveness doesn t matter a lot    x000D   x000D  function random len    x000D    let result   Math floor Math random     Math pow 10  len    x000D   x000D    return  result toString   length  lt  len    random len    result  x000D    x000D   x000D  console log random 6    x000D   x000D   x000D

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generate a random number that must have a fixed length of exactly 6 digits     000000  Math floor  Math random   1000000  1   slice -6

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You can use the below code to generate a random number that will always be 6 digits  Math random   toString   substr 2  6   Hope this works for everyone    Briefly how this works is Math random   generates a random number between 0 and 1 which we convert to a string and using  toString   and take a 6 digit sample from said string using  substr   with the parameters 2  6 to start the sample from the 2nd char and continue it for 6 characters  This can be used for any length number  If you want to do more reading on this here are some links to the docs to save you some googling  Math random    https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Math random  toString    https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Object toString  substr    https   developer mozilla org en-US docs Web JavaScript Reference Global Objects String substr

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100000   Math floor Math random     900000     will give a number from 100000 to 999999  inclusive

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Only fully reliable answer that offers full randomness  without loss  The other ones prior to this answer all looses out depending on how many characters you want  The more you want  the more they lose randomness    They achieve it by limiting the amount of numbers possible preceding the fixed length    So for instance  a random number of fixed length 2 would be 10 - 99  For 3  100 - 999  For 4  1000 - 9999  For 5 10000 - 99999 and so on  As can be seen by the pattern  it suggests 10  loss of randomness because numbers prior to that are not possible  Why   For really large numbers   18  24  48   10  is still a lot of numbers to loose out on    function generate n            var add   1  max   12 - add       12 is the min safe number Math random   can generate without it starting to pad the end with zeros              if   n  gt  max                     return generate max    generate n - max                      max          Math pow 10  n add           var min      max 10     Math pow 10  n  basically         var number   Math floor  Math random      max - min   1      min           return       number  substring add        The generator allows for  infinite length without lossy precision and with minimal performance cost    Example   generate 2   03  generate 2   72  generate 2   20  generate 3   301  generate 3   436  generate 3   015    As you can see  even the zero are included initially which is an additional 10  loss just that  besides the fact that numbers prior to 10 n are not possible    That s now a total of 20     Also  the other options have an upper limit on how many characters you can actually generate    Example with cost    var start   new Date    var num   generate 1000   console log  Time     new Date   - start   ms for   num    Logs    Time  0 ms for 7884381040581542028523049580942716270617684062141718855897876833390671831652069714762698108211737288889182869856548142946579393971303478191296939612816492205372814129483213770914444439430297923875275475120712223308258993696422444618241506074080831777597175223850085606310877065533844577763231043780302367695330451000357920496047212646138908106805663879875404784849990477942580056343258756712280958474020627842245866908290819748829427029211991533809630060693336825924167793796369987750553539230834216505824880709596544701685608502486365633618424746636614437646240783649056696052311741095247677377387232206206230001648953246132624571185908487227730250573902216708727944082363775298758556612347564746106354407311558683595834088577220946790036272364740219788470832285646664462382109714500242379237782088931632873392735450875490295512846026376692233811845787949465417190308589695423418373731970944293954443996348633968914665773009376928939207861596826457540403314327582156399232931348229798533882278769760   More hardcore   generate 100000  length     100000 - gt  true

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npm install --save randomatic  var randomize   require  randomatic    randomize pattern  length  options     Example   To generate a 10-character randomized string using all available characters   randomize      10       gt   x2  -5 T     randomize  Aa0    10       gt   LV3u BSGhw    a  Lowercase alpha characters  abcdefghijklmnopqrstuvwxyz   A  Uppercase alpha characters  ABCDEFGHIJKLMNOPQRSTUVWXYZ    0  Numeric characters  0123456789       Special characters          amp     -                All characters  all of the above combined      Custom characters  pass a string of custom characters to the options   NPM repo

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This is another random number generator that i use often  it also prevent the first digit from been zero 0    x000D   x000D    function randomNumber length    x000D      var text       x000D      var possible    123456789   x000D      for  var i   0  i  lt  length  i      x000D        var sup   Math floor Math random     possible length   x000D        text    i  gt  0  amp  amp  sup    i    0    possible charAt sup   x000D        x000D      return Number text   x000D      x000D   x000D   x000D

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I was thinking about the same today and then go with the solution   var generateOTP   function otpLength 6      let baseNumber   Math pow 10  otpLength -1      let number   Math floor Math random   baseNumber          Check if number have 0 as first digit        if  number  lt  baseNumber        number    baseNumber        return number       Let me know if it has any bug  Thanks

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In case you also want the first digit to be able to be 0 this is my solution    x000D   x000D  const getRange    size  start   0    gt  Array size  fill 0  map     i    gt  i   start   x000D   x000D  const getRandomDigit        gt  Math floor Math random     10   x000D   x000D  const generateVerificationCode        gt  getRange 6  map getRandomDigit  join      x000D   x000D  console log generateVerificationCode    x000D   x000D   x000D

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x000D   x000D  console log Math floor 100000   Math random     900000    x000D   x000D   x000D    Will always create a number of 6 digits and it ensures the first digit will never be 0   The code in your question will create a number of less than 6 digits

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Based on link you ve provided  right answer should be   Math floor Math random   899999 100000    Math random   returns float between 0 and 1  so minimum number will be 100000  max - 999999  Exactly 6 digits  as you wanted

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parseInt Math random   toString   slice 2 Math min length 2  18    10      18 - gt  due to max digits in Math random  Update  This method has few flaws  - Sometimes the number of digits might be lesser if its left padded with zeroes

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To Generate Random Number Using JS    x000D   x000D  console log  x000D  Math floor Math random     1000000  x000D     x000D   lt  DOCTYPE html gt  x000D   lt html gt  x000D   lt body gt  x000D   x000D   lt h2 gt JavaScript Math random   lt  h2 gt  x000D   x000D   lt p id  demo  gt  lt  p gt  x000D   x000D   lt  body gt  x000D   lt  html gt  x000D   x000D   x000D

[javascript] How to Generate a random number of fixed length using JavaScript?

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