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

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?

Solution 1:

console.log(Math.floor(100000 + Math.random() * 900000));

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.

Solution 2:

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 > 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 -> true

Solution 3:

I would go with this solution:

Math.floor(Math.random() * 899999 + 100000)

Solution 4:

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);

Solution 5:

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