Generate big numbers with Math random in Javascript - Stack Overflow

I need to generate 26 digit numbers with Math.random, but when I use this: Math.floor(Math.random() * 100000000000000000000000000) + 900000000000000000000000000 I gets 9.544695043285823e+26

I need to generate 26 digit numbers with Math.random, but when I use this: Math.floor(Math.random() * 100000000000000000000000000) + 900000000000000000000000000 I gets 9.544695043285823e+26

• javascript

Share Improve this question Follow edited Dec 22, 2018 at 8:50 radarbob 5,10122 gold badges2525 silver badges3838 bronze badges asked Dec 22, 2018 at 8:46 user10713925user10713925 3

• Remember, such large numbers, if represented as numbers rather than strings, will not be accurate to the integer (check out MAX_SAFE_INTEGER) – CertainPerformance Commented Dec 22, 2018 at 8:48
• Exactly what seems to be the problem? 9.544695043285823e+26 is a darn big number with actually 27 digits, as e+26 means that you should move the decimal separator 26 places to the right. – Chri.s Commented Dec 22, 2018 at 8:57
• you can use a library like BigInteger.js to generate random big integers (as strings) in any base and with any number of digits (up to cpu memory limit of course) – Nikos M. Commented Dec 22, 2018 at 10:53

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3 Answers 3

Sorted by: Reset to default 6

Modern browsers support BigInt and bigint primitive type and we can bine it with a random generated array containing 8 bytes (the sizeof bigint is 8 bytes (64 bits)).

1. Generating Random BigInt Performance Wise

We can generate a random hex string of 16 characters length and apply it directly to BigInt:

const hexString = Array(16)
  .fill()
  .map(() => Math.round(Math.random() * 0xF).toString(16))
  .join('');

const randomBigInt = BigInt(`0x${hexString}`);

// randomBigInt will contain a random Bigint

document.querySelector('#generate').addEventListener('click', () => {
  const output = [];
  let lines = 10;

  do {
    const hexString = Array(16)
      .fill()
      .map(() => Math.round(Math.random() * 0xF).toString(16))
      .join('');
    const number = BigInt(`0x${hexString}`);
 
    output.push(`${
        number.toString().padStart(24)
      } : 0x${
        hexString.padStart(16, '0')
      }`);
  } while (--lines > 0);

  document.querySelector('#numbers').textContent = output.join('\n');
});

<button id="generate">Generate</button>
<pre id="numbers"><pre>

2. Generating Random BigInt from random bytes array

If we want to use Uint8Array or if we want more control over the bits manipulation, we can bine Array.prototype.fill with Array.prototype.map to generate an array containing 8 random byte number values (beware this is around 50% slower than the above method):

const randomBytes = Array(8)
  .fill()
  .map(() => Math.round(Math.random() * 0xFF));

// randomBytes will  contain something similar to this:
// [129, 59, 98, 222, 20, 7, 196, 244]

Then we use Array.prototype.reduce to initialize a BigInt of zero value and left shift each randum byte value its position X 8 bits and applying bitwise or to the current value of each reduce iteration:

const randomBigInt = randomBytes
  .reduce((n, c, i) => n | BigInt(c) << BigInt(i) * 8n, 0n);

// randomBigInt will contain a random Bigint

Working example generating 10 random BigInt values

document.querySelector('#generate').addEventListener('click', () => {
  const output = [];
  let lines = 10;

  do {
    const number = Array(8)
      .fill()
      .map(() => Math.round(Math.random() * 0xFF))
      .reduce((n, c, i) => n | BigInt(c) << BigInt(i) * 8n, 0n);
 
    output.push(`${
        number.toString().padStart(24)
      } : 0x${
        number.toString(16).padStart(16, '0')
      }`);
  } while (--lines > 0);

  document.querySelector('#numbers').textContent = output.join('\n');
});

<button id="generate">Generate</button>
<pre id="numbers"><pre>

Floating point numbers in JavaScript (and a lot of other languages) can contain only about 15.955 digits without losing precision. For bigger numbers you can look into JS libraries, or concatenate few numbers as strings. For example:

console.log( Math.random().toString().slice(2, 15) + Math.random().toString().slice(2, 15) )

If I use a 6 decillion LG about 72.9 million out of 300 but when I switch to 10 centillion it es like this 10 ^ 999 - 99999 + 26 just like how 9 - 9 - 9 - 9 - googolplex is equal to 0 googolplex