Function randomInt

lib/internal/crypto/random.js:211–279

(min, max, callback)

// Generates an integer in [min, max) range where min is inclusive and max is
// exclusive.
function randomInt(min, max, callback) {
  // Detect optional min syntax
  // randomInt(max)
  // randomInt(max, callback)
  const minNotSpecified = typeof max === 'undefined' ||
    typeof max === 'function';

  if (minNotSpecified) {
    callback = max;
    max = min;
    min = 0;
  }

  const isSync = typeof callback === 'undefined';
  if (!isSync) {
    validateFunction(callback, 'callback');
  }
  if (!NumberIsSafeInteger(min)) {
    throw new ERR_INVALID_ARG_TYPE('min', 'a safe integer', min);
  }
  if (!NumberIsSafeInteger(max)) {
    throw new ERR_INVALID_ARG_TYPE('max', 'a safe integer', max);
  }
  if (max <= min) {
    throw new ERR_OUT_OF_RANGE(
      'max', `greater than the value of "min" (${min})`, max,
    );
  }

  // First we generate a random int between [0..range)
  const range = max - min;

  if (!(range <= RAND_MAX)) {
    throw new ERR_OUT_OF_RANGE(`max${minNotSpecified ? '' : ' - min'}`,
                               `<= ${RAND_MAX}`, range);
  }

  // For (x % range) to produce an unbiased value greater than or equal to 0 and
  // less than range, x must be drawn randomly from the set of integers greater
  // than or equal to 0 and less than randLimit.
  const randLimit = RAND_MAX - (RAND_MAX % range);

  // If we don't have a callback, or if there is still data in the cache, we can
  // do this synchronously, which is super fast.
  while (isSync || (randomCacheOffset < randomCache.length)) {
    if (randomCacheOffset === randomCache.length) {
      // This might block the thread for a bit, but we are in sync mode.
      randomFillSync(randomCache);
      randomCacheOffset = 0;
    }

    const x = randomCache.readUIntBE(randomCacheOffset, 6);
    randomCacheOffset += 6;

    if (x < randLimit) {
      const n = (x % range) + min;
      if (isSync) return n;
      process.nextTick(callback, undefined, n);