nanoid/non-secure/index.js

// This alphabet uses a-z A-Z 0-9 _- symbols.
// Symbols are generated for smaller size.
// -_zyxwvutsrqponmlkjihgfedcba9876543210ZYXWVUTSRQPONMLKJIHGFEDCBA
var url = '-_'
// Loop from 36 to 0 (from z to a and 9 to 0 in Base36).
var i = 36
while (i--) {
  // 36 is radix. Number.prototype.toString(36) returns number
  // in Base36 representation. Base36 is like hex, but it uses 0–9 and a-z.
  url += i.toString(36)
}
// Loop from 36 to 10 (from Z to A in Base36).
i = 36
while (i-- - 10) {
  url += i.toString(36).toUpperCase()
}

/**
 * Generate URL-friendly unique ID. This method use non-secure predictable
 * random generator with bigger collision probability.
 *
 * @param {number} [size=21] The number of symbols in ID.
 *
 * @return {string} Random string.
 *
 * @example
 * const nanoid = require('nanoid/non-secure')
 * model.id = nanoid() //=> "Uakgb_J5m9g-0JDMbcJqL"
 *
 * @name nonSecure
 * @function
 */
module.exports = function (size) {
  var id = ''
  i = size || 21
  // Compact alternative for `for (var i = 0; i < size; i++)`
  while (i--) {
    // `| 0` is compact and faster alternative for `Math.floor()`
    id += url[Math.random() * 64 | 0]
  }
  return id
}