code: function isSafePositiveInteger(x) {
    // https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Number/MAX_SAFE_INTEGER
    const MAX_SAFE_INTEGER = Number.MAX_SAFE_INTEGER || 9007199254740991;
    // Is it a number?
    return Object.prototype.toString.call(x) === '[object Number]' &&
      // Is it an integer?
      x % 1 === 0 &&
      // Is it positive?
      x > 0 &&
      // Is it less than the maximum safe integer?
      x <= MAX_SAFE_INTEGER;
  }
docs: Test for a positive, safe integer.
  
  @param {object} x - Object under test.