// 45678901234567890123456789012345678901234567890123456789012345678901234567890
// 14 March 2019
// 18 March 2019 - support expontent notation.
// 19 March 2019 - better comment style, better return style.

/**
 * @function fraction accepts param of any type and returns the decimal fraction
 * of the value as a string, if the value is coercible to a number.
 *
 * If the coerced value results in null, undefined, or NaN, the function returns
 * "0".
 *
 * Function accepts values in exponent notation and will insert leading '0' for
 * negative decimals. For example, '9.123456789012345e-21' will result in
 * '.000000000000000000009123456789012345', and '555.1111e-10' will result in
 * '.00000005551111'.
 *
 * @param {*} num
 * @returns {string} fractional value
 */

function fraction(num) {
  var s = String(num);
  var d = s.match(/\./g) || [];
  var n = d.length > 1 ? '0' : s;
  var m = n.split('.')[1];
  var e = m ? m.split('e')[1] : 0;

  if (!e) {

    /*
     * Case 1: No exponent notation, so handle in normal course.
     * If null or NaN, return 0, else return mantissa.
     */

    return m == null || m !== m ? '0' : '.' + m;
  }


  if (+e >= 0) {
    /*
     * Case 2: An exponent notation has been found, its value is non-negative,
     * so there can only be 0's to the right.
     */

    return '0';
  }

  /*
   * Case 3: An exponent notation has been found, its value is negative, so
   * insert 0's between the decimal and the value before 'e'.
   */

  // Note that the tilde operator converts, e.g., -8 to +7.
  var a = Array(~+e).fill(0).join('');
  var r = s.split('e')[0].match(/\d/g).join('');

  return '.' + a + r;
}

var tests = [
  1, 2, 3.4, 1234.5678,
  '1,4567', NaN, null,
  undefined, , 'a', true,
  'truth', 'true.9.false',
  Math, Math.round,
  '12312231231231232112.2456789',
  Number.POSITIVE_INFINITY,
  Number.EPSILON,
  9.11111111015e20,
  9.11111111015e21,
  9.11111111015e31,
  9.1234567890123456789e-21,
  9.11111111015e-10,
  0.00001111e-10,
  555.1111e-10
];

var results = tests.map(function(test) {
  return { input: test, result: fraction(test) };
});

console.log(JSON.stringify(results, null, 2));

/*
[
  {
    "input": 1,
    "result": "0"
  },
  {
    "input": 2,
    "result": "0"
  },
  {
    "input": 3.4,
    "result": ".4"
  },
  {
    "input": 1234.5678,
    "result": ".5678"
  },
  {
    "input": "1,4567",
    "result": "0"
  },
  {
    "input": null,
    "result": "0"
  },
  {
    "input": null,
    "result": "0"
  },
  {
    "result": "0"
  },
  null,
  {
    "input": "a",
    "result": "0"
  },
  {
    "input": true,
    "result": "0"
  },
  {
    "input": "truth",
    "result": "0"
  },
  {
    "input": "true.9.false",
    "result": "0"
  },
  {
    "input": {},
    "result": "0"
  },
  {
    "result": "0"
  },
  {
    "input": "12312231231231232112.2456789",
    "result": ".2456789"
  },
  {
    "input": null,
    "result": "0"
  },
  {
    "input": 2.220446049250313e-16,
    "result": ".0000000000000002220446049250313"
  },
  {
    "input": 911111111015000000000,
    "result": "0"
  },
  {
    "input": 9.11111111015e+21,
    "result": "0"
  },
  {
    "input": 9.11111111015e+31,
    "result": "0"
  },
  {
    "input": 9.123456789012345e-21,
    "result": ".000000000000000000009123456789012345"
  },
  {
    "input": 9.11111111015e-10,
    "result": ".000000000911111111015"
  },
  {
    "input": 1.111e-15,
    "result": ".000000000000001111"
  },
  {
    "input": 5.551111e-8,
    "result": ".00000005551111"
  }
]
*/