64-bit Integer type for Javascript based on Int64.js but NodeJS dependency on buffer removed, plus some other small mods.

Int64.js

//     Int64.js
//
//     Copyright (c) 2012 Robert Kieffer
//     MIT License - http://opensource.org/licenses/mit-license.php

/**
 * Support for handling 64-bit int numbers in Javascript (node.js)
 *
 * JS Numbers are IEEE-754 binary double-precision floats, which limits the
 * range of values that can be represented with integer precision to:
 *
 * 2^^53 <= N <= 2^53
 *
 * Int64 objects wrap a node Buffer that holds the 8-bytes of int64 data.  These
 * objects operate directly on the buffer which means that if they are created
 * using an existing buffer then setting the value will modify the Buffer, and
 * vice-versa.
 *
 * Internal Representation
 *
 * The internal buffer format is Big Endian.  I.e. the most-significant byte is
 * at buffer[0], the least-significant at buffer[7].  For the purposes of
 * converting to/from JS native numbers, the value is assumed to be a signed
 * integer stored in 2's complement form.
 *
 * For details about IEEE-754 see:
 * http://en.wikipedia.org/wiki/Double_precision_floating-point_format
 */

//
// Int64
//

/**
 * Constructor accepts any of the following argument types:
 *
 * new Int64(buffer[, offset=0]) - Existing Buffer with byte offset
 * new Int64(string)             - Hex string (throws if n is outside int64 range)
 * new Int64(number)             - Number (throws if n is outside int64 range)
 * new Int64(hi, lo)             - Raw bits as two 32-bit values
 */
var Int64 = function(a1, offset) {
  offset= offset || 0;
  if (a1 instanceof Buffer) {
    this.storage= new Array(8);
    this.storage[0]= a1[0+offset];
    this.storage[1]= a1[1+offset];
    this.storage[2]= a1[2+offset];
    this.storage[3]= a1[3+offset];
    this.storage[4]= a1[4+offset];
    this.storage[5]= a1[5+offset];
    this.storage[6]= a1[6+offset];
    this.storage[7]= a1[7+offset];
  } else if (a1 instanceof Array) {
    this.storage = a1.slice(offset,8);
  } else {
    this.storage = this.storage || new Array(8);
    this.setValue.apply(this, arguments);
  }
};


// Max integer value that JS can accurately represent
Int64.MAX_INT = Math.pow(2, 53);

// Min integer value that JS can accurately represent
Int64.MIN_INT = -Math.pow(2, 53);

Int64.HexTable = new Array(256);
for (var i = 0; i < 256; i++) {
  Int64.HexTable[i] = (i > 0xF ? '' : '0') + i.toString(16);
}

Int64.prototype = {
  /**
   * Do in-place 2's compliment.  See
   * http://en.wikipedia.org/wiki/Two's_complement
   */
  _2scomp: function() {
    var b = this.storage, o = o, carry = 1;
    for (var i = o + 7; i >= o; i--) {
      var v = (b[i] ^ 0xff) + carry;
      b[i] = v & 0xff;
      carry = v >> 8;
    }
  },

  /**
   * Set the value. Takes any of the following arguments:
   *
   * setValue(string) - A hexidecimal string
   * setValue(number) - Number (throws if n is outside int64 range)
   * setValue(hi, lo) - Raw bits as two 32-bit values
   */
  setValue: function(hi, lo) {
    var negate = false;
    if (arguments.length == 1) {
      if (typeof(hi) == 'number') {
        // Simplify bitfield retrieval by using abs() value.  We restore sign
        // later
        negate = hi < 0;
        hi = Math.abs(hi);
        lo = hi % 0x80000000;
        hi = hi / 0x80000000;
        if (hi > 0x80000000) throw new RangeError(hi  + ' is outside Int64 range');
        hi = hi | 0;
      } else if (typeof(hi) == 'string') {
        hi = (hi + '').replace(/^0x/, '');
        lo = hi.substr(-8);
        hi = hi.length > 8 ? hi.substr(0, hi.length - 8) : '';
        hi = parseInt(hi, 16);
        lo = parseInt(lo, 16);
      } else {
        throw new Error(hi + ' must be a Number or String');
      }
    }

    // Technically we should throw if hi or lo is outside int32 range here, but
    // it's not worth the effort. Anything past the 32'nd bit is ignored.

    // Copy bytes to buffer
    var b = this.storage, o = 0;
    for (var i = 7; i >= 0; i--) {
      b[o+i] = lo & 0xff;
      lo = i == 4 ? hi : lo >>> 8;
    }

    // Restore sign of passed argument
    if (negate) this._2scomp();
  },

  /**
   * Convert to a native JS number.
   *
   * WARNING: Do not expect this value to be accurate to integer precision for
   * large (positive or negative) numbers!
   *
   * @param allowImprecise If true, no check is performed to verify the
   * returned value is accurate to integer precision.  If false, imprecise
   * numbers (very large positive or negative numbers) will be forced to +/-
   * Infinity.
   */
  toNumber: function(allowImprecise) {
    var b = this.storage, o = 0;

    // Running sum of octets, doing a 2's complement
    var negate = b[0] & 0x80, x = 0, carry = 1;
    for (var i = 7, m = 1; i >= 0; i--, m *= 256) {
      var v = b[o+i];

      // 2's complement for negative numbers
      if (negate) {
        v = (v ^ 0xff) + carry;
        carry = v >> 8;
        v = v & 0xff;
      }

      x += v * m;
    }

    // Return Infinity if we've lost integer precision
    if (!allowImprecise && x >= Int64.MAX_INT) {
      return negate ? -Infinity : Infinity;
    }

    return negate ? -x : x;
  },

  /**
   * Convert to a JS Number. Returns +/-Infinity for values that can't be
   * represented to integer precision.
   */
  valueOf: function() {
    return this.toNumber(false);
  },

  /**
   * Return string value
   *
   * @param radix Just like Number#toString()'s radix
   */
  toString: function(radix) {
    return this.valueOf().toString(radix || 10);
  },

  /**
   * Return a string showing the buffer octets, with MSB on the left.
   *
   * @param sep separator string. default is '' (empty string)
   */
  toOctetString: function(sep) {
    var out = new Array(8);
    var b = this.storage, o = 0;
    for (var i = 0; i < 8; i++) {
      out[i] = Int64.HexTable[b[o+i]];
    }
    return out.join(sep || '');
  }
};

I've created a small function which can be used if all you're interested in is validating if a string is Int64 or not.
https://gist.github.com/jrharshath/fe3887acde7d64e3e0385506f4033ad3

The implementation claims "dependency on buffer removed", but I still encounter the following issue:

var x = new Int64(0x123456789);
// throws "Uncaught ReferenceError: Buffer is not defined"

However, replacing if (a1 instanceof Buffer) { with if (typeof Buffer !== 'undefined' && a1 instanceof Buffer) { seems to have resolved the issue.