JavaScript Numbers are represented as IEEE 754 double-precision floats. Unfortunately, this means they lose integer precision for values beyond +/- 2^^53. For projects that need to accurately handle 64-bit ints, such as node-thrift, a performant, Number-like class is needed. Int64 is that class.

Int64 instances look and feel much like JS-native Numbers. By way of example ...

1// First, let's illustrate the problem ...
2> (0x123456789).toString(16)
3'123456789' // <- what we expect.
4> (0x123456789abcdef0).toString(16)
5'123456789abcdf00' // <- Ugh!  JS doesn't do big ints. :(
6
7// So let's create a couple Int64s using the above values ...
8
9// Require, of course
10> Int64 = require('node-int64')
11
12// x's value is what we expect (the decimal value of 0x123456789)
13> x = new Int64(0x123456789)
14[Int64 value:4886718345 octets:00 00 00 01 23 45 67 89]
15
16// y's value is Infinity because it's outside the range of integer
17// precision.  But that's okay - it's still useful because it's internal
18// representation (octets) is what we passed in
19> y = new Int64('123456789abcdef0')
20[Int64 value:Infinity octets:12 34 56 78 9a bc de f0]
21
22// Let's do some math.  Int64's behave like Numbers.  (Sorry, Int64 isn't
23// for doing 64-bit integer arithmetic (yet) - it's just for carrying
24// around int64 values
25> x + 1
264886718346
27> y + 1
28Infinity
29
30// Int64 string operations ...
31> 'value: ' + x
32'value: 4886718345'
33> 'value: ' + y
34'value: Infinity'
35> x.toString(2)
36'100100011010001010110011110001001'
37> y.toString(2)
38'Infinity'
39
40// Use JS's isFinite() method to see if the Int64 value is in the
41// integer-precise range of JS values
42> isFinite(x)
43true
44> isFinite(y)
45false
46
47// Get an octet string representation.  (Yay, y is what we put in!)
48> x.toOctetString()
49'0000000123456789'
50> y.toOctetString()
51'123456789abcdef0'
52
53// Finally, some other ways to create Int64s ...
54
55// Pass hi/lo words
56> new Int64(0x12345678, 0x9abcdef0)
57[Int64 value:Infinity octets:12 34 56 78 9a bc de f0]
58
59// Pass a Buffer
60> new Int64(new Buffer([0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0xf0]))
61[Int64 value:Infinity octets:12 34 56 78 9a bc de f0]
62
63// Pass a Buffer and offset
64> new Int64(new Buffer([0,0,0,0,0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0xf0]), 4)
65[Int64 value:Infinity octets:12 34 56 78 9a bc de f0]
66
67// Pull out into a buffer
68> new Int64(new Buffer([0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0xf0])).toBuffer()
69<Buffer 12 34 56 78 9a bc de f0>
70
71// Or copy into an existing one (at an offset)
72> var buf = new Buffer(1024);
73> new Int64(new Buffer([0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0xf0])).copy(buf, 512);