buffer: make byteLength work with ArrayBuffer & DataView

Convert anything to string, but Buffer, TypedArray and ArrayBuffer

```
var uint8 = new Uint8Array([0xf0, 0x9f, 0x90]);
Buffer.byteLength(uint8); // should be 3, but returns 11
Buffer.byteLength(uint8.buffer); // should be 3, but return 20
```

PR-URL: #5255
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/buffer.markdown

@@ -451,7 +451,7 @@ additional performance that `Buffer.allocUnsafe(size)` provides.
 
 ### Class Method: Buffer.byteLength(string[, encoding])
 
-* `string` {String}
+* `string` {String | Buffer | TypedArray | DataView | ArrayBuffer}
 * `encoding` {String} Default: `'utf8'`
 * Return: {Number}
 
@@ -470,6 +470,11 @@ console.log(`${str}: ${str.length} characters, ` +
 // ½ + ¼ = ¾: 9 characters, 12 bytes
 ```
 
+When `string` is a `Buffer`/[`DataView`][]/[`TypedArray`][]/`ArrayBuffer`,
+returns the actual byte length.
+
+Otherwise, converts to `String` and returns the byte length of string.
+
 ### Class Method: Buffer.compare(buf1, buf2)
 
 * `buf1` {Buffer}
@@ -1744,4 +1749,6 @@ console.log(buf);
 [buffer_from_string]: #buffer_class_method_buffer_from_str_encoding
 [buffer_allocunsafe]: #buffer_class_method_buffer_allocraw_size
 [buffer_alloc]: #buffer_class_method_buffer_alloc_size_fill_encoding
-[`TypedArray.from()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/from
+[`TypedArray.from()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/from
+[`DataView`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView
+[`TypedArray`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

lib/buffer.js

@@ -340,11 +340,12 @@ function base64ByteLength(str, bytes) {
 
 
 function byteLength(string, encoding) {
-  if (string instanceof Buffer)
-    return string.length;
+  if (typeof string !== 'string') {
+    if (ArrayBuffer.isView(string) || string instanceof ArrayBuffer)
+      return string.byteLength;
 
-  if (typeof string !== 'string')
     string = '' + string;
+  }
 
   var len = string.length;
   if (len === 0)

test/parallel/test-buffer-bytelength.js

@@ -3,19 +3,53 @@
 require('../common');
 var assert = require('assert');
 var Buffer = require('buffer').Buffer;
+var SlowBuffer = require('buffer').SlowBuffer;
 
 // coerce values to string
 assert.equal(Buffer.byteLength(32, 'binary'), 2);
 assert.equal(Buffer.byteLength(NaN, 'utf8'), 3);
 assert.equal(Buffer.byteLength({}, 'binary'), 15);
 assert.equal(Buffer.byteLength(), 9);
 
+var buff = new Buffer(10);
+assert(ArrayBuffer.isView(buff));
+var slowbuff = new SlowBuffer(10);
+assert(ArrayBuffer.isView(slowbuff));
+
 // buffer
 var incomplete = new Buffer([0xe4, 0xb8, 0xad, 0xe6, 0x96]);
 assert.equal(Buffer.byteLength(incomplete), 5);
 var ascii = new Buffer('abc');
 assert.equal(Buffer.byteLength(ascii), 3);
 
+// ArrayBuffer
+var buffer = new ArrayBuffer(8);
+assert.equal(Buffer.byteLength(buffer), 8);
+
+// TypedArray
+var int8 = new Int8Array(8);
+assert.equal(Buffer.byteLength(int8), 8);
+var uint8 = new Uint8Array(8);
+assert.equal(Buffer.byteLength(uint8), 8);
+var uintc8 = new Uint8ClampedArray(2);
+assert.equal(Buffer.byteLength(uintc8), 2);
+var int16 = new Int16Array(8);
+assert.equal(Buffer.byteLength(int16), 16);
+var uint16 = new Uint16Array(8);
+assert.equal(Buffer.byteLength(uint16), 16);
+var int32 = new Int32Array(8);
+assert.equal(Buffer.byteLength(int32), 32);
+var uint32 = new Uint32Array(8);
+assert.equal(Buffer.byteLength(uint32), 32);
+var float32 = new Float32Array(8);
+assert.equal(Buffer.byteLength(float32), 32);
+var float64 = new Float64Array(8);
+assert.equal(Buffer.byteLength(float64), 64);
+
+// DataView
+var dv = new DataView(new ArrayBuffer(2));
+assert.equal(Buffer.byteLength(dv), 2);
+
 // special case: zero length string
 assert.equal(Buffer.byteLength('', 'ascii'), 0);
 assert.equal(Buffer.byteLength('', 'HeX'), 0);