Make it possible to encode a portion of the buffer

[dfahlander]

If you want to encode a TypedArray, such as a Float64Array, Uint16Array or so, it can be backed by just a portion of an ArrayBuffer. If wanting to encode such a TypedArray without having to copy the portion into a new buffer, the encode function should take an offset and a length.

Example

var buffer = new ArrayBuffer(65536);
...fill buffer with something...

// Create a Uint16 view of portion of bytes between [1024...2047]:
var array = new Uint16Array(buffer, 1024, 512); // 512 = length in 16-bit units.

// Base64 encode that portion:
encode (array.buffer, array.byteOffset, array.byteLength);

[brettz9]

Any chance of getting this pulled?

This line:

var bytes = new Uint8Array(arraybuffer, offset || 0, length !== undefined ? length : arraybuffer.byteLength),

could be simplified, however, to:

var bytes = new Uint8Array(arraybuffer, offset, length),

as per https://tc39.github.io/ecma262/#sec-typedarray-buffer-byteoffset-length which references the ToIndex abstract operation indicating that if the byte offset is undefined, index should be 0. Likewise, for length, the steps indicate the equivalence of explicit or implicit undefined "If length is either not present or undefined, then..."

[dfahlander]

This PR is replaced by #8.