Instead of copying every element in a for loop, use typedArray.set
method, it's faster
function resizeUint8(baseArrayBuffer, newByteSize) {
var resizedArrayBuffer = new ArrayBuffer(newByteSize),
len = baseArrayBuffer.byteLength,
resizeLen = (len > newByteSize)? newByteSize : len;
(new Uint8Array(resizedArrayBuffer, 0, resizeLen)).set(new Uint8Array(baseArrayBuffer, 0, resizeLen));
return resizedArrayBuffer;
}
JSFiddle test: http://jsfiddle.net/92zsn529/