Fastest way to concatenate two JS uint8 arrays to a uint16 array?

Question

I know this is fairly easy in other languages, but what is the best way to do this in JS if I have two arrays of 8-bit unsigned integers, and would like to concatenate them to one array of unsigned 16 bit integers?

Answer 1

Assuming both Arrays have the same length, use a for loop with the bitwise left shift operator <<;

var arr8A = [2, 3, 4], // example
    arr8B = [5, 6, 7];
// code start
var arr16 = new Uint16Array(arr8A.length),
    i;
for (i = 0; i < arr8A.length; ++i)
    arr16[i] = (arr8A[i] << 8) + arr8B[i];
arr16;

---

The reverse is similar but you use the bitwise zero-fill right shift operator >>> and the bitwise AND operator &;

// arr16 as above
var arr8A = new Uint8Array(arr16.length),
    arr8B = new Uint8Array(arr16.length),
    i;
for (i = 0; i < arr16.length; ++i) {
    arr8A[i] = arr16[i] >>> 8;
    arr8B[i] = arr16[i] & 255;
}
console.log(arr8A, arr8B);

The maximum size numbers you can safely use these operators with are 32-bit integers.