How to append bytes, multi-bytes and buffer to ArrayBuffer in javascript?

Question

Javascript ArrayBuffer or TypedArrays dont have any kind of appendByte(), appendBytes(), or appendBuffer() methods. So if I want to fill an ArrayBuffer one value at a time, how do I do it?

var firstVal = 0xAB;              // 1 byte
var secondVal = 0x3D7F            // 2 bytes
var anotherUint8Array = someArr;

var buffer = new ArrayBuffer();   // I don't know the length yet
var bufferArr = new UInt8Array(buffer);

// following methods do not exist. What are the alternatives for each??
bufferArr.appendByte(firstVal);
bufferArr.appendBytes(secondVal);
bufferArr.appendBuffer(anotherUint8Array);

Answer 1

You can create a new TypedArray with a new ArrayBuffer, but you can't change the size of an existing buffer

function concatTypedArrays(a, b) { // a, b TypedArray of same type
    var c = new (a.constructor)(a.length + b.length);
    c.set(a, 0);
    c.set(b, a.length);
    return c;
}

Now can do

var a = new Uint8Array(2),
    b = new Uint8Array(3);
a[0] = 1; a[1] = 2;
b[0] = 3; b[1] = 4;
concatTypedArrays(a, b); // [1, 2, 3, 4, 0] Uint8Array length 5

If you want to use different types, go via Uint8Array as the smallest unit is a byte, i.e.

function concatBuffers(a, b) {
    return concatTypedArrays(
        new Uint8Array(a.buffer || a), 
        new Uint8Array(b.buffer || b)
    ).buffer;
}

This means .length will work as expected, you could now convert this to your typed array of choice (make sure it's a type that would accept the .byteLength of the buffer though)

---

From here, you could now implement any method you like for concatenating your data, e.g.

function concatBytes(ui8a, byte) {
    var b = new Uint8Array(1);
    b[0] = byte;
    return concatTypedArrays(ui8a, b);
}

var u8 = new Uint8Array(0);
u8 = concatBytes(u8, 0x80); // [128]

Answer 2

Paul's answer allows you to concatenate one TypedArray to an existing TypedArray. In ES6, you can use the following function to concatenate multiple TypedArrays:

function concatenate(resultConstructor, ...arrays) {
    let totalLength = 0;
    for (const arr of arrays) {
        totalLength += arr.length;
    }
    const result = new resultConstructor(totalLength);
    let offset = 0;
    for (const arr of arrays) {
        result.set(arr, offset);
        offset += arr.length;
    }
    return result;
}

const ta = concatenate(Uint8Array,
    Uint8Array.of(1, 2), Uint8Array.of(3, 4));
console.log(ta); // Uint8Array [1, 2, 3, 4]
console.log(ta.buffer.byteLength); // 4

To append a new byte is:

const byte = 3;
concatenate(Uint8Array, Uint8Array.of(1, 2), Uint8Array.of(byte));

This method is found in ExploringJS.