SOURCE

const a1 = new Uint8Array([1, 2, 3, 4, 5]);
const a2 = new Uint8Array([6, 7, 8, 9, 10, 20, 30]);

const arr = [a1, a2];

function merge(arr) {
    const bytes = [];
    const sizes = [];
    const length = arr.length;

    for (let a of arr) {
        sizes.push(a.length);
        for (let e of a) {
            bytes.push(e);
        }
    }

    return { bytes, sizes, length};
}

const result = merge([a1, a2]);
console.log(result);