Reading stream from "fetch" return chunks of random size, this function transforms the same to chunks of a given size in the browser

ChunkerTransformStream.js

// Reading stream from "fetch" return chunks of random size, this function transforms the same to chunks of a given size.

import { decrypt } from "./utils";

export const ChunkerTransformStream = (chunkSize, encryptionKey) => {
    chunkSize = chunkSize || 1024 * 1024 * 5;
    const SERVER_ENCRYPTION_IV_LENGTH = 12;
    const AES_GCM_TAG_SIZE = 16
    chunkSize = chunkSize + SERVER_ENCRYPTION_IV_LENGTH + AES_GCM_TAG_SIZE;
    let chunk = new Uint8Array();

    const transform = async (newChunk, controller) => {
        const freeSpaceLength = chunkSize - chunk.length;
        if (newChunk.length > freeSpaceLength) {
            chunk = Uint8Array.from([...chunk, ...newChunk.slice(0, freeSpaceLength)])
            let decryptedData = await decrypt(chunk.buffer, encryptionKey);
            controller.enqueue(decryptedData);
            chunk = new Uint8Array();
            chunk = newChunk.slice(freeSpaceLength)
        } else {
            chunk = Uint8Array.from([...chunk, ...newChunk])
            if (chunk.length == chunkSize) {
                let decryptedData = await decrypt(chunk.buffer, encryptionKey);
                controller.enqueue(decryptedData);
                chunk = new Uint8Array(0);
            }
        }
    }

    const flush = async (controller) => {
        if (chunk.length != 0) {
            let decryptedData = await decrypt(chunk.buffer, encryptionKey);
            controller.enqueue(decryptedData);
        }
        controller.terminate();
    }

    var chunker = new TransformStream({ transform, flush });

    return chunker;
}