JavaScript File Worker

Process files asynchronously by Web Worker in browsers.

Web Worker is a browser technology that allow some JavaScript code to be executed in the background, and very likely in another thread.

For example, if you want to compute the MD5 hash for a file at browser, the computation could take seconds depends on the size of the file, but you don't want the user interface to be frozen.

In this kind of situations, you can consider create a web worker for the job.

This package (file-worker) allows you to write file processing code easily that will be run in the background.

Usage

Step 1: Import the file

You can download the latest release for browser and import it in HTML.

<script src="FileWorker.js"></script>

Or using NPM if you use webpack or TypeScript. This package is available on NPM called file-worker as well.

npm install file-worker --save-dev

To use this package in JavaScript, you can require it

var FileWorker = require("file-worker");

or import it.

import FileWorker from "file-worker";

Because this library is written in TypeScript, if you are using an editor like WebStorm, you shall be able to get the handy auto-completion.

Step 2: Write file processor

Assuming you want to compute the MD5 of your file, you can write a processor like this. However there is an existing library md5-webworker that does exactly the same thing.

function md5Processor(reader, writer) {
    reader.onData = function (bytes) {
        writer.writeOnce(computeMD5(bytes));
    };
    reader.readAll();
}

Writing a processor is very straight forward. It's a function that takes 2 parameters, one reader and one writer. However, please take now that this function is to be executed at the web worker, so you cannot access any variables or libraries directly inside the function. What you can do is to use the very handy importScripts function to load the script asynchronously.

Step 3: Get processed content

Assuming you get a File object from the browser's FileAPI. You can pass it to file worker, together with a type processor.

FileWorker.readFile is an async function, i.e. it is non-blocking and will return a promise. You can either use .then and callback to get the result.

If you use it in browser or prefer traditional .then:

FileWorker.readFile(file, md5Processor)
.then(function(result) {
    console.log(result);
});

Or call it with await.

const getMD5(file) = async () => { await FileWorker.readFile(file, md5Processor); }

License

ISC License

Copyright (c) 2017, FileWorker Authors

Permission to use, copy, modify, and/or distribute this software for any
purpose with or without fee is hereby granted, provided that the above
copyright notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
PERFORMANCE OF THIS SOFTWARE.