A tiny snippet for reading files chunk by chunk in plain JavaScript

tiny_uploader.js

/*
Copyright (c) 2015-2025 Alessandro Diaferia

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.


 * Valid options are:
 * - chunk_read_callback: a function that accepts the read chunk
                          as its only argument. If binary option
                          is set to true, this function will receive
                          an instance of ArrayBuffer, otherwise a String
 * - error_callback:      an optional function that accepts an object of type
                          FileReader.error
 * - success:             an optional function invoked as soon as the whole file has been
                          read successfully
 * - binary:              If true chunks will be read through FileReader.readAsArrayBuffer
 *                        otherwise as FileReader.readAsText. Default is false.
 * - chunk_size:          The chunk size to be used, in bytes. Default is 64K.
 */
function parseFile(file, options) {
    var opts       = typeof options === 'undefined' ? {} : options;
    var fileSize   = file.size;
    var chunkSize  = typeof opts['chunk_size'] === 'undefined' ?  64 * 1024 : parseInt(opts['chunk_size']); // bytes
    var binary     = typeof opts['binary'] === 'undefined' ? false : opts['binary'] == true;
    var offset     = 0;
    var self       = this; // we need a reference to the current object
    var readBlock  = null;
    var chunkReadCallback = typeof opts['chunk_read_callback'] === 'function' ? opts['chunk_read_callback'] : function() {};
    var chunkErrorCallback = typeof opts['error_callback'] === 'function' ? opts['error_callback'] : function() {};
    var success = typeof opts['success'] === 'function' ? opts['success'] : function() {};

    var onLoadHandler = function(evt) {
        if (evt.target.error == null) {
            offset += evt.target.result.length;
            chunkReadCallback(evt.target.result);
        } else {
            chunkErrorCallback(evt.target.error);
            return;
        }
        if (offset >= fileSize) {
            success(file);
            return;
        }

        readBlock(offset, chunkSize, file);
    }

    readBlock = function(_offset, length, _file) {
        var r = new FileReader();
        var blob = _file.slice(_offset, length + _offset);
        r.onload = onLoadHandler;
        if (binary) {
          r.readAsArrayBuffer(blob);
        } else {
          r.readAsText(blob);
        }
    }

    readBlock(offset, chunkSize, file);
}

@salazarr-js can u please elaborate, how did you do that ?

@alediaferia I would like to use this code in a project and I am wondering what is the license for it. Thanks in advance.

@bernawastaken hey! feel free to use it, I added a MIT license at the top of the file.

Thanks ! @alediaferia