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

tiny_uploader.js

/*
Copyright (c) 2015-2025 Alessandro Diaferia

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of this software and associated documentation files (the "Software"), to deal
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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);
}

I believe that readAsBinaryString should be used instead of readAsArrayBuffer

@bevancollins - readAsBinaryString is non-standard. See https://developer.mozilla.org/en-US/docs/Web/API/FileReader/readAsBinaryString:

This method has been removed from the FileAPI standard. FileReader.readAsArrayBuffer() should be used instead.

chunk_read_callback, error_callback, chunk_size: camelCase please. Great solution tho! Didn't realize files could be sliced

Here's a simpler version btw:

var EventEmitter = require("events").EventEmitter

// file - The file to read from
// options - Possible options:
    // type - (Default: "Text") Can be "Text" or "ArrayBuffer" ("DataURL" is unsupported at the moment - dunno how to concatenate dataUrls
    // chunkSize - (Default: 64K) The number of bytes to get chunks of
// Returns an EventEmitter that emits the following events:
    // data(data) - Returns a chunk of data
    // error(error) - Returns an error. If this event happens, reading of the file stops (no end event will happen).
    // end() - Indicates that the file is done and there's no more data to read
// derivedfrom here http://stackoverflow.com/questions/14438187/javascript-filereader-parsing-long-file-in-chunks
function read(file, options) {
    var emitter = new EventEmitter()

    if(options === undefined) options = {}
    if(options.type === undefined) options.type = "Text"
    if(options.chunkSize === undefined) options.chunkSize = 64000

    var offset = 0, method = 'readAs'+options.type//, dataUrlPreambleLength = "data:;base64,".length

    var onLoadHandler = function(evt) {
        if(evt.target.error !== null) {
            emitter.emit('error', evt.target.error)
            return;
        }

        var data = evt.target.result

        offset += options.chunkSize
        emitter.emit('data', data)
        if (offset >= file.size) {
            emitter.emit('end')
        } else {
            readChunk(offset, options.chunkSize, file)
        }
    }

    var readChunk = function(_offset, length, _file) {
        var r = new FileReader()
        var blob = _file.slice(_offset, length + _offset)
        r.onload = onLoadHandler
        r[method](blob)
    }

    readChunk(offset, options.chunkSize, file)

    return emitter
}

Hi,
in chrome (at least)
evt.target.result.length for binary chunks return undefined, the line 29 should be:
offset += binary ?evt.target.result.byteLength: evt.target.result.length;
otherwise you will get the first chunk in binary mode, the rest will be empty.

you really save my live ❤️ works like a charm, I use a URL.createObjectURL to preview the file (.mp4)

What's the best way of determining if the file is binary or text?

I love it! This is a great starting point for my Google Cloud Storage resumable file upload.

@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