Skip to main content

Local File Access

Reading and writing files require native support. readFile and writeFile include support for some approaches but do not support every API. When an API is not supported by readFile or writeFile, the underlying read and write methods can be used.

This demo looks at various web APIs. More specific approaches for deployments like mobile apps are covered in their respective demos.


Some snippets are also available in the "Common Use Cases" section:


Not all web APIs are supported in all browsers. For example, Firefox does not support the "File System Access API".

Even when a browser technically supports a web API, it may be disabled in the client browser. Some APIs do not give any feedback.

Binary Data

Modern browser APIs typically use typed arrays or Blob or File structures.

Reading Binary Data

Given a Blob or File, the underlying data cannot be read synchronously!

The callback-based approach uses a FileReader:

const reader = new FileReader();
reader.onload = function(e) {
/* is an ArrayBuffer */
const wb =;

The Promise-based approach uses Blob#arrayBuffer:

// usage: const wb = await blob_to_wb(blob);
async function blob_to_wb(blob) {
return blob.arrayBuffer());

Writing Binary Data

XLSX.write can generate Uint8Array results by passing type: "buffer". A Blob can be created by using the constructor:

const u8 = XLSX.write(workbook, { type: "buffer", bookType: "xlsx" });
const blob = new Blob([u8], { type: "application/" });

Web Workers


None of the browser methods work from Web Worker contexts!

Data operations with the Web APIs must happen in the browser main thread.

Web Workers and main thread can transfer ArrayBuffer or Uint8Array objects.

When generating a file, the worker will call XLSX.write with type buffer and transfer the result to the main thread to initiate a download.

When parsing a file, the main thread will use the web API to read a File or Blob, extract the underlying ArrayBuffer and transfer to the Web Worker.

HTML5 Download Attribute

Writing Files

writeFile will attempt a download in the browser using the attribute.

XLSX.writeFile(wb, "SheetJS.xlsx");

File API

Reading Files

In the change event of <input type="file">, target hold a list of files:

async function handleFileAsync(e) {
/* get first file */
const file =[0];
/* get raw data */
const data = await file.arrayBuffer();
/* data is an ArrayBuffer */
const workbook =;
/* do something with the workbook here */
input_dom_element.addEventListener("change", handleFileAsync, false);

HTML Drag and Drop API

Reading Files

The dataTransfer property of the drop event holds a list of files:

async function handleDropAsync(e) {
e.stopPropagation(); e.preventDefault();
/* get first file */
const f = e.dataTransfer.files[0];
/* get raw data */
const data = await f.arrayBuffer();
/* data is an ArrayBuffer */
const wb =;
/* do something with the workbook here */
drop_dom_element.addEventListener("drop", handleDropAsync, false);

File System Access API

Limited Browser Support

At the time of writing, browser support was fairly limited. Chrome introduced the feature in version 86. Safari did not support File System Access API.

Reading Files

window.showOpenFilePicker shows a file picker and resolves to an array of file handles. When multiple: false is set, the array has one element.

The getFile method resolves to a File object whose data can be read with the arrayBuffer method:

/* Show picker and get data */
const [hFile] = await window.showOpenFilePicker({
types: [{
description: 'Spreadsheets',
accept: { 'application/': ['.xlsx', '.xls', '.xlsb', /*...*/] }
excludeAcceptAllOption: true,
multiple: false
const ab = await (await hFile.getFile()).arrayBuffer();

/* parse */
const wb =;

/* do something with the workbook */

Writing Files

window.showSaveFilePicker shows a file picker and resolves to a file handle. The createWritable method resolves to a FileSystemWritableFileStream, which readily accepts Uint8Array data from XLSX.write:

/* Show picker and get handle to file */
const hFile = await window.showSaveFilePicker({
suggestedName: "SheetJS.xlsx",
types: [
{ description: 'Excel 2007+ (XLSX)', accept: { 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet': ['.xlsx'] } },
{ description: 'Excel 97-2004 (XLS)', accept: { 'application/': ['.xls'] } },
{ description: 'Excel 2007+ Binary (XLSB)', accept: { 'application/': ['.xlsb'] } },
/* note that each MIME type must be unique! */
const wstream = await hFile.createWritable();

/* get extension */
const ext =".")+1)
/* write */
wstream.write(XLSX.write(wb, { bookType: ext, type: "buffer" }))
/* close stream to commit file */


Live Example (click to show)

This live example reads a file then tries to save as XLSX.

Live Editor

File and Directory Entries API


In the web browser, the File and Directory Entries API has been deprecated and is not recommended for new applications.

cordova-plugin-file still uses the API patterns.

Writing Files

// Request File System Access
window.requestFileSystem(window.PERSISTENT, 0, (fs) => {
// Request a handle to "SheetJS.xlsx", making a new file if necessary
fs.root.getFile("SheetJS.xlsx", {create: true}, entry => {
// Request a FileWriter for writing data
entry.createWriter(writer => {
// The FileWriter API needs an actual Blob
const u8 = XLSX.write(wb, { type: "buffer", bookType: "xlsx" });
const data = new Blob([u8], { type: "application/" });
// `onwriteend` is called on success, `onerror` called on error
writer.onwriteend = () => {}; writer.onerror = () => {};
// write the data

Internet Explorer

Internet Explorer offered proprietary APIs that were not adopted by Chromium.

Blob API

Writing Files

IE10 and IE11 support navigator.msSaveBlob. writeFile will use the method.


Reading and Writing Files

Internet Explorer 6-9 with VBScript support Scripting.FileSystemObject. This is not supported in modern browsers.

This approach is implemented in the library readFile and writeFile methods. It requires the shim script to be loaded before the main library script:

<!-- load the shim script first -->
<script src="shim.min.js"></script>
<!-- then load the main script -->
<script src="xlsx.full.min.js"></script>

Other Platforms


fs.readFileSync and fs.writeFileSync allow for reading and writing files.

When using require, these are supported in readFile and writeFile:

var XLSX = require("xlsx");
var wb = XLSX.readFile("sheetjs.numbers");
XLSX.writeFile(wb, "sheetjs.xls");

Installation has a special note for use with NodeJS ECMAScript Modules:

import { readFile, writeFile, set_fs } from 'xlsx';
import * as fs from 'fs';

var wb = XLSX.readFile("sheetjs.numbers");
XLSX.writeFile(wb, "sheetjs.xlsx");


In Photoshop and other Adobe apps, readFile and writeFile use the File object under the hood:

#include "xlsx.extendscript.js"

var wb = XLSX.readFile("sheetjs.xlsx");
XLSX.writeFile(wb, "sheetjs.csv");


Deno.readFileSync and Deno.writeFileSync are supported by readFile and writeFile out of the box:

// @deno-types=""
import * as XLSX from '';

const wb = XLSX.readFile("sheetjs.numbers");
XLSX.writeFile(wb, "sheetjs.xlsx");

Any script using XLSX.readFile requires the --allow-read permission.

Any script using XLSX.writeFile requires the --allow-write permission.


Bun requires the fs module:

import * as XLSX from 'xlsx';
import * as fs from 'fs';

var wb = XLSX.readFile("sheetjs.numbers");
XLSX.writeFile(wb, "sheetjs.xlsx");


Desktop and mobile apps have their own specific APIs covered in separate demos: