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These are "javascript bindings" to liblouis created by cross compiling liblouis using emscripten. The Liblouis API written in C can be directly called using the ccall and cwrap functions provided by emscripten. As directly calling the C API is cumbersome, an additional API — called Easy API — is provided for most functions. This package supports NodeJS and browser environments.

npm version Bower version latest build in the npm registry latest build in the bower registry

build status of latest Easy-Api build status of latest C-Api

Table of Contents

  1. API Overview
    1. Installation
    2. List of Available Liblouis Functions
    3. List of Additional Easy-API Methods
    4. Compiling the Latest Version of Liblouis
    5. Testing a Liblouis Build
  2. Usage Examples
    1. Printing the Version Number Using the Easy API in the Browser
    2. Printing the Version Number By Directly Calling Liblouis in the Browser
    3. Printing the Version Number Using the Easy API in NodeJS
    4. Translating and Back-Translating a String Using the Easy API
    5. Altering a Table Definition on Run-Time
    6. Downloading Table Files on Demand in the Browser
    7. Loading Table Files From Disk in NodeJS
    8. Debugging and Adjusting the Log Level
    9. Persisting Log Files in NodeJS using Deprecated Liblouis Log Functions
    10. Dropping the Path Prefix of Bundled Tables
    11. Usage with Typescript
    12. Usage with Webpack
    13. Switching or Running Multiple C-API Builds in Parallel
  3. Changelog
  4. Licensing

API Overview


With NPM

npm install liblouis

This will install the latest available stable release version of liblouis' C-API and is equivalent to an installation with:

npm install liblouis-build@latest
npm install liblouis

If you want to fetch a specific version of the C-API, for example version 3.2.0, you can use the following commands:

npm install liblouis-build@3.2.0
npm install liblouis

Builds of release versions are available within minutes of the C-API release. They are however initially published as release candidates using a -rc* suffix. You can use npm view liblouis-build versions to list all release versions and all release candidates available.

If you want to install the latest available development version of liblouis' C-API execute:

npm install liblouis/js-build
npm install liblouis

If you want to install a specific development version of liblouis' C-API, you can specify the commit hash:

npm install liblouis/js-build#commit-4b4c02
npm install liblouis

You have to specify exactly 6 digits of the commit hash. Some commits won't have a prebuilt binary available. In this case you can build liblouis yourself.

Warning: While the programatic interface of liblouis-build adheres to the semantic versioning specification, table files do not. You should refrain from adding liblouis-build as dependency with a version range if you are using liblouis with custom table files.

List of Available Liblouis Functions

All liblouis C-API functions listed in the official liblouis documentation can be directly called using the ccall and cwrap functions. However, you have to explicitly copy all arguments to the heap and specify the type of the argument as described in the emscripten documentation.

Most functions in the C-API have an identically named method in the Easy-API that correctly invokes the C-API for you. The table below lists all functions that were implemented:

Method Easy API Direct Call API
lou_translateString ✔*
lou_backTranslateString ✔*
lou_logFile ✖**
lou_logPrint ✖**
lou_logEnd ✖**

* only BMP tested

** lou_logPrint, lou_logFile and lou_logEnd will not be implemented as they are deprecated.

List of Additional Easy-API Methods

The following methods do not have a corresponding C-API function, but are part of the Easy-API.


Allows you to make lightweight liblouis builds that do not include any or all required table files. Instead table files not embedded in the liblouis C-API build are downloaded from the specified url on first access. For usage examples, see Downloading Table Files on Demand in the Browser and Loading Table Files from Disk in NodeJS.


new LiblouisEasyApi(build :EmscriptenModule|string);

Create an additional instance of the Easy-API running in the same thread as the caller. Takes either a liblouis C-API build or a liblouis release version number as argument. Also see Switching or Running Multiple C-API Builds in Parallel.


new LiblouisAsyncEasyApi({
    capi: string,
    easyapi: string

Use this constructor to instantiate additional instances of the Easy-API. capi and easyapi are urls of both script files.

Warning: These scripts at the specified URLs are downloaded and executed in a worker thread. It is your responsibility to sanatize these URLs to prevent execution of malicous code.

Compiling the Latest Version of Liblouis

# obtain the latest version 
git clone
cd liblouis
# build 
emconfigure ./configure --disable-shared
emmake make
emcc ./liblouis/.libs/liblouis.a -s RESERVED_FUNCTION_POINTERS=1 -s MODULARIZE=1\
     -s EXPORT_NAME="'liblouisBuild'" -s EXTRA_EXPORTED_RUNTIME_METHODS="['FS',\
    'Runtime', 'stringToUTF16', 'Pointer_Stringify']" --pre-js ./liblouis-js/inc/pre.js\
     --post-js ./liblouis-js/inc/post.js -o build-no-tables.js
cat ./inc/append.js >> build-no-tables.js

In liblouis versions prior to release 3.2.0, you have to list all exported API functions:

emcc ./liblouis/.libs/liblouis.a -s RESERVED_FUNCTION_POINTERS=1 -s MODULARIZE=1\
     -s EXPORTED_FUNCTIONS="['_lou_version', '_lou_translateString', '_lou_translate',\
    '_lou_backTranslateString', '_lou_backTranslate', '_lou_hyphenate',\
    '_lou_compileString', '_lou_getTypeformForEmphClass', '_lou_dotsToChar',\
    '_lou_charToDots', '_lou_registerLogCallback', '_lou_setLogLevel',\
    '_lou_logFile', '_lou_logPrint', '_lou_logEnd', '_lou_setDataPath',\
    '_lou_getDataPath', '_lou_getTable', '_lou_checkTable',\
    '_lou_readCharFromFile', '_lou_free', '_lou_charSize']"\
     -s EXPORT_NAME="'liblouisBuild'" -s EXTRA_EXPORTED_RUNTIME_METHODS="['FS',\
    'Runtime', 'stringToUTF16', 'Pointer_Stringify']" --pre-js ./liblouis-js/inc/pre.js\
     --post-js ./liblouis-js/inc/post.js -o build-no-tables.js

To include a list of table files or a directory containing table files use the --embed-file flag. For example, to embed all tables in a subfolder called tables add --embed-file tables, to embed all tables in the virtual filesystem root add --embed-file tables@/.

If you build liblouis for 32-bit Unicode, execute configure with --enable-ucs4 and subsitute stringToUTF16 with stringToUTF32.

Testing a Liblouis Build

You have to make your build available as the node module liblouis-build. If you just built a binary on your system, you can execute the following command in the liblouis-js package folder. The path ../js-build should point to the builds folder:

npm link ../js-build --production

To test all builds in NodeJS and the browser, run:

npm test

To only run tests in NodeJS, run:

npm run test-node

To only run tests in the browser, run:

npm run test-browser

Note that browser tests require PhantomJS to be installed. You can pass a list of build names to only test a subset of the available builds. The command will fail if a build cannot be found. A few examples:

npm test -- build-no-tables-utf16.js build-no-tables-utf32.js
npm run test-node -- build-no-tables-utf16.js build-no-tables-utf32.js
npm run test-browser -- build-no-tables-utf16.js build-no-tables-utf32.js

Usage Examples

A lot more examples can be found in the folder tests/.

Printing the Version Number Using the Easy API in the Browser

Include a liblouis build first and the Easy-API second.

<!doctype html>
<!-- use your package manager to obtain these files: `build-no-tables.js`
     is part of the package `liblouis-build`, `easy-api.js` is the main
     file of the `liblouis-js` package. -->
<script src="build-no-tables.js"></script>
<script src="easy-api.js"></script>
<script>"Liblouis Version:", liblouis.version());
// Should print:
// Liblouis Version: 3.2.0

Printing the Version Number By Directly Calling Liblouis in the Browser

You can include any liblouis build for this example.

<!doctype html>
<script src="build-no-tables.js"></script>
<script>"Liblouis Version:", liblouisBuild.ccall("lou_version", "string"));
// Should print:
// Liblouis Version: 3.2.0

Printing the Version Number Using the Easy API in NodeJS

Using require includes the Easy API, liblouis without tables and mounts the tables folder as tables.

const liblouis = require("liblouis");"Liblouis Version using Easy API:", liblouis.version());
// Should print:
// Liblouis Version using Easy API: 3.2.0

Translating and Back-Translating a String Using the Easy API

var unicode_braille = liblouis.translateString("tables/unicode.dis,tables/de-de-g0.utb", "10 Ziegen")
// Variable should contain:
// ⠼⠁⠚ ⠵⠊⠑⠛⠑⠝
console.log(liblouis.backTranslateString("tables/unicode.dis,tables/de-de-g0.utb", unicode_braille))
// Should print:
// 10 ziegen

Altering a Table Definition on Run-Time

console.log(liblouis.translateString("tables/unicode.dis,tables/de-de-g0.utb", "1"));
// Logs: ⠼⠁
liblouis.compileString("tables/unicode.dis,tables/de-de-g0.utb", "numsign 123456");
console.log(liblouis.translateString("tables/unicode.dis,tables/de-de-g0.utb", "1"));
// Logs: ⠿⠁

Downloading Table Files on Demand in the Browser

After including a build without a bundled table folder, use the Easy-API call enableOnDemandTableLoading with an absolute or relative URL to the table directory:

// enable and set base url for tables
// call any API (direct call API or easy API) method:
var unicode_braille = liblouis.translateString("tables/unicode.dis,tables/de-de-g0.utb", "10 Ziegen")
// Makes 12 XHR-Requests for table files. The variable should contain the same
// content as above:
// ⠼⠁⠚ ⠵⠊⠑⠛⠑⠝

Note that you have to run liblouis in a worker thread for enableOnDemandTableLoading to work [1].

You should call enableOnDemandTableLoading only once on initialization. Changing the table folder location during execution causes the filesystem to be inconsistent. If you have to change the folder location, reload the whole liblouis/emscripten instance.

Loading Table Files from Disk in NodeJS

In NodeJS environments, liblouis-js automatically tries to load the tables/ folder from disk if your built does not already bundle table files.

If you do not want to load tables from disk, you can disable this feature by calling:


Or if you want to change the location of the table folder, simply call:


Providing null will reset the path to the original folder location. enableOnDemandTableLoading file not free tables that were already compiled by liblouis automatically. You have to call yourself if you want liblouis to reload table files.

Debugging and Adjusting the Log Level

The available log levels are listed in the liblouis documentation. The log level constants can be accessed using liblouis.LOG[levelname]. The default log level of liblouis is liblouis.LOG.INFO. The log messages of enabled log levels are shown in the javascript console by default [2].

// log everything including debug messages
// replace the default message handler
liblouis.registerLogCallback(function(logLevel, msg){
    // logLevel is the constant associated with the log level.
    // you may obtain a string representation of the log level as follows:
    var logLevelName = liblouis.LOG[logLevel];
    // you may check for a specific log level:
    if(logLevel === liblouis.LOG.DEBUG) {"just recieved a debug message");
    // or alternatively using a string comparison:
    if(logLevelName === "DEBUG") {"just recieved a debug message");
    console.log(logLevel, logLevelName, msg);
    // Example output:
    // 10000 "DEBUG" "found table tables/de-de-g1.ctb"
// remove the custom message handler and use the default message handler

Persisting Log Files in NodeJS using Deprecated Liblouis Log Functions

The following example creates a log file, that is persisted on the hard drive. Note that the functions of liblouis called below (lou_logFile, lou_logPrint and lou_logEnd) are all deprecated. The example only demonstrates how liblouis can be called directly from nodeJS. In production systems, you should consider implementing your own log functionality using registerLogCallback.

const path = require('path');
const capi = require('liblouis-build');
const easyapi = require('liblouis');
// map a directory on the machine to a virtual directory of emscripten:
var hdd_directory = path.resolve(__dirname, 'logs/');
liblouis.getFilesystem().mount(NODEFS, { root: hdd_directory }, '/logs');
// log all messages:
// enable liblouis' deprecated "log to file"-functionality:
capi.ccall('lou_logFile', 'void', ['string'], ['/logs/liblouis.txt']); 
// map the new log functionality of liblouis to the deprecated log
// functionality:
easyapi.registerLogCallback(function(i, str) {
    capi.ccall('lou_logPrint', 'void', ['string'], [str]);
// do something that logs messages:
easyapi.translateString("tables/unicode.dis,tables/de-de-g0.utb", "10 Ziegen");
// Example log file contents are now:
// ----------------------------------
// Performing translation: tableList=tables/unicode.dis,tables/de-de-g0.utb, inlen=38
// Inbuf=0x0031 0x0030 0x0020 0x005A 0x0069 0x0065 0x0067 0x0065 0x006E 0x0000
// 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000
// 0x0000 0x0033 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000
// 0x0000 0x0000 0x0000 0x0000 0x0000 0x0000 ~ 10 Ziegen
// Cannot resolve table 'tables/unicode.dis'
// 1 errors found.
// tables/unicode.dis,tables/de-de-g0.utb could not be found

The log file is created if it does not exist. If it exists, new log messages are appended to the end of the file.

Dropping the Path Prefix of Bundled Tables

liblouis.enableOnDemandTableLoading lets you set the location of the table folder if you are not bundling tables in the build's binary. If you want to change the folder location inside the bundled virtual filesystem, you can use liblouis' lou_setDataPath as follows:

const PREFIX = "/tables";
const capi = require('liblouis-build');
const easyapi = require('liblouis');
capi.FS.symlink("/tables", "/liblouis/tables");

Usage with Typescript

The easy api is typed and plays well with typescript and npm.

Add the library as dependency to your package.json using

$ npm install --save liblouis

and add the following line to each file that uses liblouis

/// <reference types="liblouis"/>

Usage with Webpack

The following code example demonstrates how you can use the asynchronous Easy-API in a project using webpack.

In your module file:

var liblouis = require("liblouis/easy-api");
var capi_url = require("file-loader!liblouis-build");
var easyapi_url = require("file-loader!liblouis/easy-api");
require.context("liblouis-build/tables/", false);
var table_url  = "tables/";
var asyncLiblouis = new liblouis.EasyApiAsync({
    capi: capi_url,
    easyapi: easyapi_url
asyncLiblouis.version(function(version) {"Running liblouis version ", version, "through asynchronous API.");

The file loader plugin is used to copy the Easy-API and C-API files into the destination directory.

In your webpack configuration file add the following line:

/* ... */
externals: { 'liblouis-build': 'commonjs liblouis-build'}
module: {
    loaders: [
            test: /(\.cti|\.ctb|\.utb|\.dis|\.uti|\.tbl|\.dic)$/,
            loader: 'file-loader?name=tables/[name].[ext]',
        /* ... your other loaders ... */
/* ... */

The externals property prevents webpack from unnecessarily embedding liblouis-build in the bundle. The loader setting instructs webpack to copy all table files that are required in your source file to be copied to a folder called tables/ in your output directory. In combination with the require.context call in your module file, this will copy all tables to the output directory.

Switching or Running Multiple C-API Builds in Parallel

In browser environments, each C-API build registers itself in the global variable

liblouisBuilds : { [version]: EmscriptenModule[] };

which is an object mapping liblouis release version numbers to arrays of C-API builds. The C-API build loaded first additionally registers the global variable liblouisBuild.

You can wrap each build in a LiblouisEasyApi or LiblouisAsyncEasyApi instance.


Release 0.5.0 (upcoming): Support for new table indexing feature.

Release 0.4.0: Removal of build switching liblouis.setLiblouisBuild in favor of multiple concurrent instances of the Easy-API LiblouisEasyApi; Introduction of async Easy-API LiblouisAsyncEasyApi that automatically creates a worker thread. this allows you to use on demand table loading effortlessly; support for UTF-32 builds of liblouis.

This release is backward compatible: Liblouis builds for previous versions can be used with liblouis-js 0.4.0.

Release 0.3.0: liblouis-js no longer bundles a build of the liblouis C-API [3]. Builds were moved to their own npm and bower packages - this makes build switching and liblouis C-API selection easier; Adds support for on demand table file loading in NodeJS; implements lou_setDataPath and lou_getDataPath.

This release is backward compatible: Liblouis builds for version 0.2.x can be used with liblouis-js 0.3.0.

Release 0.2.1: liblouis-js is now an official part of liblouis. The npm and bower packages were renamed to liblouis. This release updates package URLs from reiner-dolp/liblouis-js to liblouis/liblouis-js.

Release 0.2.0: Adding support for nodeJS (Issue #10, #11) and commonJS; Adding type definitions for Typescript; Updates liblouis to version 2c849bc; Renaming easy api file to easy-api.js; Implements lou_compileString; Support for build switching; Emscripten methods are no longer leaked to global scope.

This release is backward compatible: Liblouis builds for version 0.1.0 can be used with liblouis-js 0.2.0.

Release 0.1.0: Adding libouis.setLogLevel and liblouis.registerLogCallback; updating liblouis builds to commit db2a361.

Release 0.0.3: Initial public release


Emscripten is available under 2 licenses, the MIT license and the University of Illinois/NCSA Open Source License. Liblouis is licensed under LGPLv2.1+. Note that table files may have a different license. Licensing information can be found in the header of the individual table files.

Code that is not part of liblouis and not part of emscripten is licensed under GPL-3.0. The text of the license can be found in the file LICENSE of this repository.


[1] Emscripten requires the files to be loaded synchroniously. Synchronous XHR, used to fetch the files, is deprecated in the main thread as it blocks all user interaction.

[2] Liblouis writes messages to stdout and stderr by default. Emscripten redirects these to Module.print and Module.printErr, which are implemented as: function print(x) { console.log(x); } and function printErr(x) { console.warn(x); }. There is no need to overwrite these functions. You can use liblouis#registerLogCallback(int logLevel, string msg), which additionally exposes the log level. The Easy API registers a log callback by default, which maps each message level to the correct console method, e.g. liblouis warning messages to console.warn and liblouis fatal errors to console.error.

[3] Before this change, liblouis and liblouis-js versions correspond as follows: 0.1.0 = 3.0.0 (db2a361), 0.2.0 = 3.1.0 (2c849bc).



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