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Emscripten Library Generator

This packages normal JavaScript libraries into emscripten's strange, undocumented library format which can be passed to the emscripten compiler with the --js-library flag. It can also be used to automatically populate the emscripten setting EXPORTED_FUNCTIONS using the --unresolved flag. It works by computing dependency and initialization information for global symbols and automatically prefixing all resolved global symbols with an underscore to match emscripten's output. Top-level statements other than variable or function declarations are not supported (put initialization into a JavaScript function that is called as the first statement inside main() in C++).

Library Generation

Terminal commands:

npm install -g emscripten-library-generator
emscripten-library-generator input1.js input2.js ... > library.js

Example input:

var handles = {};
function Foo(data) { = data;
function Foo_new(ptr, data) {
  handles[ptr] = new Foo(data);
function Foo_delete(ptr) {
  delete handles[ptr];

Example output:

mergeInto(LibraryManager.library, {
    handles: {},
    Foo: function (data) { = data;
    Foo_new__deps: [
    Foo_new: function (ptr, data) {
        _handles[ptr] = new _Foo(data);
    Foo_delete__deps: ['handles'],
    Foo_delete: function (ptr) {
        delete _handles[ptr];

Usage from C++:

struct Foo;

extern "C" {
  void Foo_new(Foo *foo, int data);
  void Foo_delete(Foo *foo);

struct Foo {
  Foo(int data) : _data(data) {
    Foo_new(this, data);
  ~Foo() {
  int _data;

Unresolved Symbols

Finds all unresolved symbols that start with an underscore, which are assumed to be extern "C" functions in C++. If these symbol names are not specified, the emscripten compiler may omit those functions as dead code and JavaScript won't be able to access them.

Terminal commands (notice the --unresolved flag):

npm install -g emscripten-library-generator
emscripten-library-generator --unresolved input1.js input2.js ... > unresolved.json

Example input:

var timeout = 0;
function wait(value, delay) {
  if (timeout) {
  timeout = setTimeout(function() {
    timeout = 0;
  }, delay);

Example output:


Usage from C++:

extern "C" {
  void wait(int value, int delay);

  void interrupted(int value) {
    printf("interrupted: %d\n", value);

  void success(int value) {
    printf("success: %d\n", value);