An asm.js application is very fast (near native C++ speed):

enter image description here


But how is it possible to write one in C++, convert it to LLVM code, then do some trick with emscripten/asm.js ? I haven't found any tutorial about it.

And if I write the code in C++, then how to use the js API-s, for example XMLHttpRequest, WebSockets, Canvas or WebGL?

  • 3
    Sharing your research helps everyone. Tell us what you've tried and why it didn’t meet your needs. This demonstrates that you’ve taken the time to try to help yourself, it saves us from reiterating obvious answers, and most of all it helps you get a more specific and relevant answer. Also see How to Ask
    – gnat
    May 13, 2013 at 15:35
  • This third-party tutorial appears to address some of these questions: devosoft.org/an-introduction-to-web-development-with-emscripten Sep 13, 2015 at 2:33

3 Answers 3


I believe you are mistaken in your understanding of asm.js.

First off, from their FAQ

Q. Is asm.js a new language?
A. No, it's just (a subset of) JavaScript.

And you asked clarification added :

But how is it possible to write one [an asm.js application] in C++

You don't write an "asm.js application", rather asm.js is a target1 to compile your C++ code to.

This article by John Resig provides a number of details that may better explain how asm.js would be used.

Starting with this image:
C++ => clang / LLVM => emscripten => JS engine

you can see that asm.js is a translation target of emscripten. Emscripten handles translating the LLVM bytecode into JavaScript, and asm.js is a subset of JavaScript. Staying within asm.js' restricted subset of JavaScript allows the code to be optimized and run faster.

You also asked:

And if I write the code in C++, then how to use the js API-s

Again, you're kind of missing the point. Asm.js enables porting existing C/C++ applications into JavaScript so they can be run within a browser. You wouldn't normally be able to use JS APIs within your C/C++ code, and there's nothing magical about asm.js to allow that.

If you have a new application to write that needs JS APIs then you should write the application in JS and not futz with trying to write in C++ and then port to JavaScript.

And going back to Resig's article, there are two key quotes for your question:

the kind of applications that are going to target Asm.js, in the near future, are those that will benefit from the portability of running in a browser but which have a level of complexity in which a direct port to JavaScript would be infeasible


As you can probably see from the code above Asm.js isn’t designed to be written by hand. ... The most common use case for Asm.js right now is in applications complied from C/C++ to JavaScript. Almost none of these applications interact with the DOM in a meaningful way, beyond using WebGL and the like.

What you might want to consider doing instead is having a JavaScript program that calls the JS APIs that you need along with making calls to the C++ that you compiled to JavaScript. Have a look at this emscripten tutorial to see how to call C++ code from JavaScript.

For some additional research, emscripten has a tutorial that might help you get started with understanding how to take C++ code, run it through LLVM, and then target asm.js.

1 Strictly speaking, that's not true. The C/C++ code is unaware of what it's going to be compiled to, so I can't really call asm.js a target. Another tool (emscripten) takes the LLVM output and then translates to asm.js compliant JavaScript. But I'm going to call it a target because it's easier to understand.

  • ASM.js is a compile target for C/C++. So no your not writing C++ in asm.js your compiling C++ to asm.js
    – Calvin
    May 28, 2013 at 12:39
  • Only one mention comes to mind for applications started from scratch. In the case of games, having the code in C++ could be helpful for deploying on multiple platforms. Feb 13, 2017 at 19:44

Yes, you can write C++ code and compile it to asm.js, using emscripten. I haven't tried it myself, and I'm not sure how ready this is for prime time. It seems to be good enough to run a bunch of games though.

Here is a tutorial: http://kripken.github.io/emscripten-site/docs/getting_started/Tutorial.html . Looking at the tutorial, it seems quite easy to compile C++ code:

// hello.cpp

int main() {
  printf("hello, world!\n");
  return 1;
$ ./emcc tests/hello.cpp -o hello.html
  • 4
    That's actually C code. A C++ compiler is about two or three orders of magnitude more complex. Luckily, emscripten avoids that hard problem by compiling LLVM, and there's an existing C++-to-LLVM compiler.
    – MSalters
    May 13, 2013 at 21:05
  • 3
    @MSalters: It's also valid C++ code. Imagine that! Wow! Dec 10, 2014 at 5:41
  • @ThomasEding: You missed the point. The smaller the language you have to support, the easier it is to compile that language. The intersection of C and C++ is necessarily not bigger than either of those two.
    – MSalters
    Dec 10, 2014 at 23:55
  • Let's suppose that this code was pure C++, that a C compiler would not handle, would the use of emcc be valid? Jan 18, 2016 at 14:08
  • @HamzaOuaghad - yes. a simple hello world with c++'s cout & string classes works just fine with this emcc command line. using version 1.35.0. Nov 1, 2016 at 23:38

The easiest way would be to use WCPP, a package that lets you import C++ nearly directly into your Node project.

Our C++

// addTwo.cpp 

export int addTwo(int a, int b) {
  return a + b;

In the terminal (to compile our C++)

$ wcpp

Our JavaScript

const ourModule = await require('wcpp')('./addTwo.cpp')

console.log(ourModule.addTwo(2, 3))

For more information, see the NPM Package or the Git Repo

Not the answer you're looking for? Browse other questions tagged or ask your own question.