Example of calling of JavaScript code from C++ with Cheerp

compile.sh

/opt/cheerp/bin/clang++ example.cpp -o example.js -O3 -cheerp-pretty-code
#WebAssembly is the default target
#-cheerp-pretty-code is not needed, but the resulting code is more readable for this example purposes

example.cpp

#include <cmath>

namespace client
{
  //Function defined in the namespace client are only forward declared
  //and have to be defined in the JavaScript side
  //This is the same mechanism that allows to forward declare any browsers APIs
  double getRandomNumber();
};

void [[cheerp::genericjs]] printEstimatedAverageAndStdDev(double mean, double stdDev)
{
  //This is a "regular" C++ function that the attribute [[cheerp::genericjs]] forces to be compiled to JavaScript
  //The __asm__ keyworld allows to directly "inject" JavaScript code
  //With strong requirements on the types involved, it can also take input/output
  //See more details here: https://github.com/leaningtech/cheerp-meta/wiki/JavaScript-interoperability#the-_asm_-keyword
  __asm__("console.log('Estimated average            = ', %0);" :: "r" (mean));
  __asm__("console.log('Estimated standard deviation = ', %0);" :: "r" (stdDev));
}

int main()
{
        double sum = 0.0;
        double sum2 = 0.0;
        double n = 0;

        for (int i=0; i<1e6; i++)
        {
                double x = client::getRandomNumber();

                sum += x;
                sum2 += x*x;
                n += 1.0;
        }

        const double average = sum / n;
        const double variance = sum2 / n - average * average;
        const double stdDev = sqrt(variance);
  
        printEstimatedAverageAndStdDev(average, stdDev);

        return 0;
}

example.js

"use strict";
/*Compiled using Cheerp (R) by Leaning Technologies Ltd*/
var __imul=Math.imul;
var __fround=Math.fround;
var oSlot=0;var nullArray=[null];var nullObj={d:nullArray,o:0};
function fetchBuffer(p){
        var b=null,f='function';
        if(typeof fetch===f)b=fetch(p).then(r=>r.arrayBuffer());
        else if(typeof require===f){
                p=require('path').join(__dirname, p);
                b=new Promise((y,n)=>{
                        require('fs').readFile(p,(e,d)=>{
                                if(e)n(e);
                                else y(d);
                        });
                });
        }else b=new Promise((y,n)=>{
                y(read(p,'binary'));
        });
        return b;
}
function __Z30printEstimatedAverageAndStdDevdd(Lmean,LstdDev){
        console.log('Estimated average            = ', Lmean);;
        console.log('Estimated standard deviation = ', LstdDev);;
}
function assignHeaps(Ldiv){
}
var __asm=null,__heap=null;function __dummy(){throw new Error('this should be unreachable');};
__dummy.promise=
fetchBuffer('example.wasm').then(Ldiv=>
WebAssembly.instantiate(Ldiv,
{i:{
                __Z30printEstimatedAverageAndStdDevdd:__Z30printEstimatedAverageAndStdDevdd,
                __ZN6client15getRandomNumberEv:getRandomNumber,
        }})
).then(Ldiv=>{
        __asm=Ldiv.instance.exports;
        __heap=__asm.memory.buffer;
        assignHeaps(__heap);
        __asm._main();
});
/////////////END OF FILE//////////////
function getRandomNumber()
{
  //This function has to be implemented separately, is not part of the output
  return Math.random();
  
  //or try with -> return 7.0;
  //or whatever, this is just JS, so feel free to call any function
}

example.wat

module
  (type (;0;) (func (param f64 f64)))
  (type (;1;) (func (result f64)))
  (type (;2;) (func))
  (type (;3;) (func (result i32)))
  (import "i" "__ZN6client30printEstimatedAverageAndStdDevEdd" (func (;0;) (type 0)))
  (import "i" "__ZN6client15getRandomNumberEv" (func (;1;) (type 1)))
  (func $__wasm_nullptr (type 2)
    unreachable)
  (func $main (type 3) (result i32)
    (local i32 f64 f64 f64 f64)
    i32.const 0
    global.get 1
    local.tee 2
    local.tee 3
    local.set 4
    local.set 0
    loop  ;; label = @1
      local.get 0
      i32.const 1
      i32.add
      local.tee 0
      call 1
      local.tee 1
      local.get 2
      f64.add
      local.set 2
      local.get 3
      local.get 1
      local.get 1
      f64.mul
      f64.add
      local.set 3
      local.get 4
      f64.const 0x1p+0 (;=1;)
      f64.add
      local.set 4
      i32.const 1000000
      i32.ne
      br_if 0 (;@1;)
    end 
    local.get 2
    local.get 4
    f64.div
    local.tee 2
    local.get 3
    local.get 4
    f64.div
    local.get 2
    local.get 2
    f64.mul
    f64.sub
    f64.sqrt
    call 0
    i32.const 0)
  (table (;0;) 1 anyfunc)
  (memory (;0;) 17 128)
  (global (;0;) (mut i32) (i32.const 1048576))
  (global (;1;) f64 (f64.const 0x0p+0 (;=0;)))
  (export "memory" (memory 0))
  (export "_main" (func $main))
  (elem (;0;) (i32.const 0) $__wasm_nullptr))