gistfile1.hpp

// sm.cpp

#define XP_UNIX
#include "mozjs/jsapi.h"
#include <string>
#include <string.h>
#include <assert.h>
#include <malloc.h>

/* The class of the global object. */
static JSClass global_class = {
    "global", JSCLASS_GLOBAL_FLAGS,
    JS_PropertyStub, JS_PropertyStub, JS_PropertyStub, JS_PropertyStub,
    JS_EnumerateStub, JS_ResolveStub, JS_ConvertStub, JS_FinalizeStub,
    JSCLASS_NO_OPTIONAL_MEMBERS
};

/* The error reporter callback. */
void reportError(JSContext *cx, const char *message, JSErrorReport *report)
{
    fprintf(stderr, "%s:%u:%s\n",
            report->filename ? report->filename : "<no filename>",
            (unsigned int) report->lineno,
            message);
}

JSBool silly_constructor( JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval ){
    jsval x;
    JS_NewNumberValue( cx , 3 , &x );
    JS_SetProperty( cx , obj , "x" , &x );

    *rval = OBJECT_TO_JSVAL(obj);

    return JS_TRUE;
}

JSClass silly_class = {
    "Silly" , 0 , 
    JS_PropertyStub, JS_PropertyStub, JS_PropertyStub, JS_PropertyStub,
    JS_EnumerateStub, JS_ResolveStub , JS_ConvertStub, JS_FinalizeStub,
    0 , 0 , 0 , 
    silly_constructor , 
    0 , 0 , 0 , 0
};

JSBool silly_func(JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval){
    jsval x;
    assert( JS_NewNumberValue( cx , 4 , &x ) );
    *rval = x;
}

JSFunctionSpec silly_functions[] = {
    { "y" , silly_func , 0 , 0 , 0 } ,
    { 0 }
};

JSBool native_print(JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval){
    assert( argc == 1 );
    JSString * so = JS_ValueToString( cx , argv[0] );

    jschar * s = JS_GetStringChars( so );
    size_t srclen = JS_GetStringLength( so );
    
    size_t len = srclen * 2;
    char * dst = (char*)malloc( len );
    assert( JS_EncodeCharacters( cx , s , srclen , dst , &len) );
    dst[len] = 0;

    printf( "%s\n" , dst );
    free( dst );

    return JS_TRUE;
}


int main(int argc, const char *argv[])
{
    /* JS variables. */
    JSRuntime *rt;
    JSContext *cx;
    JSObject  *global;

    /* Create a JS runtime. */
    rt = JS_NewRuntime(8L * 1024L * 1024L);
    if (rt == NULL)
        return 1;

    /* Create a context. */
    cx = JS_NewContext(rt, 8192);
    if (cx == NULL)
        return 1;
    JS_SetOptions(cx, JSOPTION_VAROBJFIX);
    //JS_SetVersion(cx, JSVERSION_LATEST);
    JS_SetErrorReporter(cx, reportError);

    /* Create the global object. */
    global = JS_NewObject(cx, &global_class, NULL, NULL);
    if (global == NULL)
        return 1;

    /* Populate the global object with the standard globals,
       like Object and Array. */
    if (!JS_InitStandardClasses(cx, global))
        return 1;
    

    /* Your application code here. This may include JSAPI calls
       to create your own custom JS objects and run scripts. */

    JS_DefineFunction( cx , global , "print" , &native_print , 1 , 0 );
    
    JSObject * proto = JS_InitClass( cx , global , 0 , &silly_class , 0 , 0 , 0 , silly_functions , 0 , 0 );
    assert( proto );
    assert( JS_DefineFunctions( cx , proto , silly_functions ) );

    const char * code = 
        "print( 'A:' + new Silly() );"
        "print( 'B:' + Silly.prototype );"
        "if ( ! Silly ) throw 'no Silly!';"
        "if ( (new Silly()).x != 3 ) throw 'Silly.x is bad'; "
        "print( 'y:' + (new Silly()).y );"
        "if ( (new Silly()).y() != 4 ) throw 'silly.y() is bad';"
        "if ( ! Silly.prototype ) throw 'no Silly prototype!';"
        "x = 5 + 2 + (new Silly()).x;";
    jsval ret;
    JS_EvaluateScript( cx , global, code , strlen( code ) , "anon" , 0 , &ret );
    
    jsval x;
    JS_GetProperty( cx , global , "x" , &x );

    double d;
    JS_ValueToNumber( cx , x , &d );
    printf( "output: %f\n" , d );

    /* Cleanup. */
    JS_DestroyContext(cx);
    JS_DestroyRuntime(rt);
    JS_ShutDown();
    return 0;
}

Hi, try to change line 112:
JSObject * proto = JS_InitClass( cx , global , 0 , &silly_class , silly_constructor , 0 , 0 , silly_functions , 0 , 0 );