userData dont work about box2d.js HOT 3 OPEN

// some typedef
var char = "i8";

// Malloc space for input string and returns pointer.
function mallocString(str){
    str = String(str);
    var strptr = Box2D._malloc(str.length + 1, char, Box2D.ALLOC_STACK);
    for(var offset = 0; offset < str.length; offset++){
        Box2D.setValue(strptr + offset, str.charCodeAt(offset), char);
    }
    Box2D.setValue(strptr + offset, 0, char);   // null terminated
    return strptr;
}

// Get string back from a string pointer.
function getString(strptr){
    if(strptr === 0) return "";
    var builder = "", c;
    while(c = Box2D.getValue(strptr++, char)){
        builder += String.fromCharCode(c);
    }
    return builder;
}

// ...

bodyDef.set_userData(mallocString("Hello world"));  // store "Hello world"
var body = world.CreateBody(bodyDef);

getString(body.GetUserData());  // "Hello world"

Box2D._free(body.GetUserData());
Box2D.destroy(body);