Question

I am having troubles with passing an object to a function using setTimeout;

function alertObject(obj){
   alert(obj); //This is supposed to display "[object Object]"
}

function startCountdown(){
   var myObj = new myClass();
   setTimeout("alertObject("+ myObj +")",1000);
}

startCountdown();

When I run this, I get an error saying “unexpected identifier”. I know I can use an anonymous function like this;

setTimeout(function(){alertObject(myObj)},1000);

Instead of

setTimeout("alertObject("+ myObj +")",1000);

But the thing is I want to know why you can not pass an object using the eval() function. It works with strings…

Answer 1

You can’t serialise an object whilst maintaining the identity of that object. (In any language, not just JavaScript.)

'alertObject('+myObj+')' involves turning the object into a string with toString(), resulting in alertObject([object Object]) which is clearly not valid JavaScript.

You can provide a toString() implementation that returns something that is valid JavaScript, and use that to create a new object that is like the original object:

function myClass(num) {
    this.num= num;
}
myClass.prototype.toString= function() {
    return 'new myClass('+this.num+')';
};

var a= new myClass(3);
var b= eval(''+a);     // 'new myClass(3)'
alert(a.num===b.num);  // true

but it isn’t the same object instance:

alert(a===b); // false

and there’s no way to get the actual original object, short of, for example, keeping a lookup of every instance of the object, and passing a key to that lookup.

Hiding code in strings sucks. This is one of the reasons you should never use setTimeout with a string argument. Go with passing the the function object in.