Mark Hansen's Blog

I’ve been having some problems using JSON.Stringify to inspect objects while debugging inside node.js. It seems like a great choice:

var obj = { 
    "prop1" : "value1", 
    "prop2" : 5
}
print(JSON.stringify(obj));
//prints: {"prop1":"value1","prop2":5}

But there are gotchas:

var infinity = 1 / 0;
var func = function() {};
func.innerproperty = "value";
var obj = {
    "prop1" : func,
    "prop2" : undefined,
    "prop3" : infinity
}
print(JSON.stringify(obj);
//prints {"prop3":null}

Oh no! JSON.Stringify has

• ignored prop1, a function
• ignored func.innerproperty
• ignored prop2, an undefined value
• replaced prop3’s Infinity with null.

Yikes! This is because JSON, being a pure data interchange format, has no notation for functions. Or undefined values. And it replaces nonfinite numbers with null.

In node.js, there’s a dedicated function for stringifying objects for debuggings: sys.inspect(object). See the API docs. This fares somewhat better for the last example, outputting:

{
 "prop1": [Function],
 "prop2": undefined,
 "prop3": Infinity
}

EDIT: I opened an issue, and it was fixed in less than a day. Functions with properties are now listed too. Thanks, creationix!

This is pretty important, because it’s very common to use functions as property holders in popular javascript libraries.

• jQuery uses $ both as a selector function, and a library object
• underscore.js uses _ as a wrapper function, and a library object

javascript node.js