Don't debug with JSON.Stringify

Posted on 10 February 2010 by .

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
//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
//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

However, note that it doesn't check for function properties. prop1.innerproperty is ignored.

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

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