How to inspect Javascript Objects

How about alert(JSON.stringify(object)) with a modern browser?

In case of TypeError: Converting circular structure to JSON, here are more options: How to serialize DOM node to JSON even if there are circular references?

The documentation: JSON.stringify() provides info on formatting or prettifying the output.

The for-in loops for each property in an object or array. You can use this property to get to the value as well as change it.

Note: Private properties are not available for inspection, unless you use a "spy"; basically, you override the object and write some code which does a for-in loop inside the object's context.

For in looks like:

for (var property in object) loop();

Some sample code:

function xinspect(o,i){
    if(typeof i=='undefined')i='';
    if(i.length>50)return '[MAX ITERATIONS]';
    var r=[];
    for(var p in o){
        var t=typeof o[p];
        r.push(i+'"'+p+'" ('+t+') => '+(t=='object' ? 'object:'+xinspect(o[p],i+'  ') : o[p]+''));
    }
    return r.join(i+'\n');
}

// example of use:
alert(xinspect(document));

Edit: Some time ago, I wrote my own inspector, if you're interested, I'm happy to share.

Edit 2: Well, I wrote one up anyway.

Use console.dir(object) and the Firebug plugin

There are few methods :

 1. typeof tells you which one of the 6 javascript types is the object. 
 2. instanceof tells you if the object is an instance of another object.
 3. List properties with for(var k in obj)
 4. Object.getOwnPropertyNames( anObjectToInspect ) 
 5. Object.getPrototypeOf( anObject )
 6. anObject.hasOwnProperty(aProperty) 

In a console context, sometimes the .constructor or .prototype maybe useful:

console.log(anObject.constructor ); 
console.log(anObject.prototype ) ;