How to inspect Javascript Objects

Question

How can I inspect an Object in an alert box? Normally alerting an Object just throws the nodename:

alert(document);

But I want to get the properties and methods of the object in the alert box. How can I achieve this functionality, if possible? Or are there any other suggestions?

Particularly, I am seeking a solution for a production environment where console.log and Firebug are not available.

Answer 1

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.

Answer 2

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.

Answer 3

Use console.dir(object) and the Firebug plugin

Answer 4

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 ) ; 

Answer 5

Use your console:

console.log(object);

Or if you are inspecting html dom elements use console.dir(object). Example:

let element = document.getElementById('alertBoxContainer');
console.dir(element);

Or if you have an array of js objects you could use:

console.table(objectArr);

If you are outputting a lot of console.log(objects) you can also write

console.log({ objectName1 });
console.log({ objectName2 });

This will help you label the objects written to console.

Answer 6

var str = "";
for(var k in obj)
    if (obj.hasOwnProperty(k)) //omit this test if you want to see built-in properties
        str += k + " = " + obj[k] + "\n";
alert(str);

Answer 7

This is blatant rip-off of Christian's excellent answer. I've just made it a bit more readable:

/**
 * objectInspector digs through a Javascript object
 * to display all its properties
 *
 * @param object - a Javascript object to inspect
 * @param result - a string of properties with datatypes
 *
 * @return result - the concatenated description of all object properties
 */
function objectInspector(object, result) {
    if (typeof object != "object")
        return "Invalid object";
    if (typeof result == "undefined")
        result = '';

    if (result.length > 50)
        return "[RECURSION TOO DEEP. ABORTING.]";

    var rows = [];
    for (var property in object) {
        var datatype = typeof object[property];

        var tempDescription = result+'"'+property+'"';
        tempDescription += ' ('+datatype+') => ';
        if (datatype == "object")
            tempDescription += 'object: '+objectInspector(object[property],result+'  ');
        else
            tempDescription += object[property];

        rows.push(tempDescription);
    }//Close for

    return rows.join(result+"\n");
}//End objectInspector

Answer 8

Here is my object inspector that is more readable. Because the code takes to long to write down here you can download it at http://etto-aa-js.googlecode.com/svn/trunk/inspector.js

Use like this :

document.write(inspect(object));