Is there an equivalent for var_dump (PHP) in Javascript?

Question

As the others said, you can use Firebug, and that will sort you out no worries on Firefox. Chrome & Safari both have a built-in developer console which has an almost identical interface to Firebug's console, so your code should be portable across those browsers. For other browsers, there's Firebug Lite.

If Firebug isn't an option for you, then try this simple script:

function dump(obj) {
    var out = '';
    for (var i in obj) {
        out += i + ": " + obj[i] + "\n";
    }

    alert(out);

    // or, if you wanted to avoid alerts...

    var pre = document.createElement('pre');
    pre.innerHTML = out;
    document.body.appendChild(pre)
}

I'd recommend against alerting each individual property: some objects have a LOT of properties and you'll be there all day clicking "OK", "OK", "OK", "O... dammit that was the property I was looking for".

If you are using firefox then the firebug plug-in console is an excellent way of examining objects

console.debug(myObject);

Alternatively you can loop through the properties (including methods) like this:

for (property in object) {
    // do what you want with property, object[property].value
}

A lot of modern browsers support the following syntax:

JSON.stringify(myVar);

It can't be stated enough that you can use console.debug(object) for this. This technique will save you literally hundreds of hours a year if you do this for a living :p

To answer the question from the context of the title of this question, here is a function that does something similar to a PHP var_dump. It only dumps one variable per call, but it indicates the data type as well as the value and it iterates through array's and objects [even if they are Arrays of Objects and vice versa]. I'm sure this can be improved on. I'm more of a PHP guy.

/**
 * Does a PHP var_dump'ish behavior.  It only dumps one variable per call.  The
 * first parameter is the variable, and the second parameter is an optional
 * name.  This can be the variable name [makes it easier to distinguish between
 * numerious calls to this function], but any string value can be passed.
 * 
 * @param mixed var_value - the variable to be dumped
 * @param string var_name - ideally the name of the variable, which will be used 
 *       to label the dump.  If this argumment is omitted, then the dump will
 *       display without a label.
 * @param boolean - annonymous third parameter. 
 *       On TRUE publishes the result to the DOM document body.
 *       On FALSE a string is returned.
 *       Default is TRUE.
 * @returns string|inserts Dom Object in the BODY element.
 */
function my_dump (var_value, var_name)
{
    // Check for a third argument and if one exists, capture it's value, else
    // default to TRUE.  When the third argument is true, this function
    // publishes the result to the document body, else, it outputs a string.
    // The third argument is intend for use by recursive calls within this
    // function, but there is no reason why it couldn't be used in other ways.
    var is_publish_to_body = typeof arguments[2] === 'undefined' ? true:arguments[2];

    // Check for a fourth argument and if one exists, add three to it and
    // use it to indent the out block by that many characters.  This argument is
    // not intended to be used by any other than the recursive call.
    var indent_by = typeof arguments[3] === 'undefined' ? 0:arguments[3]+3;

    var do_boolean = function (v)
    {
        return 'Boolean(1) '+(v?'TRUE':'FALSE');
    };

    var do_number = function(v)
    {
        var num_digits = (''+v).length;
        return 'Number('+num_digits+') '+v;
    };

    var do_string = function(v)
    {
        var num_chars = v.length;
        return 'String('+num_chars+') "'+v+'"';
    };

    var do_object = function(v)
    {
        if (v === null)
        {
            return "NULL(0)";
        }

        var out = '';
        var num_elem = 0;
        var indent = '';

        if (v instanceof Array)
        {
            num_elem = v.length;
            for (var d=0; d<indent_by; ++d)
            {
                indent += ' ';
            }
            out = "Array("+num_elem+") \n"+(indent.length === 0?'':'|'+indent+'')+"(";
            for (var i=0; i<num_elem; ++i)
            {
                out += "\n"+(indent.length === 0?'':'|'+indent)+"|   ["+i+"] = "+my_dump(v[i],'',false,indent_by);
            }
            out += "\n"+(indent.length === 0?'':'|'+indent+'')+")";
            return out;
        }
        else if (v instanceof Object)
        {
            for (var d=0; d<indent_by; ++d)
            {
                indent += ' ';
            }
            out = "Object \n"+(indent.length === 0?'':'|'+indent+'')+"(";
            for (var p in v)
            {
                out += "\n"+(indent.length === 0?'':'|'+indent)+"|   ["+p+"] = "+my_dump(v[p],'',false,indent_by);
            }
            out += "\n"+(indent.length === 0?'':'|'+indent+'')+")";
            return out;
        }
        else
        {
            return 'Unknown Object Type!';
        }
    };

    // Makes it easier, later on, to switch behaviors based on existance or
    // absence of a var_name parameter.  By converting 'undefined' to 'empty 
    // string', the length greater than zero test can be applied in all cases.
    var_name = typeof var_name === 'undefined' ? '':var_name;
    var out = '';
    var v_name = '';
    switch (typeof var_value)
    {
        case "boolean":
            v_name = var_name.length > 0 ? var_name + ' = ':''; // Turns labeling on if var_name present, else no label
            out += v_name + do_boolean(var_value);
            break;
        case "number":
            v_name = var_name.length > 0 ? var_name + ' = ':'';
            out += v_name + do_number(var_value);
            break;
        case "string":
            v_name = var_name.length > 0 ? var_name + ' = ':'';
            out += v_name + do_string(var_value);
            break;
        case "object":
            v_name = var_name.length > 0 ? var_name + ' => ':'';
            out += v_name + do_object(var_value);
            break;
        case "function":
            v_name = var_name.length > 0 ? var_name + ' = ':'';
            out += v_name + "Function";
            break;
        case "undefined":
            v_name = var_name.length > 0 ? var_name + ' = ':'';
            out += v_name + "Undefined";
            break;
        default:
            out += v_name + ' is unknown type!';
    }

    // Using indent_by to filter out recursive calls, so this only happens on the 
    // primary call [i.e. at the end of the algorithm]
    if (is_publish_to_body  &&  indent_by === 0)
    {
        var div_dump = document.getElementById('div_dump');
        if (!div_dump)
        {
            div_dump = document.createElement('div');
            div_dump.id = 'div_dump';

            var style_dump = document.getElementsByTagName("style")[0];
            if (!style_dump)
            {
                var head = document.getElementsByTagName("head")[0];
                style_dump = document.createElement("style");
                head.appendChild(style_dump);
            }
            // Thank you Tim Down [http://stackoverflow.com/users/96100/tim-down] 
            // for the following addRule function
            var addRule;
            if (typeof document.styleSheets != "undefined" && document.styleSheets) {
                addRule = function(selector, rule) {
                    var styleSheets = document.styleSheets, styleSheet;
                    if (styleSheets && styleSheets.length) {
                        styleSheet = styleSheets[styleSheets.length - 1];
                        if (styleSheet.addRule) {
                            styleSheet.addRule(selector, rule)
                        } else if (typeof styleSheet.cssText == "string") {
                            styleSheet.cssText = selector + " {" + rule + "}";
                        } else if (styleSheet.insertRule && styleSheet.cssRules) {
                            styleSheet.insertRule(selector + " {" + rule + "}", styleSheet.cssRules.length);
                        }
                    }
                };
            } else {
                addRule = function(selector, rule, el, doc) {
                    el.appendChild(doc.createTextNode(selector + " {" + rule + "}"));
                };
            }

            // Ensure the dump text will be visible under all conditions [i.e. always
            // black text against a white background].
            addRule('#div_dump', 'background-color:white', style_dump, document);
            addRule('#div_dump', 'color:black', style_dump, document);
            addRule('#div_dump', 'padding:15px', style_dump, document);

            style_dump = null;
        }

        var pre_dump = document.getElementById('pre_dump');
        if (!pre_dump)
        {
            pre_dump = document.createElement('pre');
            pre_dump.id = 'pre_dump';
            pre_dump.innerHTML = out+"\n";
            div_dump.appendChild(pre_dump);
            document.body.appendChild(div_dump);
        }  
        else
        {
            pre_dump.innerHTML += out+"\n";
        }
    }
    else
    {
        return out;
    }
}

You want to see the entire object (all nested levels of objects and variables inside it) in JSON form. JSON stands for JavaScript Object Notation, and printing out a JSON string of your object is a good equivalent of var_dump (to get a string representation of a JavaScript object). Fortunately, JSON is very easy to use in code, and the JSON data format is also pretty human-readable.

Example:

var objectInStringFormat = JSON.stringify(someObject);
alert(objectInStringFormat);