How to Inspect a JavaScript Object

Introduction

This is a useful tool whenever you work with JavaScript and the compatibility issues between navigators.

You can inspect any JavaScript objects and list them as indented, ordered by levels.
It shows you type and property name. If an object property can't be accessed, an error message will be shown.

Using the Code

function inspect(obj [, maxLevels [, level]]) 

Input Vars

• obj: Object to inspect
• maxLevels: Optional. Number of levels you will inspect inside the object. Default MaxLevels=1
• level: RESERVED for internal use of the function

Return Value

HTML formatted string containing all values of inspected object obj.

function inspect(obj, maxLevels, level)
{
  var str = '', type, msg;

    // Start Input Validations
    // Don't touch, we start iterating at level zero
    if(level == null)  level = 0;

    // At least you want to show the first level
    if(maxLevels == null) maxLevels = 1;
    if(maxLevels < 1)     
        return '<font color="red">Error: Levels number must be > 0</font>';

    // We start with a non null object
    if(obj == null)
    return '<font color="red">Error: Object <b>NULL</b></font>';
    // End Input Validations

    // Each Iteration must be indented
    str += '<ul>';

    // Start iterations for all objects in obj
    for(property in obj)
    {
      try
      {
          // Show "property" and "type property"
          type =  typeof(obj[property]);
          str += '<li>(' + type + ') ' + property + 
                 ( (obj[property]==null)?(': <b>null</b>'):('')) + '</li>';

          // We keep iterating if this property is an Object, non null
          // and we are inside the required number of levels
          if((type == 'object') && (obj[property] != null) && (level+1 < maxLevels))
          str += inspect(obj[property], maxLevels, level+1);
      }
      catch(err)
      {
        // Is there some properties in obj we can't access? Print it red.
        if(typeof(err) == 'string') msg = err;
        else if(err.message)        msg = err.message;
        else if(err.description)    msg = err.description;
        else                        msg = 'Unknown';

        str += '<li><font color="red">(Error) ' + property + ': ' + msg +'</font></li>';
      }
    }

      // Close indent
      str += '</ul>';

    return str;
}

History

• 22nd March, 2008: Article posted