JavaScript data formatting/pretty printer

Question

I'm trying to find a way to "pretty print" a JavaScript data structure in a human-readable form for debugging.

I have a rather big and complicated data structure being stored in JS and I need to write some code to manipulate it. In order to work out what I'm doing and where I'm going wrong, what I really need is to be able to see the data structure in its entirety, and update it whenever I make changes through the UI.

All of this stuff I can handle myself, apart from finding a nice way to dump a JavaScript data structure to a human-readable string. JSON would do, but it really needs to be nicely formatted and indented. I'd usually use Firebug's excellent DOM dumping stuff for this, but I really need to be able to see the entire structure at once, which doesn't seem to be possible in Firebug.

Answer 1

Use Crockford's JSON.stringify like this:

var myArray = ['e', {pluribus: 'unum'}];
var text = JSON.stringify(myArray, null, '\t'); //you can specify a number instead of '\t' and that many spaces will be used for indentation...

Variable text would look like this:

[
  "e",
   {
      "pluribus": "unum"
   }
]

---

By the way, this requires nothing more than that JS file - it will work with any library, etc.

Answer 2

I wrote a function to dump a JS object in a readable form, although the output isn't indented, but it shouldn't be too hard to add that: I made this function from one I made for Lua (which is much more complex) which handled this indentation issue.

Here is the "simple" version:

function DumpObject(obj)
{
  var od = new Object;
  var result = "";
  var len = 0;

  for (var property in obj)
  {
    var value = obj[property];
    if (typeof value == 'string')
      value = "'" + value + "'";
    else if (typeof value == 'object')
    {
      if (value instanceof Array)
      {
        value = "[ " + value + " ]";
      }
      else
      {
        var ood = DumpObject(value);
        value = "{ " + ood.dump + " }";
      }
    }
    result += "'" + property + "' : " + value + ", ";
    len++;
  }
  od.dump = result.replace(/, $/, "");
  od.len = len;

  return od;
}

I will look at improving it a bit.
Note 1: To use it, do od = DumpObject(something) and use od.dump. Convoluted because I wanted the len value too (number of items) for another purpose. It is trivial to make the function return only the string.
Note 2: it doesn't handle loops in references.

EDIT

I made the indented version.

function DumpObjectIndented(obj, indent)
{
  var result = "";
  if (indent == null) indent = "";

  for (var property in obj)
  {
    var value = obj[property];
    if (typeof value == 'string')
      value = "'" + value + "'";
    else if (typeof value == 'object')
    {
      if (value instanceof Array)
      {
        // Just let JS convert the Array to a string!
        value = "[ " + value + " ]";
      }
      else
      {
        // Recursive dump
        // (replace "  " by "\t" or something else if you prefer)
        var od = DumpObjectIndented(value, indent + "  ");
        // If you like { on the same line as the key
        //value = "{\n" + od + "\n" + indent + "}";
        // If you prefer { and } to be aligned
        value = "\n" + indent + "{\n" + od + "\n" + indent + "}";
      }
    }
    result += indent + "'" + property + "' : " + value + ",\n";
  }
  return result.replace(/,\n$/, "");
}

Choose your indentation on the line with the recursive call, and you brace style by switching the commented line after this one.

... I see you whipped up your own version, which is good. Visitors will have a choice.

Answer 3

You can use the following

<pre id="dump"></pre>
<script>
   var dump = JSON.stringify(sampleJsonObject, null, 4); 
   $('#dump').html(dump)
</script>

Answer 4

In Firebug, if you just console.debug ("%o", my_object) you can click on it in the console and enter an interactive object explorer. It shows the entire object, and lets you expand nested objects.

Answer 5

For Node.js, use:

util.inspect(object, [options]);

API Documentation

Answer 6

For those looking for an awesome way to see your object, check prettyPrint.js

Creates a table with configurable view options to be printed somewhere on your doc. Better to look than in the console.

var tbl = prettyPrint( myObject, { /* options such as maxDepth, etc. */ });
document.body.appendChild(tbl);

Answer 7

I'm programming in Rhino and I wasn't satisfied with any of the answers that were posted here. So I've written my own pretty printer:

function pp(object, depth, embedded) { 
  typeof(depth) == "number" || (depth = 0)
  typeof(embedded) == "boolean" || (embedded = false)
  var newline = false
  var spacer = function(depth) { var spaces = ""; for (var i=0;i<depth;i++) { spaces += "  "}; return spaces }
  var pretty = ""
  if (      typeof(object) == "undefined" ) { pretty += "undefined" }
  else if ( typeof(object) == "boolean" || 
            typeof(object) == "number" ) {    pretty += object.toString() } 
  else if ( typeof(object) == "string" ) {    pretty += "\"" + object + "\"" } 
  else if (        object  == null) {         pretty += "null" } 
  else if ( object instanceof(Array) ) {
    if ( object.length > 0 ) {
      if (embedded) { newline = true }
      var content = ""
      for each (var item in object) { content += pp(item, depth+1) + ",\n" + spacer(depth+1) }
      content = content.replace(/,\n\s*$/, "").replace(/^\s*/,"")
      pretty += "[ " + content + "\n" + spacer(depth) + "]"
    } else { pretty += "[]" }
  } 
  else if (typeof(object) == "object") {
    if ( Object.keys(object).length > 0 ){
      if (embedded) { newline = true }
      var content = ""
      for (var key in object) { 
        content += spacer(depth + 1) + key.toString() + ": " + pp(object[key], depth+2, true) + ",\n" 
      }
      content = content.replace(/,\n\s*$/, "").replace(/^\s*/,"")
      pretty += "{ " + content + "\n" + spacer(depth) + "}"
    } else { pretty += "{}"}
  }
  else { pretty += object.toString() }
  return ((newline ? "\n" + spacer(depth) : "") + pretty)
}

The output looks like this:

js> pp({foo:"bar", baz: 1})
{ foo: "bar",
  baz: 1
}
js> var taco
js> pp({foo:"bar", baz: [1,"taco",{"blarg": "moo", "mine": "craft"}, null, taco, {}], bleep: {a:null, b:taco, c: []}})
{ foo: "bar",
  baz: 
    [ 1,
      "taco",
      { blarg: "moo",
        mine: "craft"
      },
      null,
      undefined,
      {}
    ],
  bleep: 
    { a: null,
      b: undefined,
      c: []
    }
}

I've also posted it as a Gist here for whatever future changes may be required.

Answer 8

jsDump

jsDump.parse([
    window,
    document,
    { a : 5, '1' : 'foo' },
    /^[ab]+$/g,
    new RegExp('x(.*?)z','ig'),
    alert, 
    function fn( x, y, z ){
        return x + y; 
    },
    true,
    undefined,
    null,
    new Date(),
    document.body,
    document.getElementById('links')
])

becomes

[
   [Window],
   [Document],
   {
      "1": "foo",
      "a": 5
   },
   /^[ab]+$/g,
   /x(.*?)z/gi,
   function alert( a ){
      [code]
   },
   function fn( a, b, c ){
      [code]
   },
   true,
   undefined,
   null,
   "Fri Feb 19 2010 00:49:45 GMT+0300 (MSK)",
   <body id="body" class="node"></body>,
   <div id="links">
]

QUnit (Unit-testing framework used by jQuery) using slightly patched version of jsDump.

---

JSON.stringify() is not best choice on some cases.

JSON.stringify({f:function(){}}) // "{}"
JSON.stringify(document.body)    // TypeError: Converting circular structure to JSON

Answer 9

Taking PhiLho's lead (thanks very much :)), I ended up writing my own as I couldn't quite get his to do what I wanted. It's pretty rough and ready, but it does the job I need. Thank you all for the excellent suggestions.

It's not brilliant code, I know, but for what it's worth, here it is. Someone might find it useful:

// Usage: dump(object)
function dump(object, pad){
    var indent = '\t'
    if (!pad) pad = ''
    var out = ''
    if (object.constructor == Array){
        out += '[\n'
        for (var i=0; i<object.length; i++){
            out += pad + indent + dump(object[i], pad + indent) + '\n'
        }
        out += pad + ']'
    }else if (object.constructor == Object){
        out += '{\n'
        for (var i in object){
            out += pad + indent + i + ': ' + dump(object[i], pad + indent) + '\n'
        }
        out += pad + '}'
    }else{
        out += object
    }
    return out
}

Answer 10

For anyone checking this question out in 2021 or post-2021

Check out this Other StackOverflow Answer by hassan

TLDR:

JSON.stringify(data,null,2)

here the third parameter is the tab/spaces

Answer 11

Lots of people writing code in this thread, with many comments about various gotchas. I liked this solution because it seemed complete and was a single file with no dependencies.

browser

nodejs

It worked "out of the box" and has both node and browser versions (presumably just different wrappers but I didn't dig to confirm).

The library also supports pretty printing XML, SQL and CSS, but I haven't tried those features.

Answer 12

This is really just a comment on Jason Bunting's "Use Crockford's JSON.stringify", but I wasn't able to add a comment to that answer.

As noted in the comments, JSON.stringify doesn't play well with the Prototype (www.prototypejs.org) library. However, it is fairly easy to make them play well together by temporarily removing the Array.prototype.toJSON method that prototype adds, run Crockford's stringify(), then put it back like this:

  var temp = Array.prototype.toJSON;
  delete Array.prototype.toJSON;
  $('result').value += JSON.stringify(profile_base, null, 2);
  Array.prototype.toJSON = temp;

Answer 13

I thought J. Buntings response on using JSON.stringify was good as well. A an aside, you can use JSON.stringify via YUIs JSON object if you happen to be using YUI. In my case I needed to dump to HTML so it was easier to just tweak/cut/paste PhiLho response.

function dumpObject(obj, indent) 
{
  var CR = "<br />", SPC = "&nbsp;&nbsp;&nbsp;&nbsp;", result = "";
  if (indent == null) indent = "";

  for (var property in obj)
  {
    var value = obj[property];

    if (typeof value == 'string')
    {
      value = "'" + value + "'";
    }
    else if (typeof value == 'object')
    {
      if (value instanceof Array)
      {
        // Just let JS convert the Array to a string!
        value = "[ " + value + " ]";
      }
      else
      {
        var od = dumpObject(value, indent + SPC);
        value = CR + indent + "{" + CR + od + CR + indent + "}";
      }
    }
    result += indent + "'" + property + "' : " + value + "," + CR;
  }
  return result;
}

Answer 14

A simple one for printing the elements as strings:

var s = "";
var len = array.length;
var lenMinus1 = len - 1
for (var i = 0; i < len; i++) {
   s += array[i];
   if(i < lenMinus1)  {
      s += ", ";
   }
}
alert(s);

Answer 15

My NeatJSON library has both Ruby and JavaScript versions. It is freely available under a (permissive) MIT License. You can view an online demo/converter at:
http://phrogz.net/JS/neatjson/neatjson.html

Some features (all optional):

• Wrap to a specific width; if an object or array can fit on the line, it is kept on one line.
• Align the colons for all keys in an object.
• Sort the keys to an object alphabetically.
• Format floating point numbers to a specific number of decimals.
• When wrapping, use a 'short' version that puts the open/close brackets for arrays and objects on the same line as the first/last value.
• Control the whitespace for arrays and objects in a granular manner (inside brackets, before/after colons and commas).
• Works in the web browser and as a Node.js module.

Answer 16

flexjson includes a prettyPrint() function that might give you what you want.