Can comments be used in JSON?

Can I use comments inside a JSON file? If so, how?

//comments

{"__comment": ...}

44 Answers
44

No.

The JSON should all be data, and if you include a comment, then it will be data too.

You could have a designated data element called "_comment" (or something) that would be ignored by apps that use the JSON data.

"_comment"

You would probably be better having the comment in the processes that generates/receives the JSON, as they are supposed to know what the JSON data will be in advance, or at least the structure of it.

But if you decided to:

{
"_comment": "comment text goes here...",
"glossary": {
"title": "example glossary",
"GlossDiv": {
"title": "S",
"GlossList": {
"GlossEntry": {
"ID": "SGML",
"SortAs": "SGML",
"GlossTerm": "Standard Generalized Markup Language",
"Acronym": "SGML",
"Abbrev": "ISO 8879:1986",
"GlossDef": {
"para": "A meta-markup language, used to create markup languages such as DocBook.",
"GlossSeeAlso": ["GML", "XML"]
},
"GlossSee": "markup"
}
}
}
}
}


"__comment":"comment text goes here...",

Accronym

Abbrev

__comment__

No, comments of the form //… or /*…*/ are not allowed in JSON. This answer is based on:

//…

/*…*/

application/json

Include comments if you choose; strip them out with a minifier before parsing or transmitting.

I just released JSON.minify() which strips out comments and whitespace from a block of JSON and makes it valid JSON that can be parsed. So, you might use it like:

JSON.parse(JSON.minify(my_str));


When I released it, I got a huge backlash of people disagreeing with even the idea of it, so I decided that I'd write a comprehensive blog post on why comments make sense in JSON. It includes this notable comment from the creator of JSON:

Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser. - Douglas Crockford, 2012

Hopefully that's helpful to those who disagree with why JSON.minify() could be useful.

Comments were removed from JSON by design.

I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.

Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.

Source: Public statement by Douglas Crockford on G+

DISCLAIMER: YOUR WARRANTY IS VOID

As has been pointed out, this hack takes advantage of the implementation of the spec. Not all JSON parsers will understand this sort of JSON. Streaming parsers in particular will choke.

It's an interesting curiosity, but you should really not be using it for anything at all. Below is the original answer.

I've found a little hack that allows you to place comments in a JSON file that will not affect the parsing, or alter the data being represented in any way.

It appears that when declaring an object literal you can specify two values with the same key, and the last one takes precedence. Believe it or not, it turns out that JSON parsers work the same way. So we can use this to create comments in the source JSON that will not be present in a parsed object representation.

({a: 1, a: 2});
// => Object {a: 2}
Object.keys(JSON.parse('{"a": 1, "a": 2}')).length;
// => 1


If we apply this technique, your commented JSON file might look like this:

{
"api_host" : "The hostname of your API server. You may also specify the port.",
"api_host" : "hodorhodor.com",

"retry_interval" : "The interval in seconds between retrying failed API calls",
"retry_interval" : 10,

"auth_token" : "The authentication token. It is available in your developer dashboard under 'Settings'",
"auth_token" : "5ad0eb93697215bc0d48a7b69aa6fb8b",

"favorite_numbers": "An array containing my all-time favorite numbers",
"favorite_numbers": [19, 13, 53]
}


The above code is valid JSON. If you parse it, you'll get an object like this:

{
"api_host": "hodorhodor.com",
"retry_interval": 10,
"auth_token": "5ad0eb93697215bc0d48a7b69aa6fb8b",
"favorite_numbers": [19,13,53]
}


Which means there is no trace of the comments, and they won't have weird side-effects.

Happy hacking!

JSON does not support comments. It was also never intended to be used for configuration files where comments would be needed.

Hjson is a configuration file format for humans. Relaxed syntax, fewer mistakes, more comments.

See hjson.org for JavaScript, Java, Python, PHP, Rust, Go, Ruby and C# libraries.

You can't. At least that's my experience from a quick glance at json.org.

JSON has its syntax visualized on that page. There isn't any note about comments.

Consider using YAML. It's nearly a superset of JSON (virtually all valid JSON is valid YAML) and it allows comments.

yaml

You should write a JSON schema instead. JSON schema is currently a proposed Internet draft specification. Besides documentation, the schema can also be used for validating your JSON data.

Example:

{
"description":"A person",
"type":"object",
"properties":
{
"name":
{
"type":"string"
},
"age":
{
"type":"integer",
"maximum":125
}
}
}


You can provide documentation by using the description schema attribute.

If you are using Jackson as your JSON parser then this is how you enable it to allow comments:

ObjectMapper mapper = new ObjectMapper().configure(Feature.ALLOW_COMMENTS, true);


Then you can have comments like this:

{
key: "value" // comment
}


And you can also have comments starting with # by setting:

#

mapper.configure(Feature.ALLOW_YAML_COMMENTS, true);


But in general (as answered before) the spec does not allow comments.

The connection to the server was reset while the page was loading.

Comments are not an official standard. Although some parsers support C-style comments. One that I use is JsonCpp. In the examples there is this one:

// Configuration options
{
// Default encoding for text
"encoding" : "UTF-8",

// Plug-ins loaded at start-up
"plug-ins" : [
"python",
"c++",
"ruby"
],

// Tab indent size
"indent" : { "length" : 3, "use_space": true }
}


jsonlint does not validate this. So comments are a parser specific extension and not standard.

Another parser is JSON5.

An alternative to JSON TOML.

Sorry, we can't use comments in JSON... See the syntax diagram for JSON on JSON.org.

Douglas Crockford says "why he removed comments in JSON and providing an alternative way to do that":

I removed comments from JSON because I saw people were using them to
hold parsing directives, a practice which would have destroyed
interoperability. I know that the lack of comments makes some people
sad, but it shouldn't.

Suppose you are using JSON to keep configuration files, which you
would like to annotate. Go ahead and insert all the comments you like.
Then pipe it through JSMin before handing it to your JSON parser.

If your text file, which is a JSON string, is going to be read by some program, how difficult would it be to strip out either C or C++ style comments before using it?

Answer: It would be a one liner. If you do that then JSON files could be used as configuration files.

Here is what I found in the Google Firebase documentation that allows you to put comments in JSON:

{
"//": "Some browsers will use this to enable push notifications.",
"//": "It is the same for all projects, this is not your project's sender ID",
"gcm_sender_id": "1234567890"
}


If you are using the Newtonsoft.Json library with ASP.NET to read/deserialize you can use comments in the JSON content:

//"name": "string"

//"id": int

or

/* This is a

comment example */

PS: Single-line comments are only supported with 6+ versions of Newtonsoft Json.

Additional note for people who can't think out of the box: I use the JSON format for basic settings in an ASP.NET web application I made. I read the file, convert it into the settings object with the Newtonsoft library and use it when necessary.

I prefer writing comments about each individual setting in the JSON file itself, and I really don't care about the integrity of the JSON format as long as the library I use is OK with it.

I think this is an 'easier to use/understand' way than creating a separate 'settings.README' file and explaining the settings in it.

If you have a problem with this kind of usage; sorry, the genie is out of the lamp. People would find other usages for JSON format, and there is nothing you can do about it.

The idea behind JSON is to provide simple data exchange between applications. These are typically web based and the language is JavaScript.

It doesn't really allow for comments as such, however, passing a comment as one of the name/value pairs in the data would certainly work, although that data would obviously need to be ignored or handled specifically by the parsing code.

All that said, it's not the intention that the JSON file should contain comments in the traditional sense. It should just be the data.

Have a look at the JSON website for more detail.

It depends on your JSON library. Json.NET supports JavaScript-style comments, /* commment */.

/* commment */

See another Stack Overflow question.

JSON does not support comments natively, but you can make your own decoder or at least preprocessor to strip out comments, that's perfectly fine (as long as you just ignore comments and don't use them to guide how your application should process the JSON data).

JSON does not have comments. A JSON encoder MUST NOT output comments.
A JSON decoder MAY accept and ignore comments.

Comments should never be used to transmit anything meaningful. That is
what JSON is for.

Cf: Douglas Crockford, author of JSON spec.

I just encountering this for configuration files. I don't want to use XML (verbose, graphically, ugly, hard to read), or "ini" format (no hierarchy, no real standard, etc.) or Java "Properties" format (like .ini).

JSON can do all they can do, but it is way less verbose and more human readable - and parsers are easy and ubiquitous in many languages. It's just a tree of data. But out-of-band comments are a necessity often to document "default" configurations and the like. Configurations are never to be "full documents", but trees of saved data that can be human readable when needed.

I guess one could use "#": "comment", for "valid" JSON.

"#": "comment"

JSON makes a lot of sense for config files and other local usage because it's ubiquitous and because it's much simpler than XML.

If people have strong reasons against having comments in JSON when communicating data (whether valid or not), then possibly JSON could be split into two:

JSON-DOC will allow comments, and other minor differences might exist such as handling whitespace. Parsers can easily convert from one spec to the other.

With regards to the remark made by Douglas Crockford on this issues (referenced by @Artur Czajka)

Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.

We're talking about a generic config file issue (cross language/platform), and he's answering with a JS specific utility!

Sure a JSON specific minify can be implemented in any language,
but standardize this so it becomes ubiquitous across parsers in all languages and platforms so people stop wasting their time lacking the feature because they have good use-cases for it, looking the issue up in online forums, and getting people telling them it's a bad idea or suggesting it's easy to implement stripping comments out of text files.

The other issue is interoperability. Suppose you have a library or API or any kind of subsystem which has some config or data files associated with it. And this subsystem is
to be accessed from different languages. Then do you go about telling people: by the way
don't forget to strip out the comments from the JSON files before passing them to the parser!

The Dojo Toolkit JavaScript toolkit (at least as of version 1.4), allows you to include comments in your JSON. The comments can be of /* */ format. Dojo Toolkit consumes the JSON via the dojo.xhrGet() call.

/* */

dojo.xhrGet()

Other JavaScript toolkits may work similarly.

This can be helpful when experimenting with alternate data structures (or even data lists) before choosing a final option.

dojo.xhrGet()

You can have comments in JSONP, but not in pure JSON. I've just spent an hour trying to make my program work with this example from Highcharts: http://www.highcharts.com/samples/data/jsonp.php?filename=aapl-c.json&callback=?

If you follow the link, you will see

?(/* AAPL historical OHLC data from the Google Finance API */
[
/* May 2006 */
[1147651200000,67.79],
[1147737600000,64.98],
...
[1368057600000,456.77],
[1368144000000,452.97]
]);


Since I had a similar file in my local folder, there were no issues with the Same-origin policy, so I decided to use pure JSON... and, of course, $.getJSON was failing silently because of the comments.

$.getJSON

Eventually I just sent a manual HTTP request to the address above and realized that the content-type was text/javascript since, well, JSONP returns pure JavaScript. In this case comments are allowed. But my application returned content-type application/json, so I had to remove the comments.

text/javascript

application/json

If you use JSON5 you can include comments.

JSON5 is a proposed extension to JSON that aims to make it easier for humans to write and maintain by hand. It does this by adding some minimal syntax features directly from ECMAScript 5.

JSON is not a framed protocol. It is a language free format. So a comment's format is not defined for JSON.

As many people have suggested, there are some tricks, for example, duplicate keys or a specific key _comment that you can use. It's up to you.

_comment

This is a "can you" question. And here is a "yes" answer.

No, you shouldn't use duplicative object members to stuff side channel data into a JSON encoding. (See "The names within an object SHOULD be unique" in the RFC).

And yes, you could insert comments around the JSON, which you could parse out.

But if you want a way of inserting and extracting arbitrary side-channel data to a valid JSON, here is an answer. We take advantage of the non-unique representation of data in a JSON encoding. This is allowed^* in section two of the RFC under "whitespace is allowed before or after any of the six structural characters".

^*The RFC only states "whitespace is allowed before or after any of the six structural characters", not explicitly mentioning strings, numbers, "false", "true", and "null". This omission is ignored in ALL implementations.

First, canonicalize your JSON by minifying it:

$jsonMin = json_encode(json_decode($json));


Then encode your comment in binary:

$hex = unpack('H*', $comment);
$commentBinary = base_convert($hex[1], 16, 2);


Then steg your binary:

$steg = str_replace('0', ' ', $commentBinary);
$steg = str_replace('1', "t", $steg);


Here is your output:

$jsonWithComment = $steg . $jsonMin;


There is a good solution (hack), which is valid JSON.
Just make the same key twice (or more). For example:

{
"param" : "This is the comment place",
"param" : "This is value place",
}


So JSON will understand this as:

{
"param" : "This is value place",
}


We are using strip-json-comments for our project. It supports something like:

strip-json-comments

/*
* Description
*/
{
// rainbows
"unicorn": /* ❤ */ "cake"
}


Simply npm install --save strip-json-comments to install and use it like:

npm install --save strip-json-comments

var strip_json_comments = require('strip-json-comments')
var json = '{/*rainbows*/"unicorn":"cake"}';
JSON.parse(strip_json_comments(json));
//=> {unicorn: 'cake'}


To cut a JSON item into parts I add "dummy comment" lines:

{

"#############################" : "Part1",

"data1" : "value1",
"data2" : "value2",

"#############################" : "Part2",

"data4" : "value3",
"data3" : "value4"

}


{ "comment-001":"This is where you do abc...", "comment-002":"This is where you do xyz..." }

The author of JSON wants us to include comments in the JSON, but strip them out before parsing them (see link provided by Michael Burr.) If JSON should have comments, why not standardize them, and let the JSON parser do the job? I don't agree with the logic there, but, alas, that's the standard. Using YAML solution as suggested by others is good, but requires library dependency.

If you want to strip out comments, but don't want to have a library dependency, here is a two-line solution, which works for C++-style comments, but can be adapted to others:

var comments=new RegExp("//.*", 'mg');
data = JSON.parse(fs.readFileSync(sample_file, 'utf8').replace(comments, ''));


Note that this solution can only be used in cases where you can be sure that the JSON data does not contain the comment initiator, e.g. ('//').

Another way to achieve JSON parsing, stripping of comments, and no extra library, is to evaluate the JSON in a JS interpreter. The caveat with that approach, of course, is that you would only want to evaluate untainted data (no untrusted user-input.) Here is an example of this approach in node.js -- another caveat, following example will only read the data once and then it will be cached:

data = require(fs.realpathSync(doctree_fp));


You can use JSON with comments in it, if you load it as a text file, and then remove comments from it.

You can use decomment library for that. Below is a complete example.

Input JSON (file input.js):

/*
* multi-line comments
**/
{
"value": 123 // one-line comment
}


Test Application:

var decomment = require('decomment');
var fs = require('fs');

fs.readFile('input.js', 'utf8', function (err, data) {
if (err) {
console.log(err);
} else {
var text = decomment(data); // removing comments
var json = JSON.parse(text); // parsing JSON
console.log(json);
}
});


Output:

{ value: 123 }


See also: gulp-decomment, grunt-decomment

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