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The secret power of JSON stringify

Gábor Soós on October 22, 2019

There are many functions in Javascript that do their job. We use them daily, but we don't know about their extra features. At one day, we look at i...

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Sid Vishnoi • Oct 22 '19 • Edited

Regarding the second items white list (array) argument:
We can use it to create a simple object hashing function (similar to object-hash, but with limitation of maximum object depth 1). The important thing is, JSON.stringify(obj) may not follow property order, which matters when the serialization is input for hashing/checksum. Instead we can pass Object.keys(obj).sort() as the 2nd argument, and the JSON will be stringified in that property order only.

function objectHash(obj: object): string {
  const str = JSON.stringify(obj, Object.keys(obj).sort());
  return createHash('sha1').update(str).digest('hex');
}

View code snippet for client-side at GitHub
View code snippet for Node.js at GitHub and tests

Neil • Oct 23 '19

This is kind of horrifying and I love it.

Martin Ratinaud • Oct 24 '19

Brilliant! thanks

Neville Franks • Oct 28 '20 • Edited

I'd been looking everywhere to find a simple way to get JSON.stringify() to output items in a specific order. You post provided exactly what I needed to do what I wanted.

Unfortunately upon reading the JSON.stringify() docs I can't find any information on using a replacer array to set keys order. Only info about whitelists. Can you point to any docs on this?

Some other info I found that may be if interest to others: "sort object properties and JSON.stringify" - tfzx.net/article/499097.html

Thanks so much for your post.

Sid Vishnoi • Oct 28 '20 • Edited

From tc39.es/ecma262/#sec-json.stringify :

4.b.ii: If Type(replacer) is [Array]:
4.b.ii.g: If item is not undefined and item is not currently an element of PropertyList, then Append item to the end of PropertyList.

and later tc39.es/ecma262/#sec-serializejson... :

5.a: Let K be state.[[PropertyList]].
8: For each element P of K, do:
8.v: Append member to partial.

Neville Franks • Oct 29 '20

Thanks for that. Pity it is missing from the MDN Doc's. For some reason the links didn't take me to the relevant sections.

Sid Vishnoi • Oct 29 '20

Updated links. DEV included trailing : in links, so they broke.

Neville Franks • Oct 29 '20

Thanks, the links now work.

I didn't pick up on the ", but with limitation of maximum object depth 1" issue and I need to handle objects with depth > 1.

After some more searching I found this article which is a collection of code snippets. tfzx.net/article/499097.html

The code that worked for me is:

    var allKeys = [];
    JSON.stringify( json, function( key, value ){ allKeys.push( key ); return value; } )
    allKeys.sort();

which is from: stackoverflow.com/a/53593328/91300

Gábor Soós • Oct 23 '19

Wow, didn't know it. Nice snippet! I always learn something new.

APerfectCircle • Oct 30 '19

Nice to know. However, being a java developer, this brings me to think that this should be a built-in functionality. Doesn't plain JS have hashing function for all it's objects?

Sid Vishnoi • Oct 30 '19

No. JS doesn't expose any hashing function for any of its supported types.

AJ Clemente • Oct 22 '19

Wow! I'm a big fan of writing applications with JavaScript and its wide-varieties of libraries. I also don't post at all, ever, on any social platforms.

But this post is the first time that made we want to login and comment something.

Thanks for this amazing post! It's amazing that these simple methods are overlooked by many developers. Learning about this just now is the reason why I love programming.

Gábor Soós • Oct 23 '19

Comments like this keep me motivated to keep on writing 👍 Thanks

Rose • Oct 23 '19

Great post, I use JSON.stringify all the time and I never new about this 😳love finding out about these nooks and crannies of JS.

Gábor Soós • Oct 23 '19

Glad it showed something new 👍

Ken Bellows • Oct 29 '19

I've been using JavaScript deeply for a long time and it's very rare that I learn something new about old parts of JavaScript, but you've taught me two new things! I knew about the replacer function, I knew about the indentation, I even knew about toJSON, but I had never heard of either the property whitelist array or the indentation string options before! Thanks for the knowledge! 👏👏👏

Gábor Soós • Oct 30 '19

I have used the indentation for a long time, but that null value before it always bothered me 😀

Sam Benskin • Oct 23 '19

Excellent article, I've never looked into the other arguments and the toJSON functionality is especially brilliant. Thanks for sharing!

Gábor Soós • Oct 30 '19

toJSON was also absolutely new for me also

Tanaka • Dec 18 '20

Thanks for the great post! Is there a way to easily unstringify text with a JavaScript? A kind of opposite of the stringify function? eg. how to remove the quotation marks in a dictionary of key-value pairs like {"name":"Tim", "age": "22"}. In this case, removing them just from the age value?

I have a more detailed version of this question on Stack Overflow that's been a little daunting to work around:

stackoverflow.com/questions/653516...

Thanks for the great article again!

John Harding • Oct 29 '19

I am especially excited to be able to prefix my stringified objects with a unicorn!! 🦄🦄🦄

Seriously though, great article - every time I typed that null I wondered what that second argument was for and never got around to looking...

Thanks.

Gábor Soós • Oct 30 '19

Thanks, unicorn all the way!!! 🦄🦄🦄

Wayne Patterson • Oct 29 '19

Another super power is deep copying objects dassur.ma/things/deep-copy/

Mohammed Sardar • Oct 26 '19

Thanks for the detailed sharing. Nice reading. It turned me very hunger for Java Script knowledge.

Gábor Soós • Oct 30 '19

Stay hungry 👍

fceruti • Oct 22 '19

Thank you so much! Thanks to people like you we learn something new every day :)

Torben74 • Oct 29 '19

I've been using stringify for years and had no idea how powerful it is. Great article. Thanks.

Matthieu Desprez • Oct 29 '19

Thank you, I learned some useful tips reading your article :)

Panayiotis Georgiou • Oct 29 '19

Thank you so much!

Brian De Sousa • Oct 29 '19

Well done! Didn't know about those other arguments. The toJSON method is an interesting capability too. Thanks!

Gábor Soós • Oct 30 '19

Thanks, yes indeed a real game changer

nalani5210 • Dec 28 '20 • Edited

The writing is very good, and I hope this online JSON tool website can help you in peacetime.
jsonformatting.com/

codeblogmoney • Jan 2 '20

Awesome article, I think so these kinds of tools are using the same functions. codebeautify.org/jsonviewer and jsonformatter.org/json-parser

Jorge Andrade • Oct 29 '19

awesome, new thing learned!

Anton Rusak • Oct 30 '19

Ha! The method looks so simple that I never even thought of reading MDN on it :) Thank you man for pointing us out to these awesome features. toJSON is the most promising part, I believe.