loading...
Cover image for The secret power of JSON stringify

The secret power of JSON stringify

blacksonic profile image Gábor Soós ・2 min read

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 its documentation and realize it has many more features then we have imagined. The same thing has happened with JSON.stringify and me. In this short tutorial, I'll show you what I've learned.

Basics

The JSON.stringify method takes a variable and transforms it into its JSON representation.

const firstItem = { 
  title: 'Transformers', 
  year: 2007 
};

JSON.stringify(firstItem);
// {'title':'Transformers','year':2007}

The problem comes when there is an element that can not serialize to JSON.

const secondItem = { 
  title: 'Transformers', 
  year: 2007, 
  starring: new Map([[0, 'Shia LaBeouf'],[1, 'Megan Fox']]) 
};

JSON.stringify(secondItem);
// {'title':'Transformers','year':2007,'starring':{}}

The second argument

JSON.stringify has a second argument, which is called the replacer argument.

You can pass an array of strings that act as a whitelist for properties of the object to be included.

JSON.stringify(secondItem, ['title']);
// {'title':'Transformers'}

We can filter out values that we don't want to display. These values can be too large items (like an Error object) or something that doesn't have a readable JSON representation.

The replacer argument can also be a function. This function receives the actual key and value on which the JSON.stringify method is iterating. You can alter the representation of the value with the function's return value. If you don't return anything from this function or return undefined, that item will not be present in the result.

JSON.stringify(secondItem, (key, value) => {
  if (value instanceof Set) {
    return [...value.values()];
  }
  return value;
});
// {'title':'Transformers','year':2007,'starring':['Shia LaBeouf','Megan Fox']}

By returning undefined in the function, we can remove those items from the result.

JSON.stringify(secondItem, (key, value) => {
  if (typeof value === 'string') {
    return undefined;
  }
  return value;
});
// {"year":2007,"starring":{}}

Third argument

The third argument controls the spacing in the final string. If the argument is a number, each level in the stringification will be indented with this number of space characters.

JSON.stringify(secondItem, null, 2);
//{
//  "title": "Transformers",
//  "year": 2007,
//  "starring": {}
//}

If the third argument is a string, it will be used instead of the space character.

JSON.stringify(secondItem, null, '🦄');
//{
//🦄"title": "Transformers",
//🦄"year": 2007,
//🦄"starring": {}
//}

The toJSON method

If the object what we stringify has a property toJSON, it will customize the stringification process. Instead of serializing the object, you can return a new value from the method, and this value will be serialized instead of the original object.

const thirdItem = { 
  title: 'Transformers', 
  year: 2007, 
  starring: new Map([[0, 'Shia LaBeouf'],[1, 'Megan Fox']]),
  toJSON() {
    return { 
      name: `${this.title} (${this.year})`, 
      actors: [...this.starring.values()] 
    };
  }
};

console.log(JSON.stringify(thirdItem));
// {"name":"Transformers (2007)","actors":["Shia LaBeouf","Megan Fox"]}

Demo time

Here is a Codepen where I included the above examples, and you can fiddle with it.

Final thoughts

It can be rewarding sometimes to look at the documentation of those functions we use daily. They might surprise us, and we learn something.
Stay hungry for knowledge and read the documentation 🦄.

Discussion

pic
Editor guide
Collapse
sidvishnoi profile image
Sid Vishnoi

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');
}
Enter fullscreen mode Exit fullscreen mode

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

Collapse
neilcodes profile image
Neil

This is kind of horrifying and I love it.

Collapse
getclibu profile image
Neville Franks

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.

Collapse
sidvishnoi profile image
Sid Vishnoi

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

  1. 4.b.ii: If Type(replacer) is [Array]:
  2. 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... :

  1. 5.a: Let K be state.[[PropertyList]].
  2. 8: For each element P of K, do:
  3. 8.v: Append member to partial.
Thread Thread
getclibu profile image
Neville Franks

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.

Thread Thread
sidvishnoi profile image
Sid Vishnoi

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

Thread Thread
getclibu profile image
Neville Franks

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();
Enter fullscreen mode Exit fullscreen mode

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

Collapse
martinratinaud profile image
Martin Ratinaud

Brilliant! thanks

Collapse
blacksonic profile image
Gábor Soós Author

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

Collapse
thirdy profile image
APerfectCircle

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?

Collapse
sidvishnoi profile image
Sid Vishnoi

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

Collapse
areljannc profile image
AJ Clemente

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.

Collapse
blacksonic profile image
Gábor Soós Author

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

Collapse
rose profile image
Rose

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.

Collapse
blacksonic profile image
Gábor Soós Author

Glad it showed something new 👍

Collapse
kenbellows profile image
Ken Bellows

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! 👏👏👏

Collapse
blacksonic profile image
Gábor Soós Author

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

Collapse
sambenskin profile image
Sam Benskin

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

Collapse
blacksonic profile image
Gábor Soós Author

toJSON was also absolutely new for me also

Collapse
suprsidr profile image
Wayne Patterson

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

Collapse
jdhinvicara profile image
John Harding

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.

Collapse
blacksonic profile image
Gábor Soós Author

Thanks, unicorn all the way!!! 🦄🦄🦄

Collapse
fceruti profile image
fceruti

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

Collapse
briandesousa1 profile image
Brian De Sousa

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

Collapse
blacksonic profile image
Gábor Soós Author

Thanks, yes indeed a real game changer

Collapse
torben74 profile image
Torben74

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

Collapse
matthieudesprez profile image
Matthieu Desprez

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

Collapse
panayiotisgeorgiou profile image
Panayiotis Georgiou

Thank you so much!

Collapse
mdsardar profile image
Mohammed Sardar

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

Collapse
blacksonic profile image
Gábor Soós Author

Stay hungry 👍

Collapse
yorchi profile image
Jorge Andrade

awesome, new thing learned!

Collapse
antonrusak profile image
Anton Rusak

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.

Collapse
cbmgit profile image
cbmgit

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