Gábor Soós

Posted on Oct 22, 2019 • Updated on Feb 25 • Originally published at sonicoder.com

The secret power of JSON stringify

#javascript #webdev #beginners

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 🦄.

Top comments (36)

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!