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JavaScript JSON.stringify

last modified February 15, 2021

JavaScript JSON.stringify tutorial shows how to convert JavaScript objects into JSON strings.

JSON

JSON (JavaScript Object Notation) is a lightweight data-interchange format. People can easily read and write JSON. It is also easy to create algorithms for parsing and generating JSON. The official Internet media type for JSON is application/json. JSON files have extension .json.

JavaScript provides the following methods for working with JSON:

JS JSON.stringify

The JSON.stringify method converts a JavaScript object or value to a JSON string. It can optionally modify or filter values if a replacer function/array is specified.

let json = JSON.stringify(value [, replacer, space])

The value is the value to convert to a JSON string. The replacer is either a function that alters the behavior of the stringification process or an array which servers as a filter for the properties of the value object to be included in the JSON string.

JSON.stringify simple values

In the first example, we stringify simple values.

simple_values.js
console.dir(JSON.stringify(1));
console.dir(JSON.stringify(5.9));
console.dir(JSON.stringify(true));
console.dir(JSON.stringify(false));
console.dir(JSON.stringify('falcon'));
console.dir(JSON.stringify("sky"));
console.dir(JSON.stringify(null));

The example stringifies simple values, including numbers, booleans, and strings.

$ node simple_values.js 
'1'
'5.9'
'true'
'false'
'"falcon"'
'"sky"'
'null'

JSON.stringify objects

In the next example, we stringify objects.

objects.js
console.dir(JSON.stringify({ x: 5, y: 6 }));
console.dir(JSON.stringify(new Number(6)));
console.dir(JSON.stringify(new String('falcon'))); 
console.dir(JSON.stringify(new Boolean(false)));
console.dir(JSON.stringify(new Date(2020, 0, 6, 21, 4, 5)));
console.dir(JSON.stringify(new Int8Array([1, 2, 3])));
console.dir(JSON.stringify(new Int16Array([1, 2, 3])));
console.dir(JSON.stringify(new Int32Array([1, 2, 3])));
console.dir(JSON.stringify({ x: 2, y: 3, toJSON() { return this.x + this.y; }}));

The example converts simple custom and built-in objects into JSON strings.

$ node objects.js 
'{"x":5,"y":6}'
'6'
'"falcon"'
'false'
'"2020-01-06T20:04:05.000Z"'
'{"0":1,"1":2,"2":3}'
'{"0":1,"1":2,"2":3}'
'{"0":1,"1":2,"2":3}'
'5'

JSON.stringify array of objects

The next example transforms an array of objects into a JSON string.

stringify.js
let users = [
    {
        id: 1,
        first_name: 'Robert',
        last_name: 'Schwartz',
        email: 'rob23@gmail.com'
    },
    {
        id: 2,
        first_name: 'Lucy',
        last_name: 'Ballmer',
        email: 'lucyb56@gmail.com'
    },
    {
        id: 3,
        first_name: 'Anna',
        last_name: 'Smith',
        email: 'annasmith23@gmail.com'
    }
];

let data = JSON.stringify(users, null, 2);

console.log(typeof data);
console.log(typeof users);
console.log('------------------');
console.dir(data);
console.log('------------------');
console.dir(users);

In the example, we have an array of users. We transform the array into a JSON string with the JSON.stringify function.

node stringify.js 
string
object
------------------
'[\n' +
  '  {\n' +
  '    "id": 1,\n' +
  '    "first_name": "Robert",\n' +
  '    "last_name": "Schwartz",\n' +
  '    "email": "rob23@gmail.com"\n' +
  '  },\n' +
  '  {\n' +
  '    "id": 2,\n' +
  '    "first_name": "Lucy",\n' +
  '    "last_name": "Ballmer",\n' +
  '    "email": "lucyb56@gmail.com"\n' +
  '  },\n' +
  '  {\n' +
  '    "id": 3,\n' +
  '    "first_name": "Anna",\n' +
  '    "last_name": "Smith",\n' +
  '    "email": "annasmith23@gmail.com"\n' +
  '  }\n' +
  ']'
------------------
[
  {
    id: 1,
    first_name: 'Robert',
    last_name: 'Schwartz',
    email: 'rob23@gmail.com'
  },
  {
    id: 2,
    first_name: 'Lucy',
    last_name: 'Ballmer',
    email: 'lucyb56@gmail.com'
  },
  {
    id: 3,
    first_name: 'Anna',
    last_name: 'Smith',
    email: 'annasmith23@gmail.com'
  }
]

JSON.stringify replacer example

In the following example, we use the replacer function to transform the data.

replacer.js
function replacer(key, value) {
  if (typeof value === 'string') {
    return value.toUpperCase();
  }
  return value;
}

var user = { name: 'John Doe', occupation: 'gardener', age: 34, 
  dob: new Date('1992-12-31') };

console.dir(JSON.stringify(user, replacer));

The replacer function turns all strings in the user object into uppercase.

$ node replacer.js 
'{"name":"JOHN DOE","occupation":"GARDENER","age":34,"dob":"1992-12-31T00:00:00.000Z"}'

Another form of a replacer is an array, which filters out object properties.

replacer2.js
var user = { name: 'John Doe', occupation: 'gardener', dob: new Date('1992-12-31') };

console.dir(JSON.stringify(user, ['name', 'occupation']));

In the example, we only include properties in the stringification process specified in the replacer array: name and occupation.

$ node replacer2.js 
'{"name":"John Doe","occupation":"gardener"}'

JSON.stringify pretty print

The space option is used to prettify the output. Note that console.log or console.dir already prettifies the output.

test.html
<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

    <script>
        fetch('https://jsonplaceholder.typicode.com/todos/1')
            .then(response => response.json())
            .then(json =>
                document.body.appendChild(document.createElement('pre')).innerHTML = JSON.stringify(json, null, 4));
    </script>
</body>

</html>

In the example, we retrieve data with the fetch function inside a browser. We prettify the output with JSON.stringify.

In this tutorial, we have converted JavaScript objects into JSON strings with the JSON.stringify function.

List all JavaScript tutorials.