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JavaScript - JSON.stringify() Method

The JavaScript JSON.stringify() method is used to convert a JavaScript value (such as an object or an array) into a JSON string. When using this method, you have the option to include an optional parameter called replacer, which can be specified as a function.

If you provide a replacer function, it allows you to modify the value being stringified. Additionally, by returning a different value from the replacer function, you can customize the output of the resulting JSON string.

The JSON.stringify() method is a static method in JavaScript, which means you can always use it as JSON.stringify() no need to create an object instance to invoke it.

Syntax

Following is the syntax of JavaScript JSON.stringify() method −

JSON.stringify(value, replacer, space)

Parameters

This method accepts three parameters named 'value', 'replacer', and 'space', which are described below −

  • value − The value you want to convert to a JSON string.

  • replacer (optional) − A function that modifies the stringification process or an array of strings and numbers specifying properties to include in the output. If the 'replacer' is an array, non-string and non-number elements (including symbols) are ignored. If 'replacer' is not a function or an array (e.g., null or not provided), all string-keyed properties of the object are included in the resulting JSON string.

  • space (optional) − A string or number is used to insert white space (indentation, line breaks, etc.) into the output JSON string for readability.

Return value

This method returns a JSON string representing the given value, or 'undefined'.

Examples

Example 1

If we pass only a value parameter to this method, it will convert this JavaScript value to a JSON string.

In the following example, we are using the JavaScript JSON.stringify() method to convert this JavaScript value {a: 10, b: 20, c: 30, d: 40, e: 50} to a JSON string.

<html>
<head>
   <title>JavaScript JSON.stringify() Method</title>
</head>
<body>
   <script>
      const value = {a: 10, b: 20, c: 30, d: 40, e: 50};
      document.write("The given value: ", value.a, value.b, value.c, value.d, value.e);
      document.write("<br>After converting to JSON srtring: ", JSON.stringify(value));
   </script>
</body>
</html>

Output

The above program returns a JSON string after converting JavaScript value as −

The given value: 1020304050
After converting to JSON srtring: {"a":10,"b":20,"c":30,"d":40,"e":50}

Example 2

If we pass the value and space parameters to this method, it will convert the value to a JSON string and add a specified number of space of indentations before each character.

The following is another example of the JavaScript JSON.stringify() method. We use this method to convert the JavaScript value {"name": "Rahul", "age": 22, "city": "Lucknow"} to a formatted JSON string with 3, spaces of indentation for better readability.

const value = {"name": "Rahul", "age": 22, "city": "Lucknow"};
console.log("The given value: ", value.name, " ", value.age, " ", value.city);
const space = 3;
console.log("Number of spaces: ", space);
      
//using the JSON.stringify() method
const json_string = JSON.stringify(value, null, space);
console.log(json_string);

Output

After executing the above program, it will return a JSON string in the console with 3, indentations (the changes only can be seen in the console).

The given value:  Rahul   22   Lucknow
Number of spaces:  3
{
   "name": "Rahul",
   "age": 22,
   "city": "Lucknow"
}

Example 3

If we pass the replacer (as a function) as an argument to this method, it will modify the stringification process.

In the example below, we use the JSON.stringify() method to convert a JavaScript value to a JSON string. We define a replacer function named modifyAge(), which increases the age by 5 for each record. We pass this function as an argument to the method to modify the result before it is returned.

//custom replacer function
function modifyAge(key, value){
   if(key === "age"){
      return value+5;
   } else {
      return value;
   }
}
const value = [{"name": "Rahul", "age": 22, "city": "Lucknow"}, {"name": "Vikash", "age": 20, "city": "Hyderabad"}];
console.log("The JavaScript value (before converting): ", JSON.stringify(value), " ");
let space = 5;
console.log("The number space: ", space);

//using the JSON.stringify() method
const jsonString = JSON.stringify(value, modifyAge, space);
console.log("The JSON string (after converting): ");
console.log(jsonString);

Output

Once the above program is executed, it will return a JSON string (with modified age).

The JavaScript value (before converting):  [{"name":"Rahul","age":22,"city":"Lucknow"},{"name":"Vikash","age":20,"city":"Hyderabad"}]  
The number space:  5
The JSON string (after converting): 
[
   {
      "name": "Rahul",
      "age": 27,
      "city": "Lucknow"
   },
   {
      "name": "Vikash",
      "age": 25,
      "city": "Hyderabad"
   }
]
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