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JS Web APIsLesson #185

JavaScript JSON Stringify

The JSON.stringify() method converts a JavaScript value into JSON text.

The JSON.stringify() Method

The JSON.stringify() method converts a JavaScript value into JSON text.

JavaScript variables must be converted to plain text before they can be:

Sent to a server over HTTP
Transmitted between applications
Stored in a text file

Note > The JSON.stringify() method returns a text string.

Syntax

Example Code

javascript

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JSON.stringify(value, replacer, space)

Parameter	Description
value	The JavaScript value to convert.
replacer	An optional function or array that selects or transforms values.
space	An optional number or string used to indent the JSON text.

Converting an Object

Example

Example Code

javascript

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// Create a JavaScript object

const person = {

  name: "John",

  age: 30,

  city: "New York"

};

// Convert the object to a JSON text

const text = JSON.stringify(person);

Converting an Array

Example

Example Code

javascript

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// Create a JavaScript array

const cars = ["Ford", "Volvo", "BMW"];

// Convert the array to a JSON text

const text = JSON.stringify(cars);

Converting Other Values

JSON.stringify() can convert all JavaScript values that JSON supports.

JavaScript Value	JSON Text
String	"John"
Number	42
Boolean	true
null	null
Object	{"name":"John","age":30,"city":"New York"}
Array	["Ford","Volvo","BMW"]

Example

Example Code

javascript

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JSON.stringify("John");

JSON.stringify(42);

JSON.stringify(false);

JSON.stringify(Boolean(0));

JSON.stringify(true);

JSON.stringify(Boolean(1));

JSON.stringify(undefined);

// These returns null

JSON.stringify(null);

JSON.stringify(NaN);

JSON.stringify(Infinity);

Selecting Properties

A replacer array can choose which properties to include.

Example

Example Code

javascript

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const person = {

  name: "John",

  age: 30,

  city: "New York"

};

let text = JSON.stringify(person, ["name", "city"]);

The resulting JSON contains only the selected properties.

Transforming Values

A replacer function can modify values before they are converted.

Example

Example Code

javascript

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const person = {

  name: "John",

  age: 30

};

const text = JSON.stringify(person,
function(key, value) {

  if (key == "age") {

    return value + 1;

  }

  return value;

});

Formatting JSON

The optional space parameter makes JSON easier to read.

Example

Example Code

javascript

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const person = {

  name: "John",

  age: 30,

  city: "New York"

};

let text = JSON.stringify(person, null, 1);

The value null selects NO replacer.

The value 1 indents each level with one space.

Unsupported Values

Some JavaScript values cannot be represented in JSON.

Value	Result
BigInt	Throws a TypeError.
Infinity	Converted to null in objects and in arrays.
NaN	Converted to null in objects and in arrays.
undefined	Omitted from objects. Converted to null in arrays.
Functions	Omitted from objects. Converted to null in arrays.
Symbol	Omitted from objects. Converted to null in arrays.

Example

Example Code

javascript

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const person = {

  name: "John",

  greet: function() {},

  age: undefined

};

let text = JSON.stringify(person);

Note > The resulting JSON syntax contains only the supported values.

Example

Example Code

javascript

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const person = {

  name: "John",

  greet: NAN,

  age: Infinity

};

let text = JSON.stringify(person);

Example

Example Code

javascript

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// JavaScript Array

const cars = ["Ford", "Volvo", function() {}, undefined, NaN, Infinity];

// Convert the array to JSON

let text = JSON.stringify(cars);

Stringifying Dates

The JSON.stringify() function will convert Date objects into strings.

Example

Example Code

javascript

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const person = {name:"John", today:date, city:"New York"};

let text = JSON.stringify(person);

You can convert the string back into a date object at the receiver.

Storing Data

When storing data, the data has to be a certain format, and regardless of where you choose to store it, text is always one of the legal formats.

JSON makes it possible to store JavaScript objects as text.

Example

Example Code

javascript

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// Store data:

const myObj = {name: "John", age: 31, city: "New York"};

const myJSON = JSON.stringify(myObj);
localStorage.setItem("testJSON", myJSON);

// Retrieve data:

let text = localStorage.getItem("testJSON");

let obj = JSON.parse(text);

Common Stringify Mistakes

Calling JSON.stringify() twice, adds escaped quotation marks.

Wrong

Example Code

javascript

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const person = {name:"John"};

const text = JSON.stringify(person);

const textAgain = JSON.stringify(text);

JSON.stringify() cannot convert objects that contain circular references.

Example

Example Code

javascript

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const person = {};

person.self = person;

JSON.stringify(person);

The example above throws a TypeError.