JavaScript for Handling JSON

JSON Data Types

In JSON, values must be one of the following data types:

Parsing JSON*

A common use of JSON is to exchange data to/from a web server. When receiving data from a web server, the data is always a string. Parse the data with JSON.parse(), and the data becomes a JavaScript object.

Imagine we received this text from a web server:

'{"name":"John", "age":30, "city":"New York"}'

Use the JavaScript function JSON.parse() to convert text into a JavaScript object:

const obj = JSON.parse('{"name":"John", "age":30, "city":"New York"}');

When using the JSON.parse() on a JSON derived from an array, the method will return a JavaScript array, instead of a JavaScript object. An example:

const text = '["Ford", "BMW", "Audi", "Fiat"]';
const myArr = JSON.parse(text);

Parsing Dates

Date objects are not allowed in JSON. If you need to include a date, write it as a string. You can convert it back into a date object later, as shown in the example:

const text = '{"name":"John", "birth":"1986-12-14", "city":"New York"}';
const obj = JSON.parse(text);
obj.birth = new Date(obj.birth);

document.getElementById("demo").innerHTML = obj.name + ", " + obj.birth;

Alternatively, you can use the second parameter, of the JSON.parse() function, called reviver. The reviver parameter is a function that checks each property, before returning the value.

const text = '{"name":"John", "birth":"1986-12-14", "city":"New York"}';
const obj = JSON.parse(text, function (key, value) {
  if (key == "birth") {
    return new Date(value);
  } else {
    return value;
  }
});

document.getElementById("demo").innerHTML = obj.name + ", " + obj.birth;
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Parsing Functions

Functions are not allowed in JSON. If you need to include a function, write it as a string. You can convert it back into a function later:

const text = '{"name":"John", "age":"function () {return 30;}", "city":"New York"}';
const obj = JSON.parse(text);
obj.age = eval("(" + obj.age + ")");

document.getElementById("demo").innerHTML = obj.name + ", " + obj.age();

JSON.stringify()*

You can convert any JavaScript datatype into a string with JSON.stringify().

Imagine we have this object in JavaScript:

const obj = {name: "John", age: 30, city: "New York"};

Use the JavaScript function JSON.stringify() to convert it into a string like so:

const myJSON = JSON.stringify(obj);

You can also stringify JavaScript arrays.

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. In the following example data is stored in local storage:

// Storing data:
const myObj = {name: "John", age: 31, city: "New York"};
const myJSON = JSON.stringify(myObj);
localStorage.setItem("testJSON", myJSON);

// Retrieving data:
let text = localStorage.getItem("testJSON");
let obj = JSON.parse(text);
document.getElementById("demo").innerHTML = obj.name;

Stringify a Function

In JSON, functions are not allowed as object values. The JSON.stringify() function will remove any functions from a JavaScript object, both the key and the value:

const obj = {name: "John", age: function () {return 30;}, city: "New York"};
const myJSON = JSON.stringify(obj);

This can be omitted if you convert your functions into strings before running the JSON.stringify() function.

const obj = {name: "John", age: function () {return 30;}, city: "New York"};
obj.age = obj.age.toString();
const myJSON = JSON.stringify(obj);