16 JSON Interview Questions You Must Be Prepared For

JSON (JavaScript Object Notation) is a lightweight format that is used for data interchanging. It is based on a subset of JavaScript language (the way objects are built in JavaScript). Some JavaScript is not JSON, and some JSON is not JavaScript.

The standard way to parse JSON in JavaScript is JSON.parse()

The JSON API was introduced with ES5 (2011) and has since been implemented in >99% of browsers by market share, and Node.js. Its usage is simple:

const json = '{ "fruit": "pineapple", "fingers": 10 }';
const obj = JSON.parse(json);
console.log(obj.fruit, obj.fingers);

The MIME media type for JSON text is application/json. The default encoding is UTF-8. (Source: RFC 4627).

People often assume all Javascript objects are JSON and that JSON is a Javascript object. This is incorrect.

In Javascript var x = {x:y} is not JSON, this is a Javascript object. The two are not the same thing. The JSON equivalent (represented in the Javascript language) would be var x = '{"x":"y"}'. x is an object of type string not an object in it's own right. To turn this into a fully fledged Javascript object you must first parse it, var x = JSON.parse('{"x":"y"}');, x is now an object but this is not JSON anymore.

Remember: JSON (in Javascript) is a string!

No.

The JSON should all be data, and if you include a comment, then it will be data too.

You could have a designated data element called "_comment" (or something) that would be ignored by apps that use the JSON data.

{
   "_comment": "comment text goes here...",
   "glossary": {
      "title": "example glossary",
      "GlossDiv": {
         "title": "S",
         "GlossList": {
            "GlossEntry": {
               "ID": "SGML",
               "SortAs": "SGML",
               "GlossTerm": "Standard Generalized Markup Language",
               "Acronym": "SGML",
               "Abbrev": "ISO 8879:1986",
               "GlossDef": {
                  "para": "A meta-markup language, used to create markup languages such as DocBook.",
                  "GlossSeeAlso": ["GML", "XML"]
               },
               "GlossSee": "markup"
            }
         }
      }
   }
}

JSON.stringify turns a JavaScript object into JSON text and stores that JSON text in a string, eg:

var my_object = {
    key_1: "some text",
    key_2: true,
    key_3: 5
};

var object_as_string = JSON.stringify(my_object);
// "{"key_1":"some text","key_2":true,"key_3":5}"  

typeof(object_as_string);
// "string" 

JSON.parse turns a string of JSON text into a JavaScript object, eg:

var object_as_string_as_object = JSON.parse(object_as_string);
// {key_1: "some text", key_2: true, key_3: 5} 

typeof(object_as_string_as_object);
// "object" 

JSON is built on two structures:

• A collection of name/value pairs. In various languages, this is realized as an object, record, struct, dictionary, hash table, keyed list, or associative array.
• An ordered list of values. In most languages, this is realized as an array, vector, list, or sequence.

JSONP is JSON with padding, that is, you put a string at the beginning and a pair of parenthesis around it. For example:

//JSON
{
    "name": "stackoverflow",
    "id": 5
}
//JSONP
func({
    "name": "stackoverflow",
    "id": 5
});

The result is that you can load the JSON as a script file. If you previously set up a function called func, then that function will be called with one argument, which is the JSON data, when the script file is done loading. This is usually used to allow for cross-site AJAX with JSON data. If you know that example.com is serving JSON files that look like the JSONP example given above, then you can use code like this to retrieve it, even if you are not on the example.com domain:

function func(json) {
     alert(json.name);
}
var elm = document.createElement("script");
elm.setAttribute("type", "text/javascript");
elm.src = "http://example.com/jsonp";
document.body.appendChild(elm);

There are some:

• JSON is not suitable for handling very large and complex data. When the data gets complex with several nested and hierarchical structures, it becomes complex for human readability.
• JSON does not support the comments.
• It does not support to handle the multimedia formats like image or rich text format.

• It is faster and lighter than XML as on the wire data format
• XML data is typeless while JSON objects are typed
• JSON types: Number, Array, Boolean, String
• XML data are all string
• Data is readily available as JSON object is in your JavaScript
• Fetching values is as simple as reading from an object property in your JavaScript code