JSON is a coding language, but it's not a programming language. Its purpose is to represent data with no behavior attached. It's useful for saving state, transferring information across a network, writing config files or editing settings. And since it's a strict subset of the JavaScript language syntax, it's natural to use it in JavaScript programs and JavaScript-aware text editors.


JavaScript Object Notation

  • a proper subset of JavaScript
  • basically, it's plain JavaScript without functions or variables
    • (or comments, or single-quoted strings, or a few other limitations)
  • used for data sans behavior
    • saving/loading to disk or database
    • transmitting information across network


  • efficient to parse and compress
  • human-readable, for the most part
  • well-defined rules for whitespace and character encoding
    • always Unicode, usually UTF-8
  • very flexible data format
    • allows arbitrary nesting of arrays and objects (hashes)

Example JSON object

    "Image": {
        "Width":  800,
        "Height": 600,
        "Title":  "View from 15th Floor",
        "Thumbnail": {
            "Url":    "",
            "Height": 125,
            "Width":  100
        "Animated" : false,
        "IDs": [116, 943, 234, 38793]

Its Image member is an object whose Thumbnail member is an object and whose IDs member is an array of numbers.

(from the spec)

Example JSON array

       "precision": "zip",
       "Latitude":  37.7668,
       "Longitude": -122.3959,
       "Address":   "",
       "City":      "SAN FRANCISCO",
       "State":     "CA",
       "Zip":       "94107",
       "Country":   "US"
       "precision": "zip",
       "Latitude":  37.371991,
       "Longitude": -122.026020,
       "Address":   "",
       "City":      "SUNNYVALE",
       "State":     "CA",
       "Zip":       "94085",
       "Country":   "US"

(from the spec)

Viewing JSON in Browser

  • Its mime-type is application/json which most browsers will display all on one line :-(
  • There are browser extensions that will render it better
    • Chrome: JSON Viewer (click on "Chrome Web Store" button to install)

json viewer screenshot

Viewing JSON in NodeJS Console

  • JSON is JavaScript
  • so if you copy a JSON blob and paste it into the Node REPL
  • it will look like this:

$ node
> { "Image": { "Width":  800, "Height": 600, "Title":  "View from 15th Floor", "Thumbnail": { "Url":    "", "Height": 125, "Width":  100 }, "Animated" : false, "IDs": [116, 943, 234, 38793] } }
{ Image:
   { Width: 800,
     Height: 600,
     Title: 'View from 15th Floor',
      { Url: '',
        Height: 125,
        Width: 100 },
     Animated: false,
     IDs: [ 116, 943, 234, 38793 ] } }

(beware multi-line strings though: )

Parsing & Producing JSON

  • Assume the following is in a string named text :
let text = '{ "name": "Ada Lovelace", "id": 1, "title": "The Queen of Numbers" }'

The following code converts the String data into a JavaScript object:

let data = JSON.parse(text)

And this converts a JavaScript object back into a String:

let newText = JSON.stringify(data)


  • First you will need to install Postman:

  • create a new GET request to the url

  • hit send and see the response.

There are also many plugins for Postman that give you collections of comon requests for certain sites and APIs. These collections are stored on the sites themselves rather than on Postman.