JSON Syntax

  • JSON is often used to communicated between web browsers and servers
  • it is always sent as text, AKA a string
  • the JavaScript method JSON.parse() exists to parse the data from strings into JavaScript making the translation from JSON text to useable JavaScript data easier for us

Parse Method

  • The JSON.parse() method can be used on any JSON data you want to convert to JS, whether it was received from a server, or it was stored in the JS code.
  • The first requirement of the parse method is that the JSON data must be wrapped in a string by placing single quotes around the entirety of the data


  • For example, say we declare a new JSON object on the next line like so:
let studentJSON = 
  { "firstName" : "Millie", "lastName" : "Brown", "grade" : 10 };
  • If we tried to use the parse method on studentJSON as it is, and then print the last name attribute, nothing will be printed to the console :
let studentJS = JSON.parse(studentJSON);


  • The parameter of the JSON.parse() method must be a string
studentJSON = 
  '{"firstName" : "Millie", "lastName" : "Brown", "grade" : 10}';
  • Reassigning the value of studentJSON to the string of JSON data now allows us to successfully implement the parse method:
let student = JSON.parse(studentJSON);


  • Remember that JSON is useful for exchanging data from AND to a web server.
  • In the event that we want to convert JS data and SEND JSON data to a web server, we essentially perform the opposite action of the JSON.parse method
  • Fortunately, there is also a method that makes this task easy for us, called Stringify.

Stringify Method

  • Declare a new variable to store the string returned by the Stringify method:
let newString;
  • Set the new variable equal to the Stringify method with the JavaScript object passed in as a parameter:
newString = JSON.stringify(student);


  • The string is now ready to be sent to a server, which will read the JSON text
  • exceptions:
    • Remember that JSON does not support functions, dates, or undefined values.
    • if it has a date, function, or undefined value, JSON will alter the object according to the unsupported data type.

dates & functions

  • Dates are simply converted to a string. Let’s look at an example:
let obj = { name: "John", today: new Date(), city : "New York" };
let myJSON = JSON.stringify(obj);
  • Functions are removed from the object entirely unless the return value of a function is converted to a string before the stringify method is implemented


  • How do we loop over the data?
  • Looping is extremely useful when working with larger JSON data sets; for example when populating a table of the JSON data instead of having to manually enter each cell
  • Looping through JSON data is something that will look familiar, as it just uses a simple for-in loop

For-in loop

  • used to loop through the properties of an object; in this case, we’re looping through the properties of a JSON object
  • For JSON data, we can loop through and print each key like so:
television = { "brand":"Samsung", "model":"QF Series", "make":"QN82Q6FNAFXZA", "price":2999};
for (key in television) {


  • Note that the iterator, key, can be named anything. It could be i, x, etc. The name is irrelevant
  • To loop through the JSON data and retrieve the values of the key:value pairs, we use bracket notation like so:
for (key in television) {


  • Sometimes you want to store a JSON object within a JSON object.
  • For example, if we’re making a list of our friends and their respective attributes, such as hair color, eye color, age, name, etc. all of these keys typically require one value


  • we use nesting for more complex data, such as pets
friend = {
  "hairColor": "Blonde",
  "eyeColor": "Brown",
  "pets": {
    "bird": "Mack",
    "iguana": "Beemer",
    "cat": "Ozzy",
    "hamster": "Retsuko"


  • Nested JSON data is simply accessed using dot or bracket notation like so:
  • or:
let pet2 = friend.pets["iguana"];