JSON

Question 1

Convert an object to a string

What gets lost? (5 things)

Answer 1

JSON.stringify()
JSON-encoded or serialized or stringified or marshalled object

Function properties (methods).
Symbolic keys and values.
Properties that store undefined.
Array properties
Map,  weakMap, Set, weakSet objects

Question 2

What does JSON stand for?

Answer 2

JavaScript Object Notation

Question 3

What is JSON?

Answer 3

is a general format to represent values and objects

Question 4

IS JSON just for javascript?

Answer 4

Initially it was made for JavaScript, but many other languages have libraries to handle it as well
So it’s easy to use JSON for data exchange when the client uses JavaScript and the server is written on Ruby/PHP/Java/Whatever.

Question 5

Convert a JSON string into an object

Answer 5

JSON.parse()

Question 6

What happens to:
' 
`
and 
" 
when stringified?

Answer 6

all become “

Question 7

JSON is
data-only language-independent specification
What does this mean?

Answer 7

some JavaScript-specific object properties are skipped by JSON.stringify.

Namely:

Function properties (methods).
Symbolic keys and values.
Properties that store undefined.

let user = {
  sayHi() { // ignored
    alert("Hello");
  },
  [Symbol("id")]: 123, // ignored
  something: undefined // ignored
};
alert( JSON.stringify(user) ); // {} (empty object)

Question 8

What happens with this code?
let room = {
number: 23
};

let meetup = {
  title: "Conference",
  participants: ["john", "ann"]
};

meetup. place = room;
room. occupiedBy = meetup;

JSON.stringify(meetup);

Answer 8

// Error: Converting circular structure to JSON

Question 9

What does the replacer do?
JSON.stringify(value, replacer, space)
What are it’s limitations?
How do you fix it?

Answer 9

Makes it so only those keys that match an element of the replacer array will be stringified (it can be a function too)
Including any nested keys:
let meetup = {
  title: "Conference",
  participants: [{name: "John"}, {name: "Alice"}],
  place: "Quebec"
};

alert( JSON.stringify(meetup, ['title', 'participants']) );
// {"title":"Conference","participants":[{},{}]}

‘name’ and ‘name’ are skipped

Have to do this:
let room = {
  number: 23
};

let meetup = {
  title: "Conference",
  participants: [{name: "John"}, {name: "Alice"}],
  place: room // meetup references room
};

room.occupiedBy = meetup; // room references meetup

alert( JSON.stringify(meetup, ['title', 'participants', 'place', 'name', 'number']) );
/*
{
  "title":"Conference",
  "participants":[{"name":"John"},{"name":"Alice"}],
  "place":{"number":23}
}
*/

Question 10

Write a replacer function

JSON.stringify(value, replacer, space)

Answer 10

let room = {
  number: 23
};

let meetup = {
  title: "Conference",
  participants: [{name: "John"}, {name: "Alice"}],
  place: room // meetup references room
};

room.occupiedBy = meetup;

JSON.stringify(meetup, function replacer(key, value) {
return (key == ‘occupiedBy’) ? undefined : value;
})

Question 11

What happens to dates that get stringified?

Answer 11

let meetup = {
  title: "Conference",
  date: new Date(Date.UTC(2017, 0, 1)),
  room
};

alert( JSON.stringify(meetup) );
/*
  {
    "title":"Conference",
    "date":"2017-01-01T00:00:00.000Z",  // (1)
    "room": {"number":23}               // (2)
  }
*/

Question 12

What does a toJSON() method on an object do?

Answer 12

Defines what is returned when JSON.stringify is called.

let room = {
  number: 23,
  toJSON() {
    return this.number;
  }
};

let meetup = {
  title: "Conference",
  room
};

alert( JSON.stringify(room) ); // 23

alert( JSON.stringify(meetup) );
/*
  {
    "title":"Conference",
    "room": 23
  }
*/

Question 13

What's wrong (or right) which each of the JSON properties?
let json = {
  name: "John",                     
  "surname": 'Smith',               
  'isAdmin': false                  
  "birthday": new Date(2000, 2, 3),
  "friends": [0,1,2,3]              
}`;

Answer 13

// mistake: property name without quotes
// mistake: single quotes in value (must be double)
// mistake: single quotes in key (must be double)
 // mistake: no "new" is allowed, only bare values
// here all fine

Question 14

What happens with this?

let user = {
  date: new Date(),
}

let apple = JSON.stringify(user);

let pear = JSON.parse(apple);

console.log(pear.date.getTime());

Answer 14

error.
Dates are converted to strings and they are not converted to date objects when parsed.

How do you fix this? (scroll down)
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Use a reviver
JSON.parse(text, reviver)

let meetup = JSON.parse(str, function(key, value) {
  if (key == 'date') return new Date(value);
  return value;
});