`JSON.stringify`

JSON-encoded object has several important differences from the object literal:
- Strings use double quotes.
- Object property names are double-quoted.
JSON is data-only cross-language specification, so some JavaScript-specific object properties are skipped by JSON.stringify:
- Function properties (methods).
- Symbolic properties.
- Properties that store undefined.
Nested objects are supported and converted automatically.
- The important limitation is that there must be no circular references.

JSON.stringify({}); // '{}'
JSON.stringify(true); // 'true'
JSON.stringify('foo'); // '"foo"'
JSON.stringify([1, 'false', false]); // '[1,"false",false]'
JSON.stringify({ x: 5 }); // '{"x":5}'
JSON.stringify(new Date(2006, 0, 2, 15, 4, 5)) // '"2006-01-02T15:04:05.000Z"'
JSON.stringify({ x: 5, y: 6 }); // '{"x":5,"y":6}' or '{"y":6,"x":5}'
JSON.stringify([new Number(1), new String('false'), new Boolean(false)]); // '[1,"false",false]'
JSON.stringify({ x: [10, undefined, function(){}, Symbol('')] }); // '{"x":[10,null,null,null]}'

// Symbols:
JSON.stringify({ x: undefined, y: Object, z: Symbol('') }); // '{}'
JSON.stringify({ [Symbol('foo')]: 'foo' }); // '{}'
JSON.stringify({ [Symbol.for('foo')]: 'foo' }, [Symbol.for('foo')]); // '{}'
JSON.stringify({ [Symbol.for('foo')]: 'foo' }, function(k, v) { 
	if (typeof k === 'symbol') {
		return 'a symbol';	
	}
}); // '{}'

// Non-enumerable properties:
JSON.stringify(Object.create(null, { 
		x: { 
			value: 'x', 
			enumerable: false 
		}, 
		y: { 
			value: 'y', 
			enumerable: true 
		} 
})); // '{"y":"y"}'

The full syntax of JSON.stringify is includes two extra parameters:
- **replacer** is an array of properties to encode or a mapping function function(key, value).
- **space** is an amount of space to use for formatting

let room = {
  number: 23
};

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

room.occupiedBy = meetup; // room references meetup

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

Here we are probably too strict. The property list is applied to the whole object structure.
So participants are empty, because name is not in the list.
Let’s include every property except room.occupiedBy that would cause the circular reference:

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}
}
*/

Now everything except occupiedBy is serialized. But the list of properties is quite long.
Fortunately, we can use a function instead of an array as the replacer.
The function will be called for every (key, value) pair and should return the “replaced” value, which will be used instead of the original one.
In our case, we can return value “as is” for everything except occupiedBy.
To ignore occupiedBy, the code below returns undefined:

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, function replacer(key, value) {
  alert(`${key}: ${value}`); // to see what replacer gets
  return (key == 'occupiedBy') ? undefined : value;
}));

/* key:value pairs that come to replacer:
:             [object Object]
title:        Conference
participants: [object Object],[object Object]
0:            [object Object]
name:         John
1:            [object Object]
name:         Alice
place:        [object Object]
number:       23
*/

Please note that replacer function gets every key/value pair including nested objects and array items. It is applied recursively. The value of this inside replacer is the object that contains the current property.
The first call is special. It is made using a special “wrapper object”: {"": meetup}. In other words, the first (key, value) pair has an empty key, and the value is the target object as a whole. That’s why the first line is ":[object Object]" in the example above.
The idea is to provide as much power for replacer as possible: it has a chance to analyze and replace/skip the whole object if necessary.