JSON Stringification

The JSON.stringify(..) utility will automatically omit undefined, function, and symbol values when it comes across them. If such a value is found in an array, that value is replaced by null (so that the array position information isn't altered). If found as a property of an object, that property will simply be excluded.

JSON.stringify( undefined );					// undefined
JSON.stringify( function(){} );					// undefined

JSON.stringify( [1,undefined,function(){},4] );	// "[1,null,null,4]"
JSON.stringify( { a:2, b:function(){} } );		// "{"a":2}"

But if you try to JSON.stringify(..) an object with circular reference(s) in it, an error will be thrown.

JSON stringification has the special behavior that if an object value has a toJSON() method defined, this method will be called first to get a value to use for serialization.

If you intend to JSON stringify an object that may contain illegal JSON value(s), or if you just have values in the object that aren't appropriate for the serialization, you should define a toJSON() method for it that returns a JSON-safe version of the object.

For example:

var o = { };

var a = {
	b: 42,
	c: o,
	d: function(){}
};

// create a circular reference inside `a`
o.e = a;

// would throw an error on the circular reference
// JSON.stringify( a );

// define a custom JSON value serialization
a.toJSON = function() {
	// only include the `b` property for serialization
	return { b: this.b };
};

JSON.stringify( a ); // "{"b":42}"

It's a very common misconception that toJSON() should return a JSON stringification representation. That's probably incorrect, unless you're wanting to actually stringify the string itself (usually not!). toJSON() should return the actual regular value (of whatever type) that's appropriate, and JSON.stringify(..) itself will handle the stringification.

In other words, toJSON() should be interpreted as "to a JSON-safe value suitable for stringification," not "to a JSON string" as many developers mistakenly assume.