JSON.stringify()

The JSON.stringify() method converts a JavaScript value to a JSON string, optionally replacing values if a replacer function is specified or optionally including only the specified properties if a replacer array is specified.

console.log(JSON.stringify({ x: 5, y: 6 }));
// expected output: "{"x":5,"y":6}"

console.log(JSON.stringify([new Number(3), new String('false'), new Boolean(false)]));
// expected output: "[3,"false",false]"

console.log(JSON.stringify({ x: [10, undefined, function(){}, Symbol('')] }));
// expected output: "{"x":[10,null,null,null]}"

console.log(JSON.stringify(new Date(2006, 0, 2, 15, 4, 5)));
// expected output: ""2006-01-02T15:04:05.000Z""

JSON.parse()

The JSON.parse() method parses a JSON string, constructing the JavaScript value or object described by the string. An optional reviver function can be provided to perform a transformation on the resulting object before it is returned.
var json = '{"result":true, "count":42}';
obj = JSON.parse(json);

console.log(obj.count);
// expected output: 42

console.log(obj.result);
// expected output: true

JSON.stringify, avoid TypeError: Converting circular structure to JSON

Use JSON.stringify with a custom replacer. For example:

// Demo: Circular reference
var o = {};
o.o = o;

// Note: cache should not be re-used by repeated calls to JSON.stringify.
var cache = [];
JSON.stringify(o, function(key, value) {
    if (typeof value === 'object' && value !== null) {
        if (cache.indexOf(value) !== -1) {
            // Circular reference found, discard key
            return;
        }
        // Store value in our collection
        cache.push(value);
    }
    return value;
});
cache = null; // Enable garbage collection

The replacer in this example is not 100% correct (depending on your definition of "duplicate"). In the following case, a value is discarded:

var a = {b:1}
var o = {};
o.one = a;
o.two = a;
// one and two point to the same object, but two is discarded:
JSON.stringify(o, ...);

But the concept stands: Use a custom replacer, and keep track of the parsed object values.