How does moment.js know when it's object is being serialised?

Question

From the moment.js docs

moment().toJSON(); When serializing an object to JSON, if there is a Moment object, it will be represented as an ISO8601 string.

JSON.stringify({
    postDate : moment()
}); // {"postDate":"2013-02-04T22:44:30.652Z"}

I don't understand how the moment object can detect the function operating on it. How is it able to return a different value when serialised, and when simply stored in an object, or returned as a string?

Answer 1

When using stringify, an object may define how it gets represented, as shown in this documentation:

From https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/JSON/stringify

toJSON behavior

If an object being stringified has a property named toJSON whose value is a function, then the toJSON method customizes JSON stringification behavior: instead of the object being serialized, the value returned by the toJSON method when called will be serialized.

For example:

var x = {
  foo: 'foo',
  toJSON: function () {
    return 'bar';
  }
};
var json = JSON.stringify({x: x});
//json will be the string '{"x":"bar"}'.

moment.js's documentation (seen here: https://raw.github.com/timrwood/moment/2.0.0/moment.js ) shows that this is indeed supported, here is the exact code

toJSON : function () {
 return moment.utc(this).format('YYYY-MM-DD[T]HH:mm:ss.SSS[Z]');
}

So, that is how it is aware of how to represent itself when being stringified.