Preserving undefined that JSON.stringify otherwise removes

The JSON spec does not allow undefined values, but does allow null values.

You can pass a replacer function to JSON.stringify to automatically convert undefined values to null values, like this:

var string = JSON.stringify(
  obj,
  function(k, v) { return v === undefined ? null : v; }
);

This works for undefined values inside arrays as well, as JSON.stringify already converts those to null.