This would generally be called #toString, so that coercion will work correctly:

var obj = {
  toString() { return 'test'; }
};
String(obj); // => 'test'
'' + obj; // => 'test'

I don't really like coercion to be honest. Usually I try to steer away. You feel strongly about it, I'll switch.

Need to document here why this is safe.

Ok. I made it explicit to data: