5.3.3 Converter

Converter classes convert between JavaScript types and MySQL types. If the user supplies a JavaScript converter, it used to read and write to the database.

Converters have several purposes, including the following:

To convert between MySQL DECIMAL types and a user's preferred JavaScript fixed-precision utility library
To convert between MySQL BIGINT types and a user's preferred JavaScript big number utility library
To serialize arbitrary application objects into character or binary columns

The ndb back end also uses converters to support SET and ENUM columns. (The mysql back end does not use these.)

A Converter class has the interface defined here:

function Converter() {}:

Converter.prototype = {
  "toDB"    : function(obj) {  },
  "fromDB"  : function(val) {  }
};

The Converter must implement the following two functions:

toDB(obj): Convert an application object obj into a form that can be stored in the database.
fromDB(val): Convert a value val read from the database into application object format.

Each function returns the result of the conversion.

Converter invocations are chained in the following ways:

When writing to the database, first the registered FieldConverter, if any, is invoked. Later, any registered TypeConverter is invoked.
When reading from the database, first the registered TypeConverter, if any, is invoked. Later, any registered FieldConverter is invoked.