@function can-stache.registerConverter registerConverter
@description Register a helper for bidirectional value conversion.
@parent can-stache/deprecated

@deprecated {4.15.0} Use [can-stache.addConverter] instead. It will always be passed
observables, removing the need for the user to pass values with `~`.

@signature `stache.registerConverter(converterName, getterSetter)`

Creates a helper that can do two-way conversion between two
values.  This is especially useful with
[can-stache-bindings.twoWay two-way bindings] like:

```html
<input value:bind='numberToString(~age)'/>
```

A converter helper provides:

 - a `get` method that returns the value
  of the `left` value given the arguments passed on the `right`.
 - a `set` method that updates one or multiple of the `right` arguments
   computes given a new `left` value.

`numberToString` might converts a number (`age`)
to a string (`value`), and the string (`value`) to a number (`age`)
as follows:


```js
import {stache, Reflect as canReflect} from "can";

stache.registerConverter( "stringToNumber", {
	get: function( numberObservable ) {
		return "" + canReflect.getValue(numberObservable);
	},
	set: function( string, numberObservable ) {
		canReflect.setValue(numberObservable, +string);
	}
} );
```

@param {String} converterName The name of the converter helper.
@param {can-stache.getterSetter} getterSetter An object containing get() and set() functions.

@body

## Use

> __NOTE__: Before creating your own converter, you may want to look at what’s provided by [can-stache-converters].

These helpers are useful for avoiding creating [can-define/map/map] getters and setters that do similar conversions on the view model.  Instead,
a converter can keep your viewModels more ignorant of the demands of the
view.  Especially as the view’s most common demand is that everything
must be converted to a string.

That being said, the following is a richer example of a converter,
but one that should probably be part of a view model.

```handlebars
<input value:bind='fullName(~first, ~last)'/>
```

The following might converts both ways `first` and `last` to `value`.

```js
import canBatch from "can-event/batch/batch";

stache.registerConverter( "fullName", {
	get: function( first, last ) {
		return first() + last();
	},
	set: function( newFullName, first, last ) {
		canBatch.start();
		const parts = newFullName.split( " " );
		first( parts[ 0 ] );
		last( parts[ 1 ] );
		canBatch.stop();
	}
} );
```