Class “Conversion”

:point_right: Make sure to first read the introduction to data binding.

Instances of the Conversion class are given as the second argument to any converter functions used in data binding.

The property converter functions of @property/@prop do not receive this object.

In all Tabris.js data binding APIs source values may be converted before the framework applies them to the target property. To do so a converter function is used, both in one-way bindings via JSX and one-way/two-way bindings via @bind or @bindAll.

The TypeScript type of this function is exported as BindingConverter and is defined as:

type BindingConverter<From = any, Target = any, TargetProperty extends string = any>
  = (fromValue: From, conversion: Conversion<Target, TargetProperty>) => any | void;`

In most cases such a function can simply take the original value it gets passed as the first parameter and return the converted value:

const toUpperCase: BindingConverter<string> = value => value.toUpperCase();

However, the function can also take in to consideration who will receive the converted value. This is especially relevant in the case of two-way bindings, as the same function is responsible for conversions in both directions. The Conversion object allows to identify the target of the conversion and provides a callback that the result of the conversion may be passed to.


A BindingConverter that converts to upper case when it targets a TextInput widget, otherwise to lower case:

const toUpperCase: BindingConverter = (value: string, conversion) =>
  conversion.resolve(conversion.targets(TextInput, 'text') ? value.toUpperCase() : value.toLowerCase());

:point_right: See example app “bind-two-way-convert”.


All methods of Conversion are bound to the instance, meaning parameter destructuring may be used to call them:

(value, {targets, resolve}) => resolve(targets(Type) ? x : y);

targets(type, property?)

Returns true if the target of the conversion is of the given type (constructor) and property (string) of the target. If no property is given only the type is checked. The target type needs to be an exact match, subclasses or implementations of the given type do not count.


An alternative way to pass the converted value to the framework is to call this method. If this is done the function must not return a value. It must also not be called twice.


All properties are read-only.


The prototype object of the conversion target.


The name (string) of the targeted property.