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

Extends the behavior of a property. Most importantly it will generate change events, thereby making it a valid source for one-way data bindings on widgets decorated with @component. If the type of the property is known at runtime it can also check and convert the incoming value. In JavaScript the type can be made known using the type option, while in TypeScript is provided automatically by the compiler.

The @property decorator can be used in any class, not just subclasses of Widget. On a non-widget class change events may be listened to via an instance of ChangeListeners attached to an appropriately named property.

@property (no parameter)

:point_right: See example apps “property-change-events” (TypeScript) and “property-change-events-jsx” (JavaScript/JSX).

Triggers change events and (in TypeScript) performs implicit runtime checks on any value the property is set to.

class Foo {
  @property myText: string = 'foo';
  @event onMyTextChanged: ChangeListeners<Foo, 'myText'>;

const foo = new Foo();
foo.onMyTextChanged(ev => console.log(ev.value));
foo.myText = 'bar'; // logs "bar" due to the change event
(foo as any).myText = 23; // throws due to the implicit value check

In JavaScript these checks do not happen unless the type option is set.

The implicit runtime check only works with primitive types and classes. Advanced type and interfaces can not be checked:

class Foo {
  @property myItem: {bar: string};

const foo = new Foo();
foo.myItem = {bar: 'foo'}; // OK
(foo as any).myItem = {foo: 'bar'}; // runtime check passes despite incorrect type

In these cases it is recommended to use a type guard.


A shorthand for @property({typeGuard: fn}). See “config.typeGuard”.


Like @property, but with more options.


Where type is a constructor function.

This option is usually not required in TypeScript, only in JavaScript/JSX files. A possible exception is using it with a converter.

When providing this option the property will check that every assigned value is an instance of the given constructor function, such as Date. Primitives (number, string, boolean) are represented by the constructors of their boxed/wrapped values.

JavaScript Example:

class Foo {

  /** @type {Date} */
  @property({type: Date})
  myDate = new Date();

  /** @type {string} */
  @property({type: String})
  myText = 'foo';


This is the exact equivalent of the following TypeScript code:

class Foo {
  @property myDate: Date = new Date();
  @property myText: string = 'foo';

It is currently not possible to use type to describe interfaces (for “duck typing”) or mixed types (e.g. “string or number”). However, type checking for such cases can be achieved using a type guard.


Where typeGuard is of the type (value: any) => boolean.

:point_right: See example app “property-value-checks”.

Uses the given function (type guard) to perform type checks. The type guard may be more strict than the TypeScript compiler (e.g. allowing only positive numbers for a number property), but should never be less strict.

The function may return either a boolean (true indicates the value passes), or throw an error explaining why the value did not pass.

Example for a type guard more strict than the compiler:

  class CustomComponent extends Composite {

    @property(v => v instanceof Array || (!isNaN(v) && v >= 0))
    mixedType: number[] | number = 0; // compiler would allow -1, but not the type guard


By using multiple @property decorator on the same property you can give multiple type guards. They are executed in the given order.


Where default can be any value except undefined, which turns the feature off.

Defines the initial value of this property. It will also be used as the fallback value if the property is not nullable. If a converter is defined it will be applied to the default value as well.

Using default differs from setting the property with the declaration, which changes the value only after super has been called. Thus it allows setting the property in a super-constructor:

class MyComponent extends Composite {

  // Bad:
  // @property num: number = 0;

  // Good:
  @property({default: 0})
  num: number;

  constructor(properties: Properties<MyComponent>) {


console.log(new MyComponent({}).num); // "0"
console.log(new MyComponent({num: 1}).num); // "1"

Alternatively you can use this.set(properties); instead of super(properties);. In that case it doesn’t matter how the property is initialized.


Where nullable is a boolean. The default is true.

Dictates whether the property can be set to null or undefined. If set to false the initial value may be null/undefined (assuming no default value is defined), but it can not change to have these values later.

How this is achieved depends on the “default” and “convert” options. Here is what happens when a non-nullable property is set to null or undefined:

Target type Option “default” Option “convert” Result
any type set any value value reset to default
any type not set 'off' Exception thrown
primitive not set 'auto' or function Value converted
object type not set 'auto' or function Exception thrown


Where observe is a boolean. The default is false. Has no effect on properties containing primitives.

If set to true, the property will fire a change event for itself whenever its contained object fires a change event for any of its own properties:

class MyData {

  @property({observe: true})
  data: OtherData;

  onOtherDataChanged: ChangeListeners<ObserveExample, 'data'>;


const data = new MyData();
data.onOtherDataChanged(ev => console.log(ev.value?.foo));

const otherData = new OtherData();
otherData.foo = 1;
data.otherData = otherData; // logs "1"
otherData.foo = 2; // logs "2"

The change event fired by the “observing” property (here: data) will have a reference to the change event that caused it:

data.onOtherDataChanged(ev => console.log(ev.originalEvent?.value));

If the change event was caused by the “observing” property itself receiving a new value (e.g. another object or null) the originalEvent property will be null.

The data property of Widget and the item property of Cell both are “observing” properties. All properties of an ObservableData instance are as well, and any property decorated with @bind, @bindAll or @prop.

This feature is intended to be used with the apply attribute/method or the @bind/@bindAll decorators in scenarios where values for the UI are computed based on models that are nested within each other.

Examples can be found here:


Where equals is 'strict', 'shallow', 'auto' or a function. Default is 'strict'.

Controls how the property determines whether a new value would be equal to the current one. If both are considered equal the new value will be discarded in favor of the current one and no change event will fire.

When set to 'strict' both values are only considered equal if they are identical, i.e. a === b. Two objects or arrays will never be considered equal, even if they contain the same values.

When set to 'shallow' two object values will be considered equal if they have the exact same properties set to the exactly (strictly equal) same values. If used with arrays they also have to have the same length, even if all the additional slots of the longer array are empty. As the name implies this is not a “deep equal” check, so it is best used with arrays of primitives or simple models.

When set to 'auto' it will use the 'strict' strategy for primitives and 'shallow' for arrays and plain objects, i.e. object literals like {foo: 'bar'}. Also, two objects will be considered equal if they have the same constructor and return the same primitives when the valueOf() method is called. Lastly, if an object implements a method called equals that takes exactly one parameter it will be called with the other object to test equality. If it returns true the objects are considered equal.

When set to a function it will be called with the two values to compare. They will be considered equal if the return value is true. If the function throws the exception will be propagated to the code that is setting the property.


Where convert is 'off', 'auto' or a function. Default is 'off'.

Allows to convert a value that is assigned to the property to the specified type of the property. This works regardless of how the value is set - directly (obj.prop = value), using the set() method, on declaration, via the default option, or by data binding. In JavaScript it requires the type option to be set. In TypeScript this is not required unless the runtime type differs from the compile time type. (See “Usage with TypeScript” section below.) If the type of the property is unknown a warning will be printed and no conversion will be applied.

If set to 'off' no conversion is applied. All values will be set (or rejected) as-is.

If set to 'auto' the property will attempt to convert any new value to the expected type if the result would be semantically similar. See “Conversion strategies” below.

The 'auto' conversion does not work with interfaces, mixed types or plain objects as the target type.

If set to a function, it will be called with the incoming value and must return a value of the expected type. If the value is already of the expected type the function will not be called, so it can for example not be used to convert a string to another string. The function may throw an exception if the incoming value can not be converted. The exception will be propagated to the code that is setting the property. If a type guard is present it will be called after the converter with the result of the conversion.

The converter function will never be called with null or undefined.

Usage with TypeScript

In TypeScript the compiler should prevent the code from directly assigning a value of the wrong type to the property. However, in data bindings there is no compile time check, so in this case a converter makes a property bindable to other properties of different types. Without a proper converter an exception would be thrown due to the type mismatch. The feature may also be useful when dealing with the any type, for example as the result of JSON.parse() or a REST call.

It is also valid to explicitly set the type option to a subset of the TypeScript type. This is necessary for the converter to work if the TypeScript (compile-time) type is an “advanced” type such as a union of mixed types. The result - assuming the converter can handle the value - will be that any value that matches the compile time type can be set, but on get the value will always be of the type specified in the “config” object. This works for most tabris built-in data types.


@property({type: Color, convert: 'auto', equals: 'auto'})
color: ColorValue;

// Or just:
color: ColorValue;

Like with built-in widgets types, this color property can be assigned any ColorValue such as 'red' or '#ff0000', but will always return a Color instance.

Conversion strategies

The 'auto' converter can convert between a number of types, but only if the value can be plausibly expressed in the target type. It therefore does NOT work like JavaScript type coercion, and it is also NOT always symmetric. If there is no plausible strategy for conversion the setter will throw an exception.

Below is a list of strategies for each target type.

  • string:
    • Primitives will be “stringified”, e.g. 1 becomes '1'
    • Arrays will be joined
    • Objects will be converted by calling toLocaleString() or toString(), whichever is provided
  • number
    • A string will be parsed as a number if it contains a valid JavaScript number expression, including signed, floating point and hex numbers. It also includes Infinity and NaN
    • Empty strings become 0
    • true becomes 1 and false becomes 0
    • Date will become a unix timestamp
  • boolean
    • String 'true' becomes true and string 'false' becomes false.
    • Empty strings become false.
    • Any number greater than 0 becomes true, all other false
  • Array
    • Strings will be separated by comma in to a string array
    • Other primitives will be wrapped in to an array with a single entry
    • Array-likes (objects with length property, e.g. arguments and List) will be converted 1-to-1 in to “real” arrays
    • Typed arrays and ArrayBuffer will be converted in to a number-array with each entry representing one byte.
    • Objects with an toArray method will be converted by calling that method.
    • All other objects will turn in to arrays containing the property values in arbitrary order.
  • Typed arrays and ArrayBuffer
    • Conversion between typed arrays will re-use the same ArrayBuffer instance
    • A plain array or array-like will be used to create a new ArrayBuffer with each entry interpreted as a number representing 1 byte (via UInt8ClampedArray).
  • Built-in types
    • Boxed (wrapper) types like Number can not be created, the result will always be the respective primitive.
    • Blob instances can be created from strings, typed arrays and ArrayBuffer.
    • Date instances are created by interpreting the value as a number representing a unix timestamp.
    • Tabris data-types are created using their respective from() method, e.g. Color.from().

Additional strategies:

  • Object to primitive
    • If an arrow function is supposed to be converted to primitive it will simply be called and the result will be used if it has the expected type.
    • An array can be converted to a primitive if it contains a single value of the desired type, or if the target type is string.
    • If the object provides a valueOf() implementation, this method will be called.
    • Lastly, if the object has a value property containing a primitive, that value will be used.
  • Primitive to object
    • If the target object type provides a static from() method it may well be called to convert the value. It needs to take exactly one parameter and return the expected type. If the type does not match or the method throws an exception, the setter will throw.
    • For conversions from string to object a static parse() method will be used over from() if available.
    • As a last resort, if no other conversion strategy is found, the constructor of the target type will be called with the incoming value as the sole argument. For this to work the constructor must have one or more parameters.