Client-Side Form Binding Reference

The key concepts behind client-side form binding are: the field() directive, the Model, the Binder and binder nodes.

The Field Directive

Client-side form binding in Vaadin works together with Lit web component library, and its underlying template rendering library lit-html.

The field() directive does the main job of binding the form field components in the template, namely:

  • sets the name attribute

  • implements two-way binding for the value state

  • sets the required (boolean) state

  • sets the invalid (boolean) state and the errorMessage (string) when the current value is invalid

 label="Full name"

Depending on the type of the bound component, the field directive selects a strategy, which defines how exactly the states above are applied on the component, for example, which attributes and properties of the element are used.

You can find more information on field strategy customization in the Using a Web Component Field article.

The field directive supports Vaadin components and HTML input elements. While Vaadin components have support for all the states, for HTML input elements the invalid, required and errorMessage are not displayed in the bound component. As a workaround, you can bind those manually in the template:

<label for="fullName">
  Full name
  ${binder.for(binder.model.fullName).required ? '*' : ''}
<input id="fullName" ...="${field(binder.model.fullName)}"/><br/>
    ? html`
    : ''

The Form Model

A form model describes the structure of the form. It allows referencing individual fields in a type-safe way (an alternative to strings like 'person.firstName').

Typically a model is used as an argument for the field() directive or the binder.for() method to specify the target form property to create a binding or to access the state. In contrast with string names like 'person.firstName', typed form models allow autocompletion and static type checking, which makes creating forms faster and safer.

Vaadin automatically generates Model classes for server-side endpoints from Java beans. Usually there is no need to define models manually.

Technically, every model instance represents either a key of a parent model, or the value of the Binder itself (for example, the form data object).


The model classes are not intended to be instantiated manually. Instead, the Binder constructor receives the form model class and takes care of creating model instances.

Primitive Models

These are the built-in models that represent the common primitive field types:

TypeValue type TEmpty value (Model.createEmptyValue() result)










Primitive models extend PrimitiveModel<T>. Primitive models are leaf nodes of the data structure, that is, they do not have nested field keys.

Object Models

Typically, the primitive editable values of the form are grouped in objects. ObjectModel<T> accommodates that. ObjectModel<T> is also a common superclass for all the models generated from Java beans.

The subclasses of ObjectModel<T> define the type argument constrains and the default type. In addition, the subclasses list all the public properties, following the shape of the described object.

For example, for the following Java bean:

public class IdEntity {
    private String idString;

    public String getIdString() {
        return idString;

    public void setIdString(String idString) {
        this.idString = idString;
import javax.validation.constraints.NotEmpty;

public class Person extends IdEntity {
    @NotEmpty(message = "Cannot be empty")
    private String fullName;

    public String getFullName() {
        return fullName;

    public void setFullName(String fullName) {
        this.fullName = fullName;

The following TypeScript interfaces are generated for type-checking endpoints:

export default interface IdEntity {
  idString: string;
import IdEntity from './IdEnity';

export default interface Person extends IdEntity {
  fullName: string;

And the following models are generated for client-side form binding:

import IdEntity from './IdEntity';

export default class IdEntityModel<T extends IdEntity = IdEntity> extends ObjectModel<T> {
  static createEmptyValue: () => IdEntity;
  readonly idString = new StringModel(this, 'idString');
import IdEntityModel from './IdEntityModel';

import Person from './Person';

export default class PersonModel<T extends Person = Person> extends IdEntityModel<T> {
  static createEmptyValue: () => Person;
  readonly fullName = new StringModel(this, 'fullName', new NotEmpty({message: 'Cannot be empty'}));

To avoid naming collisions with user-defined object model fields, the built-in models and model superclasses do not have any public instance properties or methods, aside form the toString() and valueOf() exceptions inherited from AbstractModel<T> (see below).

The properties of object models are intentionally read-only.

The Array Model

The ArrayModel<T> is used to represent array properties.

The type argument T in array models indicates the type of values in the array.

An array model instance contains the item model class reference. The item model is instantiated for every array entry, as necessary.

Array models are iterable, iterating yields binder nodes for entries:

${repeat(this.binder.model.people, personBinder => html`
     label="Full name"
    <strong>Full name:</strong>

The array entries are not available for indexing with bracket notation ([]).

The Abstract Model Superclass

All models subclass from the AbstractModel<T> TypeScript class, where the T type argument refers to the value type.

The Empty Value Definition

Model classes define an empty value, which is used to initialise binder.defaultValue and binder.value properties, and also for binder.clear().

For that purpose, AbstractModel<T>, as well as every subclass, has a method static createEmptyValue(): T, that returns the empty value of the subject model type.

const emptyPerson: Person = PersonModel.createEmptyValue();
console.log(emptyPerson); // {"fullName": ""}

Models in Expressions

As with any JavaScript object, AbstractModel<T> has toString(): string and valueOf(): T instance methods, that are handy for template expressions.

For StringModel in string expressions, the following are equivalent:


You can use NumberModel in formulas using valueOf():

  Cost: ${model.quantity.valueOf() * model.price.valueOf()}

The Binder

A form binder controls all aspects of a single form. Typically it is used to get and set the form value, access the form model, validate, reset, and submit the form.

The Binder constructor arguments are:

context: Element

The form view component instance to update.

Model: ModelConstructor<T, M>

The constructor (the class reference) of the form model. The Binder instantiates the top-level model and

config?: BinderConfiguration<T>

The options object.

onChange?: (oldValue?: T) ⇒ void

The callback that updates the form view, by default uses context.requestUpdate().

onSubmit?: (value: T) ⇒ Promise<T | void>

The endpoint for submitting the form data into.

The Binder has the following instance properties:

model: M

The form model, the top-level model instance created by the Binder.

value: T

The current value of the form, two-way bound to the field components.

defaultValue: T

The initial value of the form, before any fields are edited by the user.

readonly validating: boolean

True when there is an ongoing validation.

readonly submitting: boolean

True if the form was submitted, but the submit promise is not resolved yet.

The Binder instance methods are:

read(value: T): void

Load the given value to the form.

reset(): void

Reset the form to the previous value.

clear(): void

Sets the form to empty value, as defined in the Model.

getFieldStrategy(element: any): FieldStrategy

Determines and returns the field directive strategy for the bound element. Override to customise the binding strategy for a component. The Binder extends BinderNode, see the inherited properties and methods below.

Binder Nodes

The BinderNode<T, M> class provides the form binding related APIs with respect to a particular model instance.

Structurally, model instances form a tree, in which the object and array models have child nodes of field and array item model instances.

Every model instance has a one-to-one mapping to a corresponding BinderNode instance. The Binder itself is a BinderNode for the top-level form model.

Use the binderNode.for() method to obtain the binder node related with the model.

The binder nodes have the following properties:

model: M

The model instance mapped to this binder node.

value: T

The current value related to the model, two-way bound to the field components.

readonly defaultValue: T

The default value related to the model. Note: this is read-only here, use the top-level binder.defaultValue to change.

parent: BinderNode<any, AbstractModel<any>> | undefined

The parent node, if this binder node corresponds to a nested model, otherwise undefined for the top-level binder.

binder: Binder<any, AbstractModel<any>>

The binder for the top-level model.

readonly name: string

The name generated from the model structure, used to set the name attribute on the field components.

readonly required: boolean

True if the value is required to be non-empty. Based on presence of validators, that have impliesRequired: true flag.

dirty: boolean

True if the current value is different from the defaultValue.

visited: boolean

True if the bound field was ever focused and blurred by the user. The value is set by the field directive.

validators: ReadonlyArray<Validator<T>>

The array of validators for the model. The default value is defined in the model.

readonly ownErrors: ReadonlyArray<ValueError<T>>

The array of validation errors directly related with the model.

readonly errors: ReadonlyArray<ValueError<any>>

The combined array of all errors for this node’s model and all its nested models.

readonly invalid: boolean

True when the errors array is not empty.

The binder node has the following instance methods:

for<NM extends AbstractModel<any>>(model: NM): BinderNode<ModelType<NM>, NM>

Returns a binder node for the nested model instance.

async validate(): Promise<ReadonlyArray<ValueError<any>>>

Runs all validation callbacks potentially affecting this or any nested model. Returns the combined array of all errors as in the errors property.

addValidator(validator: Validator<T>): void

A helper method to add a validator to the validators.

appendItem(itemValue?: T): void

A helper method for array models. If the node’s model is an ArrayModel<T>, appends an item to the array, otherwise throws. If the argument is given, the argument value is used for the new item, otherwise an new empty item is created.

prependItem(itemValue?: T): void

A helper method for array modes, similar with appendItem(), but prepends an item to the array.

removeSelf(): void

A helper method for array item models. If the node’s model parent model is an ArrayModel<T>, removes the item the array, otherwise throws.