How do you define a KnockoutJS component?

1 What is a component?

A component is a reusable piece of UI that can have its own view model, template, and parameters. A component can be registered globally or locally, and can be used as a custom HTML element or a binding. A component can also communicate with its parent or child components using observables, custom events, or callbacks. A component can be defined either declaratively using HTML markup, or programmatically using JavaScript code.

2 How to define a component declaratively?

To define a component declaratively, you need to use the ko.components.register function and provide a name and a configuration object for your component. The configuration object can have the following properties: viewModel , template , params , synchronous , and createViewModel . The viewModel property specifies the constructor function or the instance of the view model for your component. The template property specifies the HTML markup for your component. The params property specifies the names and default values of the parameters that your component accepts. The synchronous property specifies whether your component should be loaded synchronously or asynchronously. The createViewModel property specifies a custom function that creates and returns the view model for your component.

3 How to define a component programmatically?

To define a component programmatically, you need to use the ko.components.register function and provide a name and a factory function for your component. The factory function can return either a plain object or a promise that resolves to a plain object. The plain object can have the same properties as the configuration object for declarative components, except that the viewModel property can also be a function that returns the view model for your component. The factory function can also accept a callback parameter that can be used to load the component asynchronously.

4 How to use a component in HTML?

To use a component in HTML, you can either use a custom HTML element or a component binding. A custom HTML element has the same name as the registered component, and can have attributes that correspond to the parameters of the component. For example, <my-component param1="value1" param2="value2"></my-component> . A component binding can be applied to any HTML element, and can have an object literal that specifies the name and the parameters of the component. For example, <div data-bind="component: {name: 'my-component', params: {param1: 'value1', param2: 'value2'}}"></div> .

5 How to use a component in JavaScript?

To use a component in JavaScript, you can either use the ko.components.get function or the ko.applyBindingsToNode function. The ko.components.get function takes the name of the registered component and a callback function, and returns the configuration object or the factory function for the component. The callback function can be used to access the view model and the template of the component. The ko.applyBindingsToNode function takes a DOM node and a binding object, and applies the bindings to the node. The binding object can have a component property that specifies the name and the parameters of the component.

6 How to communicate between components?

To communicate between components, you can either use observables, custom events, or callbacks. Observables are properties that can be subscribed to and updated by different components. For example, you can have a shared observable in the parent component that can be passed as a parameter to the child component, and then the child component can update or read the observable value. Custom events are events that can be triggered and listened to by different components. For example, you can have a custom event in the child component that can be triggered when something happens, and then the parent component can listen to the event and react accordingly. Callbacks are functions that can be passed as parameters to different components, and then the components can invoke the functions when needed. For example, you can have a callback function in the parent component that can be passed as a parameter to the child component, and then the child component can invoke the function to notify the parent component about something.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?