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KnockoutJS – Components ”; Previous Next Components are a huge way of organizing the UI code for structuring a large application and promoting code reusability. It is inherited or nested from other component. For loading and configuration, it defines its own conventions or logic. It is packaged to reuse throughout the application or the project. Represents the complete sections of application or small controls/widgets. It can be loaded or preloaded on demand. Component Registration Components can register using the ko.components.register() API. It helps to load and represent the components in KO. Component name with configuration is expected for registration. The configuration specifies how to determine the viewModel and template. Syntax Components can be registered as follows − ko.components.register(”component-name”, { viewModel: {…}, //function code template: {….) //function code }); The component-name can be any nonempty string. viewModel is optional, and can take any of the viewModel formats listed in the next sections. template is required, and can take any of the template formats listed in the next sections. Stating a ViewModel Following table lists the viewModel formats that can be used to register the components. Sr.No. viewModel Forms & Description 1 constructor function It creates a separate viewModel object for each component. The object or function is used to bind in components view. function SomeComponentViewModel(params) { this.someProperty = params.something; } ko.components.register(”component name”, { viewModel: SomeComponentViewModel, template: … }); 2 shared object instance The viewModel object instance is shared. The instance property is passed to use the object directly. var sharedViewModelInstance = { … }; ko.components.register(”component name”, { viewModel: { instance: sharedViewModelInstance }, template: … }); 3 createViewModel It calls a function which acts as a factory and can be used as view model that can return an object. ko.components.register(”component name”, { viewModel: { createViewModel: function (params, componentInfo) { … //function code … } }, template: …. }); 4 AMD module It is a module format for defining modules where module and dependencies both are loaded asynchronously. ko.components.register(”component name”, { viewModel: { require: ”some/module/name” }, template: … }); define([”knockout”], function(ko) { function MyViewModel() { // … } return MyViewModel; }); Stating a Template Following table lists the template formats that can be used to register the components. Sr.No. Template Forms 1 element ID ko.components.register(”component name”, { template: { element: ”component-template” }, viewModel: … }); 2 element instance var elemInstance = document.getElementById(”component-template”); ko.components.register(”component name”, { template: { element: elemInstance }, viewModel: … }); 3 string of markup ko.components.register(”component name”, { template: ”<input data-bind = “value: yourName” /> <button data-bind = “click: addEmp”>Add Emp </button>”, viewModel: … }); 4 DOM nodes var emp = [ document.getElementById(”node 1”), document.getElementById(”node 2”), ]; ko.components.register(”component name”, { template: emp, viewModel: … }); 5 document fragement ko.components.register(”component name”, { template: someDocumentFragmentInstance, viewModel: … }); 6 AMD module ko.components.register(”component name”, { template: { require: ”some/template” }, viewModel: … }); Components Registered as a Single AMD Module The AMD module can register a component by itself without using viewModel/template pair. ko.components.register(”component name”,{ require: ”some/module”}); Component Binding There are two ways of component binding. Full syntax − It passes the parameter and object to the component. It can pass using the following properties. name − It adds the component name. params − It can pass multiple parameters in object on the component. <div data-bind=”component: { name: “tutorials point”, params: { mode: “detailed-list”, items: productsList } }”> </div> Shorthand syntax − It passes the string as a component name and it does not include parameter in it. <div data-bind = ”component: “component name””></div> Template-only components − Components can only define template without specifing the viewModel. ko.components.register(”component name”, { template:”<input data-bind = “value: someName” />, }); Using Component without a container element − Components can be used without using extra container element. This can be done using containerless flow control which is similar as the comment tag. <!–ko.component: “”–> <!–/ko–> Custom Element Custom element is a way for rendering a component. Here, you can directly write a selfdescriptive markup element name instead of defining a placeholder, where the components are binded through it. <products-list params = “name: userName, type: userType”></products-list> Passing Parameter params attribute is used to pass the parameter to component viewModel. It is similar to data-bind attribute. The contents of the params attribute are interpreted like a JavaScript object literal (just like a data-bind attribute), so you can pass arbitrary values of any type. It can pass the parameter in following ways − Communication between parent and child components − The component is not instantiated by itself so the viewmodel properties are referred from outside of the component and thus would be received by child component viewmodel. For example, you can see in the following syntax that ModelValue is the parent viewmodel, which is received by child viewModel constructor ModelProperty. Passing observable expressions − It has three values in params parameter. simpleExpression − It is a numeric value. It does not involve any observables. simpleObservable − It is an instance that is defined on parent viewModel. The parent viewModel will automatically get the changes on observable done by child viewModel. observableExpression − Expression reads the observable when the expression is evaluated by itself. When the observable value changes, then the result of expression can also changs over time. We can pass the parameters as follows − <some-component params = ”simpleExpression: 1 + 1, simpleObservable: myObservable, observableExpression: myObservable() + 1”> </some-component> We can pass the parameters in viewModel as follows − <some-component params = ”objectValue:{a: 3, b: 2}, dateValue: new date(), stringValue: “Hi”, numericValue:123, boolValue: true/false, ModelProperty: ModelValue”> </some-component> Passing Markup into Components The received markup is used to create a component and is selected as a part of the output. Following nodes are passed as part of the output in the component template. template: { nodes: $componentTemplateNodes } Controlling custom element tag names The names which you register in the components using ko.components.register, the same name corresponds to the custom element tag names. We can change the custom element tag names by overriding it to control using getComponentNameForNode.