# JavaScript - [Syntax](#en-us_topic_0000001058562835_s6ca2e99746664509961f65b82d11ab58) - [Object](#en-us_topic_0000001058562835_s7493791622a248fbb2e03703149bb3b5) - [Function](#en-us_topic_0000001058562835_s4e1ff24ec78e41948502d8977d24e44c) - [Obtaining a DOM element](#en-us_topic_0000001058562835_section1560185062215) - [Obtaining the ViewModel](#en-us_topic_0000001058562835_section13798143717421) - [Lifecycle Functions](#en-us_topic_0000001058562835_s962b82fb67304ec9a50fb06ffa977560) You can use a **.js** file to define the service logic of an HML page. The JS UI framework supports the JavaScript language that conforms to the ECMAScript specification. With dynamic typing, JavaScript can make your application more expressive with a flexible design. The following describes the JavaScript compilation and running. ## Syntax The ES6 syntax is supported. - **Module declaration** Import functionality modules. ``` import router from '@system.router'; ``` - **Code reference** Import JavaScript code. ``` import utils from '../../common/utils.js'; ``` ## Object - **Application object**

Attribute	Type	Description
$def	Object	Object that is exposed in the app.js file and obtained by this.$app.$def. NOTE: Application objects do not support data binding. Data update should be triggered on the UI.

Example Code ``` // app.js export default { onCreate() { console.info('AceApplication onCreate'); }, onDestroy() { console.info('AceApplication onDestroy'); }, globalData: { appData: 'appData', appVersion: '2.0', }, globalMethod () { console.info('This is a global method!'); this.globalData.appVersion = '3.0'; } }; ``` ``` // index.js export default { data: { appData: 'localData', appVersion:'1.0', }, onInit() { this.appData = this.$app.$def.globalData.appData; this.appVersion = this.$app.$def.globalData.appVersion; }, invokeGlobalMethod() { this.$app.$def.globalMethod(); }, getAppVersion() { this.appVersion = this.$app.$def.globalData.appVersion; } } ``` - **Page objects**

Attribute	Type	Description
data	Object/Function	Data model of the page. If the attribute is of the function type, the return value must be of the object type. The attribute name cannot start with a dollar sign ($) or underscore (_). Do not use reserved words (for, if, show, and tid). Do not use this attribute and private or public at the same time.
$refs	Object	DOM elements or child component instances that have registered the ref attribute. For example code, see Obtaining a DOM element.
private	Object	Data model of the page. Private data attribute can be modified only on the current page.
public	Object	Data model of the page. Behaviors of public data attributes are the same as those of the data attribute.
props	Array/Object	Used for communication between components. This attribute can be transferred to components via <tag xxxx='value'>. A props name must be in lowercase and cannot start with a dollar sign ($) or underscore (_). Do not use reserved words (for, if, show, and tid). Currently, props does not support functions. For details, see Custom Components.
computed	Object	Used for pre-processing an object for reading and setting. The result is cached. The name cannot start with a dollar sign ($) or underscore (_). Do not use reserved words. For details, see Custom Components.

## Function - **Data functions**

Attribute	Type	Parameter	Description
$set	Function	key: string value: any	Adds an attribute or modifies an existing attribute. Usage: this.$set('key',value): Add an attribute.
$delete	Function	key: string	Deletes an attribute. Usage: this.$delete('key'): Delete an attribute.

Attribute

Type

Parameter

Description

$set

Function

key: string

value: any

Adds an attribute or modifies an existing attribute.

Usage:

this.$set('key',value): Add an attribute.

$delete

Function

key: string

Deletes an attribute.

Usage:

this.$delete('key'): Delete an attribute.

Example Code ``` export default { data: { keyMap: { OS: 'OpenHarmony', Version: '2.0', }, }, getAppVersion() { this.$set('keyMap.Version', '3.0'); console.info("keyMap.Version = " + this.keyMap.Version); // keyMap.Version = 3.0 this.$delete('keyMap'); console.info("keyMap.Version = " + this.keyMap); // log print: keyMap.Version = undefined } } ``` - **Public functions**

Attribute	Type	Parameter	Description
$element	Function	id: string Component ID	Obtains the component with a specified ID. If no ID is specified, the root component is returned. For example code, see Obtaining a DOM element. Usage: <div id='xxx'></div> this.$element('xxx'): Obtain the component whose ID is xxx. this.$element(): Obtain the root component.
$root	Function	N/A	Obtains the root ViewModel instance. For example code, see Obtaining the ViewModel.
$parent	Function	N/A	Obtains the parent ViewModel instance. For example code, see Obtaining the ViewModel.
$child	Function	id: string Component ID	Obtains the ViewModel instance of a custom child component with a specified ID. For example code, see Obtaining the ViewModel. Usage: this.$child('xxx'): Obtain the ViewModel instance of a custom child component whose ID is xxx.

- **Event function**

Attribute	Type	Parameter	Description
$watch	Function	data: string callback: function name. The callback has two parameters. The first one is the new attribute value, and the second is the original value.	Listens for attribute changes. If the value of the data attribute changes, the bound event is triggered. For details, see Custom Components. Usage: this.$watch('key', callback)

Attribute

Type

Parameter

Description

$watch

Function

data: string

callback: function name. The callback has two parameters. The first one is the new attribute value, and the second is the original value.

Listens for attribute changes. If the value of the data attribute changes, the bound event is triggered. For details, see Custom Components.

Usage:

this.$watch('key', callback)

## Obtaining a DOM element 1. Use **$refs** to obtain a DOM element. ```

``` ``` // index.js export default { data: { images: [ { src: '/common/frame1.png' }, { src: '/common/frame2.png' }, { src: '/common/frame3.png' }, ], }, handleClick() { const animator = this.$refs.animator; // Obtain the DOM element whose $refs attribute is animator. const state = animator.getState(); if (state === 'paused') { animator.resume(); } else if (state === 'stopped') { animator.start(); } else { animator.pause(); } }, }; ``` 2. Call **$element** to obtain a DOM element. ```

``` ``` // index.js export default { data: { images: [ { src: '/common/frame1.png' }, { src: '/common/frame2.png' }, { src: '/common/frame3.png' }, ], }, handleClick() { const animator = this.$element('animator'); // Obtain the DOM element whose ID is animator. const state = animator.getState(); if (state === 'paused') { animator.resume(); } else if (state === 'stopped') { animator.start(); } else { animator.pause(); } }, }; ``` ## Obtaining the ViewModel The following shows files of the root page: ```

``` ``` // root.js export default { data: { text: 'I am a root!', }, } ``` Customize the parent component. ```

Click this parent component Hello parent component!

``` ``` // parent.js export default { data: { show: false, text: 'I am the parent component!', }, parentClicked () { this.show = !this.show; console.info('parent component get parent text'); console.info(`${this.$parent().text}`); console.info("The parent component gets the child function."); console.info(`${this.$child('selfDefineChild').childClicked()}`); }, } ``` Customize the child component. ```

Child component clicked Hello child component

``` ``` // child.js export default { data: { show: false, text: 'I am the child component!', }, childClicked () { this.show = !this.show; console.info('child component get parent text'); console.info('${this.$parent().text}'); console.info('child component get root text'); console.info('${this.$root().text}'); }, } ``` ## Lifecycle Functions - **Page lifecycle**

Attribute	Type	Parameter	Return Value	Description	Called When
onInit	Function	None	None	Listens for page initialization.	Page initialization is complete. This function is called only once in a lifecycle.
onReady	Function	None	None	Listens for page creation.	A page is created. This function is called only once in a lifecycle.
onShow	Function	None	None	Listens for page display.	The page is displayed.
onHide	Function	None	None	Listens for page disappearance.	The page disappears.
onDestroy	Function	None	None	Listens for page destruction.	The page is destroyed.
onBackPress	Function	None	Boolean	Listens for the back button action.	The back button is tapped. true means that the page processes the return logic. false means that the default return logic is used. If no value is returned, the default return logic is used.
onActive()⁵⁺	Function	None	None	Listens for age activation.	The page is activated.
onInactive()⁵⁺	Function	None	None	Listens for page suspension.	The page is suspended.
onNewRequest()⁵⁺	Function	None	None	Listens for a new FA request.	The FA has been started and a new request is received.

The lifecycle functions of page A are called in the following sequence: - Open page A: onInit -\> onReady -\> onShow -\> onActive - Open page B on page A: onInactive -\> onHide - Go back to page A from page B: onShow -\> onActive - Exit page A: onBackPress -\> onInactive -\> onHide -\> onDestroy - Hide page A: onInactive -\> onHide - Show background page A on the foreground: onShow - **Application lifecycle**

Attribute	Type	Parameter	Return Value	Description	Called When
onCreate	Function	None	None	Listens for application creation.	The application is created.
onDestroy	Function	None	None	Listens for application exit.	The application exits.