# JavaScript


You can use a .js file in the ECMAScript compliant JavaScript language to define the service logic of an HML page. 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';
  ```


## Objects

- Application Object
    | Attribute | Type | Description | 
  | -------- | -------- | -------- |
  | $def | Object | Object that is exposed in the app.js file and obtained by this.$app.$def.<br/>> ![icon-note.gif](public_sys-resources/icon-note.gif) **NOTE**:<br/>> 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).<br/>Do not use this attribute and private or public at the same time. |
  | $refs | Object | DOM elements or child component instances that have registered the ref attribute. For example code, see [Obtaining a DOM element](#obtaining-a-dom-element). |
  | private | Object | Data model of the page. Private data attribute can be modified only on the current page. |
  | public | Object | Data model of the page. Behaviors of public data attributes are the same as those of the data attribute. |
  | props | Array/Object | Used for communication between components. This attribute can be transferred to components via &lt;tag xxxx='value'&gt;. A props name must be in lowercase and cannot start with a dollar sign ($) or underscore (_). Do not use reserved words (for, if, show, and tid). Currently, props does not support functions. For details, see [Custom Component](../reference/arkui-js/js-components-custom-props.md). |
  | 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 Component](../reference/arkui-js/js-components-custom-props.md). |

## Functions

- Data functions
    | Function | Parameter | Description | 
  | -------- | -------- | -------- |
  | $set | key: string, value: any | Adds an attribute or modifies an existing attribute.<br/>Usage:<br/>this.$set('_key_',_value_): Add an attribute. | 
  | $delete | key: string | Deletes an attribute.<br/>Usage:<br/>this.$delete('_key_'): Delete an attribute. | 

  Example Code

  
  ```
  // index.js
  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
    | Function | Parameter | Description |
  | -------- | -------- | -------- |
  | $element | id: string | Obtains the component with a specified ID. If no ID is specified, the root component is returned. For example code, see [Obtaining a DOM element](#obtaining-a-dom-element).<br/>Usage:<br/>```<div id='_xxx_'></div>```<br/>- this.$element('_xxx_'): Obtain the component whose ID is _xxx_.<br/>- this.$element(): Obtain the root component. |
  | $rootElement | None | Obtains the root element.<br/>Usage: this.$rootElement().scrollTo({ duration: 500, position: 300 }), which scrolls the page by 300 px within 500 ms. |
  | $root | N/A | Obtains the root ViewModel instance. For example code, see [Obtaining the ViewModel](#obtaining-the-viewmodel). |
  | $parent | N/A | Obtains the parent ViewModel instance. For example code, see [Obtaining the ViewModel](#obtaining-the-viewmodel). |
  | $child | id: string | Obtains the ViewModel instance of a custom child component with a specified ID. For example code, see [Obtaining the ViewModel](#obtaining-the-viewmodel).<br/>Usage:<br/>this.$child('_xxx_'): Obtain the ViewModel instance of a custom child component whose ID is _xxx_. |

- Event function
    | Function | Parameter | Description |
  | -------- | -------- | -------- |
  | $watch | data: string, callback: string \| Function | Listens for attribute changes. If the value of the data attribute changes, the bound event is triggered. For details, see [Custom Component](../reference/arkui-js/js-components-custom-props.md)<br/>Usage:<br/>this.$watch('_key_',_callback_) |

- Page function
    | Function | Parameter | Description |
  | -------- | -------- | -------- |
  | scrollTo<sup>6+</sup> | scrollPageParam:  ScrollPageParam | Scrolls the page to the target position. You can specify the position using the ID selector or scrolling distance. |

    Table1 ScrollPageParam<sup>6+</sup>
  
  | Name | Type | Default Value | Description |
  | -------- | -------- | -------- | -------- |
  | position | number | - | Position to scroll to. |
  | id | string | - | ID of the element to be scrolled to. |
  | duration | number | 300 | Scrolling duration, in milliseconds. |
  | timingFunction | string | ease | Animation curve for scrolling. Available option:<br/>[Animation Styles](../reference/arkui-js/js-components-common-animation.md) |
  | complete | () => void | - | Callback to be invoked when the scrolling is complete. |

  Example:

  
  ```
  this.$rootElement.scrollTo({position: 0})
  this.$rootElement.scrollTo({id: 'id', duration: 200, timingFunction: 'ease-in', complete: ()=>void})
  ```


## Obtaining a DOM Element

1. Use $refs to obtain a DOM element.
  
   ```
   <!-- index.hml -->
   <div class="container">
     <image-animator class="image-player" ref="animator" images="{{images}}" duration="1s" onclick="handleClick"></image-animator>
   </div>
   ```

   
   ```
   // 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.hml -->
   <div class="container">
     <image-animator class="image-player" id="animator" images="{{images}}" duration="1s" onclick="handleClick"></image-animator>
   </div>
   ```

   
   ```
   // 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.hml -->
<element name='parentComp' src='../../common/component/parent/parent.hml'></element>
<div class="container">
  <div class="container">
    <text>{{text}}</text>
    <parentComp></parentComp>
  </div>
</div>
```


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

Customize the parent component.


```
<!-- parent.hml -->
<element name='childComp' src='../child/child.hml'></element>
<div class="item" onclick="textClicked">
  <text class="text-style" onclick="parentClicked">Click this parent component</text>
  <text class="text-style" if="{{show}}">Hello parent component!</text>
  <childComp id = "selfDefineChild"></childComp>
</div>
```


```
// 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.hml -->
<div class="item" onclick="textClicked">
  <text class="text-style" onclick="childClicked">Child component clicked</text>
  <text class="text-style" if="{{show}}">Hello child component</text>
</div>
```


```
// 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}');
  },
}
```