# 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 ECMAScript 6.0 syntax is supported. Lite wearables only support the following ECMAScript 6.0 syntax:

1. let/const

2. arrow functions

3. class

4. default value

5. destructuring assignment

6. destructuring binding pattern

7. enhanced object initializer

8. for-of

9. rest parameter

10. template strings

- Module declaration
  Import functionality modules.

  
  ```
  import router from '@system.router';
  ```

- Code reference
  Import JavaScript code.

  
  ```
  import utils from '../../common/utils.js';
  ```


## Objects

- 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 name cannot start with a dollar sign ($) or underscore (_). Do not use reserved words (**for**, **if**, **show**, and **tid**). |
  | $refs | Object | DOM elements or child component instances that have registered the **ref** attribute. For an example, see [Obtaining a DOM Element](#obtaining-a-dom-element).|


## Obtaining a DOM Element

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();
    }
  },
};
```


## Lifecycle APIs

- Page lifecycle APIs
    | Name | Type| Parameter| Return Value| Description| Triggered When|
  | -------- | -------- | -------- | -------- | -------- | -------- |
  | onInit | Function | N/A| N/A| Listens for page initialization.| Page initialization is complete. This API is called only once in the page lifecycle.|
  | onReady | Function | N/A| N/A| Listens for page creation.| A page is created. This API is called only once in the page lifecycle.|
  | onShow | Function | N/A| N/A| Listens for page display. | The page is displayed.|
  | onHide | Function | N/A| N/A| Listens for page disappearance.| The page disappears.|
  | onDestroy | Function | N/A| N/A| Listens for page destruction.| The page is destroyed.|
  
    The lifecycle APIs of page A are called in the following sequence:
  - Open page A: onInit() -> onReady() -> onShow()
  
  - Open page B on page A: onHide() -> onDestroy()
  
  - Go back to page A from page B: onInit() -> onReady() -> onShow()
  
  - Exit page A: onHide() -> onDestroy()
  
  - Hide page A: onHide()
  
  - Show background page A on the foreground: onShow()

- Application lifecycle APIs
    | Name | Type| Parameter| Return Value| Description| Triggered When|
  | -------- | -------- | -------- | -------- | -------- | -------- |
  | onCreate | Function | N/A| N/A| Listens for application creation.| The application is created.|
  | onDestroy | Function | N/A| N/A| Listens for application exit.| The application exits.|