Component JavaScript File

Every component must have a JavaScript file. If the component renders UI, the JavaScript file defines the HTML element. If the component is a service component (library), the JavaScript file exports functionality for other components to use.

JavaScript files in Lightning web components are ES6 modules. By default, everything declared in a module is local—it’s scoped to the module.

To import a class, function, or variable declared in a module, use the import statement. To allow other code to use a class, function, or variable declared in a module, use the export statement.

A component’s JavaScript file can have a maximum file size of 128 KB (131,072 bytes).

UI Component

The JavaScript file follows the naming convention <component>.js, such as myComponent.js.

Every UI component must include a JavaScript file with at least this code.

import { LightningElement } from 'lwc';
export default class MyComponent extends LightningElement {

}

The core module in Lightning Web Components is lwc. The import statement imports LightningElement from the lwc module.

import { LightningElement } from 'lwc';

LightningElement is a custom wrapper of the standard HTML element.

Extend LightningElement to create a JavaScript class for a Lightning web component. You can’t extend any other class to create a Lightning web component.

export default class MyComponent extends LightningElement {
    // Your code here
}

The export default keywords export a MyComponent class for other components to use. The class must be the default export for UI components. Exporting other variables or functions in a UI component isn’t currently supported.

The convention is for the class name to be Pascal Case, where the first letter of each word is capitalized. For myComponent.js, the class name is MyComponent.

The JavaScript file can contain:

• The component’s public API via public properties and methods annotated with @api.
• Fields
• Event handlers

UI component folders can include other JavaScript files in addition to the main myComponent.js file. However, the code in these files is for the component’s own use and can’t be shared.

Tip Although you can’t extend base Lightning components, you can look at the code for many of them in the Base Components Recipes GitHub repository. See how we developed the components and get ideas for creating your own.

Service Component (Library)

To share code between components, create an ES6 module and export the variables or functions that you want to expose.

An ES6 module is a file that explicitly exports functionality that other modules can use. Modules make it easier to structure your code without polluting the global scope.

See Share JavaScript Code.

See Also

• Fields, Properties, and Attributes

https://developer.salesforce.com/docs/component-library/documentation/en/lwc/lwc.create_components_javascript