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 an API module (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 1 MB (1,000,000 bytes). If your JavaScript file size exceeds the size limit, refactor the code into multiple files. For more information, see Share JavaScript Code.

Note

UI Component

The JavaScript file follows the naming convention componentName.js, such as myComponent.js.

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

1import { LightningElement } from "lwc";
2export default class MyComponent extends LightningElement {}

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

1import { 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 Salesforce base component class to create a Lightning web component, unless otherwise stated.

1export default class MyComponent extends LightningElement {
2  // Your code here
3}

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.

API Module Component

A Lightning web component without a UI can be used as an API module component for sharing code. API module components are sometimes known as service components.

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.

An API module component follows the same folder structure and naming convention as the UI components. The difference is that there’s no HTML file. See Share JavaScript Code.

These export statements for the lwc module aren’t allowed.

1// These exports are invalid
2export {} from "lwc";
3export * from "lwc";
4export { registerTemplate } from "lwc";

LWC Import Declarations

An LWC module can use named imports and default imports. You can’t save an LWC module that contains namespace imports and re-export bindings. You can use only these named imports for the lwc module.

You can import multiple names from the lwc module.

1import { LightningElement, api, wire } from "lwc";

These import statements for the lwc module aren’t allowed.

1// These imports are invalid
2import lwc from "lwc";
3import \* as lwc from "lwc";
4import { registerTemplate } from "lwc";
5import "lwc";

In addition to importing from the lwc module, there are several ways to use named imports in a Lightning web component.

See Also