Lightning Web Components Slots

I'm trying to create a simple lightning web component that wraps a element, using an unnamed slot to populate the options. However, the options are not being rendered. If I inspect the generated DOM elements, it is just rendering slotslot as text inside the select element. If I move the slot outside of the select element, the slot. LWC Data Table is the powerful standard slds table provided by salesforce. By using 'lightning-datatable' tag we can display set of records in tabular format. If we use Data table in lightning web component we don't need code for table design and function for selecting records explicitly. Slots in Lightning Web Components Published by Avi Rai on March 30, 2020 Slots allow you to define placeholders in your template that can be filled with any markup fragment that a parent component passes into a component’s body. A component can have zero or more slots.

# Component Bundles

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To create a component, create a folder that bundles your component’s files. Strategic investment blackjack. A component folder (bundle) must live in a parent folder that defines the component's namespace. The name of the parent folder is the namespace.

A bundle can be a UI component, which renders UI, or a module, which is a library used to share code.

  • Lightning web components dispatch standard DOM events. Components can also create and dispatch custom events. Use events to communicate up the component containment hierarchy. For example, a child component, example-todo-item, dispatches an event to tell its parent, example-todo-app, that a user selected it.
  • The next post also ties together several points related to slot based composition in LWC, as well as how to write good unit tests for your Lightning Web Components. Modal Usage Included in this repository is the modal as well as an example modalwrapper component that hooks into the modal.


To understand the structure of a component and its namespace, look at the lwc-recipes-oss repo. Most components are in the recipe namespace. There are also components in the ui namespace and the data namespace. Most components are UI components. To see an example of a module, see mortgage.

# UI Components

The easiest way to create a UI component is to use the Lightning Web Components CLI, which generates boilerplate files.

A component that renders UI must have an HTML template file and a JavaScript class file. It may include a CSS file, additional JavaScript utility files, and test files. The HTML template, JavaScript class, and CSS file must have the same name. The framework uses the name to autowire the relationship between them.

The folder and its files must follow these naming rules.

  • Must begin with a lowercase letter
  • Must contain only alphanumeric or underscore characters
  • Must be unique in the namespace
  • Can’t include whitespace
  • Can’t end with an underscore
  • Can’t contain two consecutive underscores
  • Can’t contain a hyphen (dash)


Use camel case to name your component. Camel case component folder names map to kebab case in markup. Every component is part of a namespace. Use a hyphen character (-) to separate the namespace from the component name. In markup, to reference a component in the example namespace with the name myComponent, use <example-my-component>.

A bit more information about naming. Lightning web components match web standards wherever possible. The HTML standard requires that custom element names contain a hyphen. You can use underscores in component folder names, but they don’t map to kebab case in markup. The names are legal because the namespace is separated with a hyphen, but most developers expect hyphens instead of underscores in a web component name. For example, the component <example-my_component> is legal, but it looks odd and may confuse developers consuming your component.


The HTML spec mandates that tags for custom elements (components) aren’t self-closing. Self-closing tags end with />. Therefore, <example-my-component> includes a closing </example-my-component> tag.

# HTML File

Every UI component must have an HTML file with the root tag <template>.

The HTML file follows the naming convention <component>.html, such as myComponent.html.

Create the HTML for a Lightning web component declaratively, within the <template> tag. The HTML template element contains your component’s HTML.

When a component renders, the <template> tag is replaced with the namespace and name of the component. If this component were in the example namespace, the tag is <example-my-component>.

To create your template, use these techniques:

  • Create slots that component consumers can replace with content.

# JavaScript File

Every UI component must have a JavaScript file, which is a class that defines the HTML element.

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

This class must include at least this code:

The core module is lwc. The import statement imports LightningElement from the lwc module. LightningElement is a custom wrapper of the standard HTML element.

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.

Extend LightningElement to create a JavaScript class for a Lightning web component. The export default keywords export a MyComponent class 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.

The JavaScript class can contain:

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

In addition to this JavaScript file that creates the HTML element, a UI component’s folder can contain other JavaScript files that are used to share code. These utility JavaScript files must be ES6 modules and must have names that are unique within the component’s folder.

See Reactive Properties and Properties and Attributes.

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# CSS File

A component can include a CSS file. Use standard CSS syntax to style Lightning web components.

To style a component, create a style sheet in the component bundle with the same name as the component. If the component is called myComponent, the style sheet is myComponent.css. The style sheet is applied automatically.

See CSS.

# Test Files

To create Jest tests for a component, create a folder called __tests__ at the top level of the component’s folder. Save the tests inside the __tests__ folder.

Jest runs JavaScript files in the __tests__ directory. Test files must have names that end in .js, and we recommend that tests end in .test.js. You can have a single test file with all of your component tests, or you can have multiple files to organize related tests. Test files can be placed in sub folders.

See Unit Tests.

# Modules

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

The module folder and file name must follow the same naming conventions as a UI component.

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.

Within a JavaScript class, to reference a module, use namespace/module. The forward slash character (/) separates the namespace from the module name.

See Share JavaScript Code.

# JavaScript Decorators

Decorators add functionality to a property or function. The ability to create decorators is part of ECMAScript, but these decorators are unique to Lightning Web Components.


To expose a public property, decorate a field with @api. Public properties define the API for a component. An owner component that uses the component in its HTML markup can access the component’s public properties via HTML attributes.

Public properties are reactive. If the value of a property changes, the component’s template re-renders any content that references the property. For more information, see Public Properties.

To expose a public method, decorate it with @api. Public methods also define the API for a component. To communicate down the component hierarchy, owner and parent components can call JavaScript methods on child components.


All fields are reactive. When the framework observes a change to a field used in a template or used in the getter of a property used in a template, the component re-renders. However, there is a limit to the depth of changes that the framework observes.

If a field contains an object value or an array value, the framework observes changes that assign a new value to the field. If the value that you assign to the field is not to the previous value, the component re-renders.

Decorate a field with @track to tell the framework to observe changes to the properties of an object or to the elements of an array.

For code samples, see Field Reactivity.

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LWC has an elegant way to provide a stream of data to a component. Declare a component's data requirements in a wire adapter. Use the @wire decorator to connect or wire a component to the wire adapter.

Wire adapters are a statically analyzable expression of data. Wire adapters can also have dynamic configuration parameters. If a configuration value changes, new data is provisioned and the component rerenders.

See Wire Adapters.

# HTML Template Directives

Write templates using standard HTML and a few directives that are unique to Lightning Web Components. Directives are special HTML attributes that add dynamic behavior to an HTML template. Directives give you more power to manipulate the DOM in markup.

These are the directives supported in Lightning Web Components templates.


Use this directive to iterate over an array and render a list.

See Render Lists.


Use this directive to access the current item's zero-based index.

The index placeholder is a new identifier that the model injects into the current scope.

See Render Lists.

#if:true false={expression}

Use this directive to conditionally render DOM elements in a template.

The expression can be a JavaScript identifier (for example, person) or dot notation that accesses a property from an object (person.firstName). The engine doesn’t allow computed expressions (person[2].name['John']). To compute the value of expression, use a getter in the JavaScript class.

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See Render DOM Elements Conditionally.


Use this directive to apply special behavior to the first or last item in an array and render a list.

Access these properties on the iteratorName:

  • value—The value of the item in the list. Use this property to access the properties of the array. For example, iteratorName.value.propertyName.
  • index—The index of the item in the list.*first—A boolean value indicating whether this item is the first item in the list.
  • last—A boolean value indicating whether this item is the last item in the list.

See Render Lists.


Use this directive to improve rendering performance by assigning a unique identifier to each item in a list. The key must be a string or a number, it can’t be an object. The engine uses the keys to determine which items have changed.

See Render Lists.


Add this directive to a native HTML element to call appendChild() on the element from the owner’s JavaScript class and preserve styling.

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If a call to appendChild() manipulates the DOM, styling isn’t applied to the appended element. Using JavaScript to manipulate the DOM isn’t recommended when programming Lightning web components, but some third-party JavaScript libraries require it.

See Third-Party JavaScript Libraries.