lit-analyzer

CLI that type checks bindings in lit-html templates

➤ Installation

npm install lit-analyzer -g

Note:

• If you use Visual Studio Code you can also install the lit-plugin extension.
• If you use Typescript you can also install ts-lit-plugin.

➤ Usage

lit-analyzer analyzes an optional input glob and emits the output to the console as default. When the input glob is omitted it will analyze all components in src.

lit-analyzer src
lit-analyzer "src/**/*.{js,ts}"
lit-analyzer my-element.js
lit-analyzer --format markdown --outFile result.md 

➤ Configuration

You can configure the CLI with arguments:

lit-analyzer --strict --rules.no-unknown-tag-name off --format markdown

Note: You can also configure the CLI using a tsconfig.json file (see ts-lit-plugin).

Available arguments

Option	Description	Type	Default
--help	Print help message	boolean
--rules.rule-name	Enable or disable rules (example: --rules.no-unknown-tag-name off). Severity can be "off" | "warn" | "error". See a list of rules here.	{"rule-name": "off" | "warn" | "error"}
--strict	Enable strict mode. This changes the default ruleset	boolean
--format	Change the format of how diagnostics are reported	code | list | markdown	code
--maxWarnings	Fail only when the number of warnings is larger than this number	number	-1
--outFile	Emit all output to a single file	filePath
--quiet	Report only errors and not warnings	boolean
--failFast	Exit the process right after the first problem has been found	boolean
--debug	Enable CLI debug mode	boolean

➤ Rules

The default severity of each rule depend on the strict configuration option. Strict mode is disabled as default.

Each rule can have severity of off, warning or error. You can toggle rules as you like.

Validating custom elements

Rule	Description	Severity normal	Severity strict
no-unknown-tag-name	The existence of tag names are checked. Be aware that not all custom elements from libraries will be found out of the box.	off	warning
no-missing-import	When using custom elements in HTML it is checked if the element has been imported and is available in the current context.	off	warning
no-unclosed-tag	Unclosed tags, and invalid self closing tags like custom elements tags, are checked.	warning	error
no-missing-element-type-definition	This rule will ensure that custom elements are registered on the HTMLElementTagNameMap Typescript interface.	off	off

Validating binding names

Rule	Description	Severity normal	Severity strict
no-unknown-attribute no-unknown-property	You will get a warning whenever you use an unknown attribute or property within your lit-html template.	off	warning
no-unknown-event	When using event bindings it's checked that the event names are fired.	off	off
no-unknown-slot	Using the "@slot" jsdoc tag on your custom element class, you can tell which slots are accepted for a particular element.	off	warning
no-legacy-attribute	Disallows use of legacy Polymer binding syntax (e.g. foo$=).	off	warning

Validating binding types

Rule	Description	Severity normal	Severity strict
no-invalid-boolean-binding	Disallow boolean attribute bindings on non-boolean types.	error	error
no-expressionless-property-binding	Disallow property bindings without an expression.	error	error
no-noncallable-event-binding	Disallow event listener bindings with a noncallable type.	error	error
no-boolean-in-attribute-binding	Disallow attribute bindings with a boolean type.	error	error
no-complex-attribute-binding	Disallow attribute bindings with a complex type.	error	error
no-nullable-attribute-binding	Disallow attribute bindings with nullable types such as "null" or "undefined".	error	error
no-incompatible-type-binding	Disallow incompatible type in bindings.	error	error
no-invalid-directive-binding	Disallow using built-in directives in unsupported bindings.	error	error
no-unintended-mixed-binding	Disallow mixed value bindings where a character ', ", } or / is unintentionally included in the binding.	warning	warning

Validating LitElement

Rule	Description	Severity normal	Severity strict
no-incompatible-property-type	When using the @property decorator in Typescript, the property option type is checked against the declared property Typescript type	warn	error
no-invalid-attribute-name	When using the property option attribute, the value is checked to make sure it's a valid attribute name.	error	error
no-invalid-tag-name	When defining a custom element the tag name is checked to make sure it's valid.	error	error
no-property-visibility-mismatch	This rule will ensure public properties use @property and non-public properties use @internalProperty.	off	warn

Validating CSS

Rule	Description	Severity normal	Severity strict
💅 no-invalid-css	CSS within the tagged template literal css will be validated.	warning	error

Validating custom elements

All web components in your code are analyzed using web-component-analyzer which supports native custom elements and web components built with LitElement.

🤷‍ no-unknown-tag-name

Web components defined in libraries need to either extend the global HTMLElementTagNameMap (typescript definition file) or include the "@customElement tag-name" jsdoc on the custom element class.

Below you will see an example of what to add to your library typescript definition files if you want type checking support for a given html tag name.

declare global {
  interface HTMLElementTagNameMap {
    "my-element": MyElement;
  }
}

📣 no-missing-import

When using custom elements in HTML it is checked if the element has been imported and is available in the current context. It's considered imported if any imported module (or their imports) defines the custom element.

The following example is considered a warning:

// No import of "my-element"
html`<my-element></my-element>`

The following example is not considered a warning:

import "my-element.js";
html`<my-element></my-element>`

☯ no-unclosed-tag

Unclosed tags, and invalid self closing tags like custom elements tags, are checked.

The following examples are considered warnings:

html`<div>`
html`<video />`
html`<custom-element />`

The following examples are not considered warnings:

html`<div></div>`
html`<custom-element></custom-element>`
html`<video></video>`
html`<input />`

no-missing-element-type-definition

This rule is only applicable to Typescript files.

When sharing custom elements it's a good practice to add custom elements to the global interface HTMLElementTagNameMap. This rule will ensure that custom elements are registered on this interface.

The following example is considered a warning:

export class MyElement extends HTMLElement {

} 

customElements.define("my-element", MyElement)

The following example is not considered a warning:

export class MyElement extends HTMLElement {

} 

customElements.define("my-element", MyElement)

declare global {
  interface HTMLElementTagNameMap {
    "my-element": MyElement
  }
}

Validating binding names

Attributes, properties and events are picked up on custom elements using web-component-analyzer which supports native custom elements and web components built with LitElement.

✅ no-unknown-attribute, no-unknown-property

You will get a warning whenever you use an unknown attribute or property. This check is made on both custom elements and built in elements.

The following example is considered a warning:

html`<input .valuuue="${value}" unknownattribute="button" />`

The following example is not considered a warning:

html`<input .value="${value}" type="button" />`

⚡️ no-unknown-event

You can opt in to check for unknown event names. Using the @fires jsdoc or the statement this.dispatch(new CustomEvent("my-event)) will make the event name available. All event names are accepted globally because events bubble.

The following example is considered a warning:

html`<input @iinput="${console.log}" />`

The following example is not considered a warning:

html`<input @input="${console.log}" />`

📬 no-unknown-slot

Using the "@slot" jsdoc tag on your custom element class, you can tell which slots are accepted for a particular element. Then you will get warnings for invalid slot names and if you forget to add the slot attribute on elements without an unnamed slot.

/**
 * @slot - This is a comment for the unnamed slot
 * @slot right - Right content
 * @slot left
 */
class MyElement extends HTMLElement {
}
customElements.define("my-element", MyElement);

The following example is considered a warning:

html`
<my-element>
  <div slot="not a slot name"></div>
</my-element>
`

The following example is not considered a warning:

html`
<my-element>
  <div></div>
  <div slot="right"></div>
  <div slot="left"></div>
</my-element>
`

no-legacy-attribute

A common mistake when dealing with Lit in particular is to use the legacy Polymer syntax as seen in earlier versions of Polymer (the predecessor of Lit).

The following examples are considered warnings:

html`<input name$=${val} />`
html`<input disabled?=${val} />`;
html`<input name="val" />`;

The following examples are not considered warnings:

html`<input name=${val} />`
html`<input ?disabled=${val} />`;
html`<input name=${val} />`;

Validating binding types

Be aware that many checks involving analyzing bindings will work better in Typescript files because we have more information about the values being bound.

❓ no-invalid-boolean-binding

It never makes sense to use the boolean attribute binding on a non-boolean type.

The following example is considered a warning:

html`<input ?type="${"button"}" />`

The following example is not considered a warning:

html`<input ?disabled="${isDisabled}" />`

⚫️ no-expressionless-property-binding

Because of how lit-html parses bindings internally you cannot use the property binding without an expression.

The following example is considered a warning:

html`<input .value="text" />`

The following example is not considered a warning:

html`<input .value="${text}" />`

🌀 no-noncallable-event-binding

It's a common mistake to incorrectly call the function when setting up an event handler binding instead of passing a reference to the function. This makes the function call whenever the code evaluates.

The following examples are considered warnings:

html`<button @click="${myEventHandler()}">Click</button>`
html`<button @click="${{hannndleEvent: console.log()}}">Click</button>`

The following examples are not considered warnings:

html`<button @click="${myEventHandler}">Click</button>`
html`<button @click="${{handleEvent: console.log}}">Click</button>`

😈 no-boolean-in-attribute-binding

You should not be binding to a boolean type using an attribute binding because it could result in binding the string "true" or "false". Instead you should be using a boolean attribute binding.

This error is particular tricky, because the string "false" is truthy when evaluated in a conditional.

The following example is considered a warning:

html`<input disabled="${isDisabled}" />`

The following example is not considered a warning:

html`<input ?disabled="${isDisabled}" />`

☢️ no-complex-attribute-binding

Binding an object using an attribute binding would result in binding the string "[object Object]" to the attribute. In this cases it's probably better to use a property binding instead.

The following example is considered a warning:

html`<my-list listitems="${listItems}"></my-list>`

The following example is not considered a warning:

html`<my-list .listItems="${listItems}"></my-list>`

⭕️ no-nullable-attribute-binding

Binding undefined or null in an attribute binding will result in binding the string "undefined" or "null". Here you should probably wrap your expression in the "ifDefined" directive.

The following examples are considered warnings:

html`<input value="${maybeUndefined}" />`
html`<input value="${maybeNull}" />`

The following examples are not considered warnings:

html`<input value="${ifDefined(maybeUndefined)}" />`
html`<input value="${ifDefined(maybeNull === null ? undefined : maybeNull)}" />`

💔 no-incompatible-type-binding

Assignments in your HTML are typed checked just like it would be in Typescript.

The following examples are considered warnings:

html`<input type="wrongvalue" />`
html`<input placeholder />`
html`<input max="${"hello"}" />`
html`<my-list .listItems="${123}"></my-list>`

The following examples are not considered warnings:

html`<input type="button" />`
html`<input placeholder="a placeholder" />`
html`<input max="${123}" />`
html`<my-list .listItems="${listItems}"></my-list>`

💥 no-invalid-directive-binding

Directives are checked to make sure that the following rules are met:

• ifDefined is only used in an attribute binding.
• class is only used in an attribute binding on the 'class' attribute.
• style is only used in an attribute binding on the 'style' attribute.
• unsafeHTML, cache, repeat, asyncReplace and asyncAppend are only used within a text binding.

The directives already make these checks on runtime, so this will help you catch errors before runtime.

The following examples are considered warnings:

html`<input value="${unsafeHTML(html)}" />`
html`<input .value="${ifDefined(myValue)}" />`
html`<div role="${class(classMap)}"></div>`

The following examples are not considered warnings:

html`<button>${unsafeHTML(html)}</button>`
html`<input .value="${myValue}" />`
html`<input value="${myValue}" />`
html`<div class="${class(classMap)}"></div>`

🕷 no-unintended-mixed-binding

Sometimes unintended characters sneak into bindings. This often indicates a typo such as <input value=${"foo"}} /> where the expression is directly followed by a "}" which will be included in the value being bound, resulting in "foo}". Another example is self-closing tags without a space between the binding and "/" like <input value=${"foo"}/> which will result in binding the string "myvalue/".

This rule disallows mixed value bindings where a character ', ", } or / is unintentionally included in the binding.

The following examples are considered warnings:

html`<input .value=${"myvalue"}" />`
html`<input value=${"myvalue"}} />`
html`<input value=${"myvalue"}/>`
html`<input ?required=${true}/>`

The following examples are not considered warnings:

html`<input .value=${"myvalue"} />`
html`<input value="${"myvalue"}" />`
html`<input ?required=${true} />`
html`<input @input="${console.log}" />`

Validating LitElement

💞 no-incompatible-property-type

This rule checks that LitElement-controlled properties are correctly configured in accordance with the default value converter.

The following is a summary of what this rule does:

1. The type given to the LitElement property configuration is checked against the actual Typescript type of the property.
2. The default converter only accepts the types String, Boolean, Number, Array and Object, so all other values for type are considered warnings.
3. The absence of a type is only considered a warning if the property is not assignable to the string type.

This rule will not check for a given LitElement-controlled property if the property has custom converter configured.

The following examples are considered warnings:

class MyElement extends LitElement {
  @property({type: Number}) text: string;
  @property({type: Boolean}) count: number;
  @property({type: String}) disabled: boolean;
  @property({type: Object}) list: ListItem[];

  static get properties () {
    return {
      callback: {
        type: Function
      },
      text: {
        type: MyElement
      }
    }
  }
}

The following examples are not considered warnings:

class MyElement extends LitElement {
  @property({type: String}) text: string;
  @property({type: Number}) count: number;
  @property({type: Boolean}) disabled: boolean;
  @property({type: Array}) list: ListItem[];

  static get properties () {
    return {
      callback: {
        type: Function,
        converter: myCustomConverter
      },
      text: {
        type: String
      }
    }
  }

}

⁉️ no-invalid-attribute-name

When using the property option attribute, the value is checked to make sure it's a valid attribute name.

The following example is considered a warning:

class MyElement extends LitElement {
  static get properties () {
    return {
      text: {
        attribute: "invald=name"
      }
    }
  }
}

⁉️ no-invalid-tag-name

When defining a custom element, the tag name is checked to make sure it's a valid custom element name.

The following example is considered a warning:

@customElement("wrongElementName")
class MyElement extends LitElement {
}

customElements.define("alsoWrongName", MyElement);

The following example is not considered a warning:

@customElement("my-element")
class MyElement extends LitElement {
}

customElements.define("correct-element-name", MyElement);

no-property-visibility-mismatch

When using the @property decorator, your property should be publicly visible, expected to be exposed to consumers of the element. Private and protected properties however, should make use of the @internalProperty decorator instead.

This rule will ensure public properties use @property and non-public properties use @internalProperty.

The following example is considered a warning:

class MyElement extends LitElement {
	@property() private myProperty: string;
}

Validating CSS

lit-analyzer uses vscode-css-languageservice to validate CSS.

💅 no-invalid-css

CSS within the tagged template literal css will be validated.

The following example is considered a warning:

css`
  button
    background: red;
  }
`

The following example is not considered a warning:

css`
  button {
    background: red;
  }
`

➤ Documenting slots, events, attributes and properties

Code is analyzed using web-component-analyzer in order to find properties, attributes and events. Unfortunately, sometimes it's not possible to analyze these things by looking at the code, and you will have to document how your component looks using jsdoclike this:

/**
 * This is my element
 * @attr size
 * @attr {red|blue} color - The color of my element
 * @prop {String} value
 * @prop {Boolean} myProp - This is my property
 * @fires change
 * @fires my-event - This is my own event
 * @slot - This is a comment for the unnamed slot
 * @slot right - Right content
 * @slot left
 * @cssprop {Color} --border-color
 * @csspart header
 */
class MyElement extends HTMLElement { 
}

customElements.define("my-element", MyElement);

➤ Contributors

Rune Mehlsen	Andreas Mehlsen	You?

➤ License

Licensed under MIT.