lit-analyzer

CLI that type checks bindings in lit-html templates.

About this Fork

This is a maintained fork of the original lit-analyzer by Rune Mehlsen. The original project has been inactive since 2023 and lacks ongoing maintenance. This fork aims to:

• Keep the tool compatible with modern Node.js versions
• Fix bugs and security vulnerabilities
• Maintain compatibility with the latest LitElement and lit-html versions
• Provide ongoing support for the TypeScript and VS Code ecosystems

About

lit-analyzer is a powerful CLI tool that provides static analysis for your Lit templates. It type-checks bindings, validates element usage, and helps catch errors before runtime. It supports both JavaScript and TypeScript projects and integrates well with various development workflows.

Installation

npm install @jarrodek/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 @jarrodek/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 the complete list of available rules. | {"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 |

Custom element validation rules

🤷‍ 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
  }
}

Binding name validation rules

✅ 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} />`;

Binding type validation rules

❓ 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}" />`

LitElement validation rules

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;
}

CSS validation rules

💅 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

This fork is maintained to continue the excellent work started by:

• Rune Mehlsen - Original creator
• Andreas Mehlsen - Original contributor
• Pawel Uchida-Psztyc - Contributor

Contributions to this maintained fork are welcome! Please see the contributing guidelines for more information.

License

Licensed under MIT.