TypeScript in Marko

Note: Types are supported in Marko v5.22.7+ and Marko v4.24.6+

Marko’s TypeScript support offers in-editor error checking, makes refactoring less scary, verifies that data matches expectations, and even helps with API design.

Or maybe you just want more autocomplete in VSCode. That works too.

Enabling TypeScript in your Marko project

There are two (non-exclusive) ways to add TypeScript to a Marko project:

For sites and web apps, you can place a tsconfig.json file at the project root:

📁 components/
📁 node_modules/
 index.marko
📦 package.json
 tsconfig.json

If you’re publishing packages of Marko tags, add the following to your marko.json:
```
"script-lang": "ts"
```
This will automatically expose type-checking and autocomplete for the published tags.

ProTip: You can also use the script-lang method for sites and apps.

Typing a tag's `input`

A .marko file will use any exported Input type for that file’s input object.

This can be export type Input or export interface Input.

Example

PriceField.marko

export interface Input {
  currency: string;
  amount: number;
}

<label>
  Price in ${input.currency}:
  <input type="number" value=input.amount min=0 step=0.01>
</label>

export interface Input {
  currency: string;
  amount: number;
}

label
  -- Price in ${input.currency}:
  input type="number" value=input.amount min=0 step=0.01

You can also import, reuse, and extend Input interfaces from other .marko or .ts files:

import { Input as PriceInput } from "<PriceField>";
import { ExtraTypes } from "lib/utils.ts";
export type Input = PriceInput & ExtraTypes;

import { Input as PriceInput } from "<PriceField>";
import { ExtraTypes } from "lib/utils.ts";
export type Input = PriceInput & ExtraTypes;

import { Input as PriceInput } from "<PriceField>";
export interface Input extends PriceInput {
  discounted: boolean;
  expiresAt: Date;
};

import { Input as PriceInput } from "<PriceField>";
export interface Input extends PriceInput {
  discounted: boolean;
  expiresAt: Date;
}

Generic `Input`s

Generic Types and Type Parameters on Input are recognized throughout the entire .marko template (excluding static statements).

For example, if you set up a component like this:

components/my-select.marko

export interface Input<T> {
  options: T[];
  onSelect: (newVal: T) => unknown;
}

static function staticFn() {
  // can NOT use `T` here
}

$ const instanceFn = (val: T) => {
  // can use `T` here
}

// can use `as T` here
<select on-input(evt => input.onSelect(options[evt.target.value] as T))>
  <for|value, i| of=input.options>
    <option value=i>${value}</option>
  </for>
</select>

export interface Input<T> {
  options: T[];
  onSelect: (newVal: T) => unknown;
}
static function staticFn() {
  // can NOT use `T` here
}
$ const instanceFn = (val: T) => {
  // can use `T` here
};
// can use `as T` here

select on-input((evt) => input.onSelect(options[evt.target.value] as T))
  for|value, i| of=input.options
    option value=i -- ${value}

…then your editor will figure out the types of inputs to that component:

<my-select options=[1,2,3] onSelect=val => {}/>
                                 // ^^^ number

<my-select options=["M","K","O"] onSelect=val => {}/>
                                       // ^^^ string

my-select options=[1, 2, 3] onSelect=((val) => {})
// ^^^ number


my-select options=["M", "K", "O"] onSelect=((val) => {})
// ^^^ string

Built-in Marko Types

Marko exposes type definitions you can reuse in a TypeScript namespace called Marko:

Marko.Template<Input, Return>
- The type of a .marko file
- typeof import("./template.marko")
Marko.TemplateInput<Input>
- The object accepted by the render methods of a template. It includes the template's Input as well as $global values.
Marko.Body<Params, Return>
- The type of the body content of a tag (renderBody)
Marko.Component<Input, State>
- The base class for a class component
Marko.Renderable
- Values accepted by the <${dynamic}/> tag
- string | Marko.Template | Marko.Body | { renderBody: Marko.Body}
Marko.Out
- The render context with methods like write, beginAsync, etc.
- ReturnType<template.render>
Marko.Global
- The type of the object in $global and out.global that can be passed to a template's render methods as the $global property.
Marko.RenderResult
- The result of rendering a Marko template
- ReturnType<template.renderSync>
- Awaited<ReturnType<template.render>>
Marko.Emitter
- EventEmitter from @types/node
Marko.NativeTags
- Marko.NativeTags: An object containing all native tags and their types
Marko.Input<TagName> and Marko.Return<TagName>
- Helpers to extract the input and return types native tags (when a string is passed) or a custom tag.
Marko.BodyParameters<Body> and Marko.BodyReturnType<Body>
- Helpers to extract the parameters and return types from the specified Marko.Body
Marko.AttrTag<T>
- Used to represent types for attributes tags
- A single attribute tag, with a [Symbol.iterator] to consume any repeated tags.

Typing `renderBody`

The most commonly used type from the Marko namespace is Marko.Body which can be used to type input.renderBody:

child.marko

export interface Input {
  renderBody?: Marko.Body;
}

export interface Input {
  renderBody?: Marko.Body;
}

Here, the following will be acceptable values:

index.marko

<child/>
<child>Text in render body</child>
<child>
  <div>Any combination of components</div>
</child>

child
child -- Text in render body
child
  div -- Any combination of components

Passing other values (including components) will cause a type error:

index.marko

import OtherTag from "<other-tag>";
<child renderBody=OtherTag/>

import OtherTag from "<other-tag>";
child renderBody=OtherTag

Typing Tag Parameters

Tag parameters are passed to the renderBody by the child tag. For this reason, Marko.Body also allows typing of its parameters:

for-by-two.marko

export interface Input {
  to: number;
  renderBody: Marko.Body<[number]>
}

<for|i| from=0 to=input.to by=2>
  <${input.renderBody}(i)/>
</for>

export interface Input {
  to: number;
  renderBody: Marko.Body<[number]>;
}

for|i| from=0 to=input.to by=2
  ${input.renderBody}(i)

index.marko

<for-by-two|i| to=10>
  <div>${i}</div>
</for-by-two>

for-by-two|i| to=10
  div -- ${i}

Extending native tag types within a Marko tag

The types for native tags are accessed via the global Marko.Input type. Here's an example of a component that extends the button html tag:

color-button.marko

export interface Input extends Marko.Input<"button"> {
  color: string;
  renderBody?: Marko.Body;
}

$ const { color, renderBody, ...restOfInput } = input;

<button style=`color: ${color}` ...restOfInput>
  <${renderBody}/>
</button>

export interface Input extends Marko.Input<"button"> {
  color: string;
  renderBody?: Marko.Body;
}
$ const { color, renderBody, ...restOfInput } = input;

button style=`color: ${color}` ...restOfInput
  ${renderBody}

Registering a new native tag (eg for custom elements).

interface MyCustomElementAttributes {
  // ...
}

declare global {
  namespace Marko {
    interface NativeTags {
      // By adding this entry, you can now use `my-custom-element` as a native html tag.
      "my-custom-element": MyCustomElementAttributes;
    }
  }
}

Registering new "global" HTML Attributes

declare global {
  namespace Marko {
    interface HTMLAttributes {
      "my-non-standard-attribute"?: string; // Adds this attribute as available on all HTML tags.
    }
  }
}

Registering CSS Properties (eg for custom properties)

declare global {
  namespace Marko {
    namespace CSS {
      interface Properties {
        "--foo"?: string; // adds a support for a custom `--foo` css property.
      }
    }
  }
}

TypeScript Syntax in `.marko`

Any JavaScript expression in Marko can also be written as a TypeScript expression.

Tag Type Parameters

<child <T>|value: T|>
  ...
</child>

child <T>|value: T| -- ...

Tag Type Arguments

components/child.marko

export interface Input<T> {
  value: T;
}

export interface Input<T> {
  value: T;
}

index.marko

// number would be inferred in this case, but we can be explicit
<child<number> value=1 />

// number would be inferred in this case, but we can be explicit

child<number> value=1

Method Shorthand Type Parameters

<child process<T>() { /* ... */ } />

child process<T>() {
  /* ... */
}

Attribute Type Assertions

The types of attribute values can usually be inferred. When needed, you can assert values to be more specific with TypeScript’s as keyword:

<some-component
  number=1 as const
  names=[] as string[]
/>

some-component number=1 as const names=([] as string[])

JSDoc Support

For existing projects that want to incrementally add type safety, adding full TypeScript support is a big leap. This is why Marko also includes full support for incremental typing via JSDoc.

Setup

You can enable type checking in an existing .marko file by adding a // @ts-check comment at the top:

// @ts-check

If you want to enable type checking for all Marko & JavaScript files in a JavaScript project, you can switch to using a jsconfig.json. You can skip checking some files by adding a // @ts-nocheck comment to files.

Once that has been enabled, you can start by typing the input with JSDoc. Here's an example component with typed input:

// @ts-check

/**
 * @typedef {{
 *   firstName: string,
 *   lastName: string,
 * }} Input
 */

<div>${firstName} ${lastName}</div>

// @ts-check

/**
 * @typedef {{
 *   firstName: string,
 *   lastName: string,
 * }} Input
 */

div -- ${firstName}${" "}${lastName}

With a separate `component.js` file

Many components in existing projects adhere to the following structure:

📁 components/
  📁 color-rotate-button/
     index.marko
     component.js

The color-rotate-button takes a list of colors and moves to the next one each time the button is clicked:

<color-rotate-button colors=["red", "blue", "yellow"]>
  Next Color
</color-rotate-button>

color-rotate-button colors=["red", "blue", "yellow"] -- Next Color

Here is an example of how this color-rotate-button component could be typed:

components/color-rotate-button/component.js

// @ts-check

/**
 * @typedef {{
 *   colors: string[],
 *   renderBody: Marko.Renderable
 * }} Input
 * @typedef {{
 *   colorIndex: number
 * }} State
 * @extends {Marko.Component<Input, State>}
 */
export default class extends Marko.Component {
  onCreate() {
    this.state = {
      colorIndex: 0,
    };
  }

  rotateColor() {
    this.state.colorIndex =
      (this.state.colorIndex + 1) % this.input.colors.length;
  }
}

components/color-rotate-button/index.marko

// @ts-check

/* Input will be automatically imported from `component.js`! */

<button
  onClick('rotateColor')
  style=`color: ${input.colors[state.colorIndex]}`>
  <${input.renderBody}/>
</button>

// @ts-check

/* Input will be automatically imported from `component.js`! */

button onClick("rotateColor") style=`color: ${input.colors[state.colorIndex]}`
  ${input.renderBody}