Example of RunKit declaration file. These types may be out of date, so you probably want to copy the latest types from the docs directly.

index.js

// VS Code will show you the types here
window.RunKit.createNotebook({
    element: document.getElementById("runkit-embed"),
    source: "1 + 1"
})

RunKit.d.ts

// Put this file alongside your JS code and VS Code will automatically pick it up!
interface Window {
    RunKit: GlobalRunKit
}

type GlobalRunKit = {
    createNotebook: (options: EmbedOptions) => NotebookEmbed
}

type EmbedOptions = {
    element: HTMLElement // Parent element for the new notebook

    // Environment variables for the execution environment. Available under
    // `process.env`. Defaults to []
    environment?: Array<{ name: string, value: string }>
    evaluateOnLoad?: boolean
    gutterStyle?: "inside" | "none" | "outside" // Defaults to "outside"
    hidesActionButton?: boolean
    hidesEndpointLogs?: boolean
    hostURL?: string

    // Minimum height of the embed in pixels. E.g. "100px". Defaults to "73px"
    minHeight?: string

    // If "endpoint" the the notebook will be run as a server endpoint and a link to
    // it will be exposed in the embed. See runkit.com/docs/endpoint. Defaults to
    // "default"
    mode?: "endpoint" | "default"

    // A semver range that the node engine should satisfy, e.g. "4.0.x" or ">
    // 6.9.2". Defaults to "10.x.x"
    nodeVersion?: string
    source?: string // The source code of the notebook

    // The timestamp in UTC milliseconds to recreate the state of package
    // availability. No packages published to npm after this time are available in this
    // embed. Useful for reproducing bugs, or guaranteeing dependency versions. `null`
    // sets the time to when the embed is created
    packageTimestamp?: number | null

    // Code in the preamble field will not be displayed in the embed, but will be
    // executed before running the code in the embed. For example, libraries that use
    // RunKit for documentation often require their package in the preamble to avoid
    // clutter in the embed code
    preamble?: string
    readOnly?: boolean
    tabSize?: number // Minimum of 0. Defaults to 4
    theme?: string // Defaults to "runkit-light"
    title?: string // The title of the notebook when it is saved on RunKit.
}

type NotebookEmbed = {
    /// Methods
    destroy: () => void
    evaluate: () => void

    /// Events
    onEvaluate: () => void // Called when a cell is evaluated

    // Called when the embed has fully loaded. The function will be passed a
    // reference to the notebook.
    onLoad: (arg: NotebookEmbed) => void

    // Called when the embed cell is resized
    onResize: (arg: { height: number }) => void
    onSave: () => void // Called when the embed is saved

    // Called when the shareable URL or endpoint URL changes
    onURLChanged: (arg: { shareableURL: string, endpointURL: string }) => void

    /// Getters
    getEndpointURL: () => Promise<string>
    getEnvironment: () => Promise<Array<{ name: string, value: string }>>
    getEvaluateOnLoad: () => Promise<boolean>
    getGutterStyle: () => Promise<"inside" | "none" | "outside">
    getHidesActionButton: () => Promise<boolean>
    getHidesEndpointLogs: () => Promise<boolean>
    getMinHeight: () => Promise<string>
    getMode: () => Promise<"endpoint" | "default">
    getNodeVersion: () => Promise<string>
    getSource: () => Promise<string>
    getPackageTimestamp: () => Promise<number | null>
    getPreamble: () => Promise<string>
    getReadOnly: () => Promise<boolean>

    // A URL that references this exact notebook
    getShareableURL: () => Promise<string>

    // A path that you can use to require this code from other notebooks
    getRequirePath: () => Promise<string>
    getTabSize: () => Promise<number>
    getTheme: () => Promise<string>
    getTitle: () => Promise<string>

    // Setters
    setEnvironment: (environment: Array<{ name: string, value: string }>) => Promise<undefined>
    setGutterStyle: (gutterStyle: "inside" | "none" | "outside") => Promise<undefined>
    setHidesActionButton: (hidesActionButton: boolean) => Promise<undefined>
    setHidesEndpointLogs: (hidesEndpointLogs: boolean) => Promise<undefined>
    setMinHeight: (minHeight: string) => Promise<undefined>
    setMode: (mode: "endpoint" | "default") => Promise<undefined>
    setNodeVersion: (nodeVersion: string) => Promise<undefined>
    setSource: (source: string) => Promise<undefined>
    setPackageTimestamp: (packageTimestamp: number | null) => Promise<undefined>
    setPreamble: (preamble: string) => Promise<undefined>
    setReadOnly: (readOnly: boolean) => Promise<undefined>
    setTabSize: (tabSize: number) => Promise<undefined>
    setTheme: (theme: string) => Promise<undefined>
    setTitle: (title: string) => Promise<undefined>
}