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Penpal

Penpal is a promise-based library for securely communicating with iframes via postMessage. The parent window can call methods exposed by iframes, pass arguments, and receive a return value. Similarly, iframes can call methods exposed by the parent window, pass arguments, and receive a return value. Easy peasy.

This library has no dependencies.

Installation

Using npm

Preferably, you'll be able to use Penpal from npm with a bundler like Webpack, Rollup, or Parcel. If you use npm for client package management, you can install Penpal with:

npm install penpal

Using a CDN

If you don't want to use npm to manage client packages, Penpal also provides a UMD distribution in a dist folder which is hosted on a CDN:

<script src="https://unpkg.com/penpal@^6/dist/penpal.min.js"></script>

Penpal will then be installed on window.Penpal. window.Penpal will contain the following properties:

Penpal.connectToChild
Penpal.connectToParent
Penpal.ErrorCode.ConnectionDestroyed
Penpal.ErrorCode.ConnectionTimeout
Penpal.ErrorCode.NoIframeSrc

Usage is similar to if you were using a bundler, which is documented below, but instead of importing each module, you would access it on the Penpal global instead.

Usage

Parent Window

import { connectToChild } from 'penpal';

const iframe = document.createElement('iframe');
iframe.src = 'http://example.com/iframe.html';

// This conditional is not Penpal-specific. It's merely
// an example of how you can add an iframe to the document.
if (
  document.readyState === 'complete' ||
  document.readyState === 'interactive'
) {
  document.body.appendChild(iframe);
} else {
  document.addEventListener('DOMContentLoaded', () => {
    document.body.appendChild(iframe);
  });
}

// This is where the magic begins.
const connection = connectToChild({
  // The iframe to which a connection should be made.
  iframe,
  // Methods the parent is exposing to the child.
  methods: {
    add(num1, num2) {
      return num1 + num2;
    },
  },
});

connection.promise.then((child) => {
  child.multiply(2, 6).then((total) => console.log(total));
  child.divide(12, 4).then((total) => console.log(total));
});

Child Iframe

import { connectToParent } from 'penpal';

const connection = connectToParent({
  // Methods child is exposing to parent.
  methods: {
    multiply(num1, num2) {
      return num1 * num2;
    },
    divide(num1, num2) {
      // Return a promise if the value being
      // returned requires asynchronous processing.
      return new Promise((resolve) => {
        setTimeout(() => {
          resolve(num1 / num2);
        }, 1000);
      });
    },
  },
});

connection.promise.then((parent) => {
  parent.add(3, 1).then((total) => console.log(total));
});

API

connectToChild(options: Object) => Object

For Penpal to operate correctly, you must ensure that connectToChild is called before the iframe calls connectToParent. As shown in the example above, it is safe to set the src or srcdoc property of the iframe and append the iframe to the document before calling connectToChild as long as they are both done in the same JavaScript event loop. Alternatively, you can always append the iframe to the document after calling connectToChild instead of before.

Options

options.iframe: HTMLIFrameElement (required)

The iframe element to which Penpal should connect. Unless you provide the childOrigin option, you will need to have set either the src or srcdoc property on the iframe prior to calling connectToChild so that Penpal can automatically derive the child origin. In addition to regular URLs, data URIs and file URIs are also supported.

options.methods: Object (optional)

An object containing methods which should be exposed for the child iframe to call. The keys of the object are the method names and the values are the functions. Nested objects with function values are recursively included. If a function requires asynchronous processing to determine its return value, make the function immediately return a promise and resolve the promise once the value has been determined.

options.childOrigin: string (optional)

In the vast majority of cases, Penpal can automatically determine the child origin based on the src or srcdoc property that you have set on the iframe. Communication will automatically be restricted to that origin.

In some rare cases, particularly when using the file:// protocol on various devices, browsers are inconsistent in how they report and handle origins. If you receive an error saying that the parent received a handshake from an unexpected origin, you may need to manually pass the child origin using this option.

In other niche scenarios, you may want the parent to be able to communicate with any child origin. In this case, you can set childOrigin to *. This is discouraged. To illustrate the risk, if a nefarious attacker manages to create a link within the child page that another user can click (for example, if you fail to inadequately escape HTML in a message board comment), and that link navigates the unsuspecting user's iframe to a nefarious URL, then the page at the nefarious URL could start communicating with your parent window.

Regardless of how you configure childOrigin, communication will always be restricted to only the iframe to which you are connecting.

options.timeout: number (optional)

The amount of time, in milliseconds, Penpal should wait for the child to respond before rejecting the connection promise. There is no timeout by default.

options.debug: boolean (optional)

Enables or disables debug logging. Debug logging is disabled by default.

Return value

The return value of connectToChild is a connection object with the following properties:

connection.promise: Promise

A promise which will be resolved once communication has been established. The promise will be resolved with an object containing the methods which the child has exposed. Note that these aren't actual memory references to the methods the child exposed, but instead proxy methods Penpal has created with the same names and signatures. When one of these methods is called, Penpal will immediately return a promise and then go to work sending a message to the child, calling the actual method within the child with the arguments you have passed, and then sending the return value back to the parent. The promise you received will then be resolved with the return value.

connection.destroy: Function

A method that, when called, will disconnect any messaging channels. You may call this even before a connection has been established.

connectToParent([options: Object]) => Object

Options

options.parentOrigin: string | RegExp (optional but highly recommended!)

The origin of the parent window which your iframe will be communicating with. If this is not provided, communication will not be restricted to any particular parent origin resulting in any webpage being able to load your webpage into an iframe and communicate with it.

options.methods: Object (optional)

An object containing methods which should be exposed for the parent window to call. The keys of the object are the method names and the values are the functions. Nested objects with function values are recursively included. If a function requires asynchronous processing to determine its return value, make the function immediately return a promise and resolve the promise once the value has been determined.

options.timeout: number (optional)

The amount of time, in milliseconds, Penpal should wait for the parent to respond before rejecting the connection promise. There is no timeout by default.

options.debug: boolean (optional)

Enables or disables debug logging. Debug logging is disabled by default.

Return value

The return value of connectToParent is a connection object with the following properties:

connection.promise: Promise

A promise which will be resolved once communication has been established. The promise will be resolved with an object containing the methods which the parent has exposed. Note that these aren't actual memory references to the methods the parent exposed, but instead proxy methods Penpal has created with the same names and signatures. When one of these methods is called, Penpal will immediately return a promise and then go to work sending a message to the parent, calling the actual method within the parent with the arguments you have passed, and then sending the return value back to the child. The promise you received will then be resolved with the return value.

connection.destroy: Function

A method that, when called, will disconnect any messaging channels. You may call this even before a connection has been established.

Reconnection

If the child iframe attempts to reconnect with the parent, the parent will accept the new connection. This could happen, for example, if a user refreshes the child iframe or navigates within the iframe to a different page that also uses Penpal. In this case, the child object the parent received when the initial connection was established will be updated with the new methods provided by the child iframe.

NOTE: Currently there is no API to notify consumers of a reconnection. If this is important for you, please provide feedback on this issue with how you would like to see the API designed.

Errors

Penpal will throw (or reject promises with) errors in certain situations. Each error will have a code property which may be used for programmatic decisioning (e.g., do something if the error was due to a connection timing out) along with a message describing the problem. Errors may be thrown with the following codes:

ConnectionDestroyed

This error will be thrown when attempting to call a method on child or parent objects and the connection was previously destroyed.

ConnectionTimeout

The promise found at connection.promise will be rejected with this error after the timeout duration has elapsed and a connection has not been established.

NoIframeSrc

This error will be thrown when the iframe passed into connectToChild does not have src or srcdoc set.

For your convenience, these error codes can be imported as follows:

import { ErrorCode } from 'penpal';
// ErrorCode.ConnectionDestroyed
// ErrorCode.ConnectionTimeout
// ErrorCode.NoIframeSrc

TypeScript

When calling connectToChild or connectToParent, you may pass a generic type argument. This will be used to type the child or parent object that connection.promise is resolved with. This is better explained in code:

import { connectToChild } from 'penpal';

// This interace could be imported from a code library
// that both the parent and child share.
interface ChildApi {
  multiply(...args: number[]): number;
}

// Supply the interface as a generic argument.
const connection = connectToChild<ChildApi>({
  iframe: new HTMLIFrameElement(),
});

// The resulting child object will contain properly
// typed methods.
const child = await connection.promise;
// The result variable is typed as a number.
const result = await child.multiply(1, 3);

The following TypeScript types are also exported as named constants for your use:

• Connection
• Methods
• AsyncMethodReturns
• CallSender
• PenpalError

React

If you're using Penpal within a React app, please check out react-penpal.

Supported Browsers

Penpal is designed to run successfully on the most recent versions of Chrome, Firefox, Safari, and Edge. If you need to support Internet Explorer 11, feel free to use version 3.x of Penpal. See the 3.x README for documentation.

Penpal has also been reported to work within Ionic projects on iOS and Android devices.

Inspiration

This library is inspired by:

• Postmate
• JSChannel

License

MIT