3.0.4 • Public • Published

A powerful, simple, promise-based postMessage iFrame communication library.

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Postmate is a promise-based API built on postMessage. It allows a parent page to speak with a child iFrame across origins with minimal effort.

You can download the compiled javascript directly here


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  • Promise-based API for elegant and simple communication.
  • Secure two-way parent <-> child handshake, with message validation.
  • Child exposes a retrievable model object that the parent can access.
  • Child emits events that the parent can listen to.
  • Parent can call functions within a child
  • Zero dependencies. Provide your own polyfill or abstraction for the Promise API if needed.
  • Lightweight, weighing in at ~ 1.6kb (minified & gzipped).

NOTE: While the underlying mechanism is window.postMessage(), only iFrame is supported.


Postmate can be installed via NPM.


$ yarn add postmate # Install via Yarn
$ npm i postmate --save # Install via NPM


  • Parent: The top level page that will embed an iFrame, creating a Child.
  • Child: The bottom level page loaded within the iFrame.
  • Model: The object that the Child exposes to the Parent.
  • Handshake: The process by which the parent frame identifies itself to the child, and vice versa. When a handshake is complete, the two contexts have bound their event listeners and identified one another.


  1. The Parent begins communication with the Child. A handshake is sent, the Child responds with a handshake reply, finishing Parent/Child initialization. The two are bound and ready to communicate securely.

  2. The Parent fetches values from the Child by property name. The Child can emit messages to the parent. The Parent can call functions in the Child Model.



// Kick off the handshake with the iFrame
const handshake = new Postmate({
  container: document.getElementById('some-div'), // Element to inject frame into
  url: 'http://child.com/page.html' // Page to load, must have postmate.js. This will also be the origin used for communication.
  classListArray: ["myClass"] //Classes to add to the iframe via classList, useful for styling.

// When parent <-> child handshake is complete, data may be requested from the child
handshake.then(child => {

  // Fetch the height property in child.html and set it to the iFrames height
    .then(height => child.frame.style.height = `${height}px`);

  // Listen to a particular event from the child
  child.on('some-event', data => console.log(data)); // Logs "Hello, World!"


const handshake = new Postmate.Model({
  // Expose your model to the Parent. Property values may be functions, promises, or regular values
  height: () => document.height || document.body.offsetHeight

// When parent <-> child handshake is complete, events may be emitted to the parent
handshake.then(parent => {
  parent.emit('some-event', 'Hello, World!');



// parent.com or child.com
Postmate.debug = true;
new Postmate(options);
Name Type Description Default
debug Boolean Set to true to enable logging of additional information false


// parent.com or child.com
Postmate.Promise = RSVP.Promise;
new Postmate(options);
Name Type Description Default
Promise Object Replace the Promise API that Postmate uses window.Promise


// parent.com
new Postmate({
  container: document.body,
  url: 'http://child.com/',
  classListArray: ["myClass"]
  model: { foo: 'bar' }

This is written in the parent page. Creates an iFrame at the specified url. Initiates a connection with the child. Returns a Promise that signals when the handshake is complete and communication is ready to begin.

Returns: Promise(child)


Name Type Description Default
container (optional) DOM Node Element An element to append the iFrame to document.body
url String _A URL to load in the iFrame. The origin of this URL will also be used for securing message transport none_
classListArray Array _An Array to add classes to the iFrame. Useful for styling none_
model Object An object literal to represent the default values of the Childs model none


// child.com
new Postmate.Model({
  // Serializable values
  foo: "bar",
  // Functions
  height: () => document.height || document.body.offsetHeight,
  // Promises
  data: fetch(new Request('data.json'))

This is written in the child page. Calling Postmate.Model initiates a handshake request listener from the Child. Once the handshake is complete, an event listener is bound to receive requests from the Parent. The Child model is extended from the model provided by the Parent.

Returns: Promise(handshakeMeta)


Name Type Description Default
model Object An object of gettable properties to expose to the parent. Value types may be anything accepted in postMessage. Promises may also be set as values or returned from functions. {}


// parent.com
new Postmate({
  container: document.body,
  url: 'http://child.com/'
}).then(child => {
  child.get('something').then(value => console.log(value));

Retrieves a value by property name from the Childs model object.

Returns: Promise(value)


Name Type Description
key String (required) The string property to lookup in the childs model

child.call(key, data)

// parent.com
new Postmate({
  container: document.body,
  url: 'http://child.com/'
}).then(child => {
  child.call('sayHi', 'Hello, World!');

Calls the function sayHi in the Child Model with the parameter Hello, World!

Returns: undefined


Name Type Description
key String (required) The string property to lookup in the childs model
data Mixed The optional data to send to the child function


// parent.com
new Postmate({
  container: document.body,
  url: 'http://child.com/'
}).then(child => child.destroy());

Removes the iFrame element and destroys any message event listeners

Returns: undefined


new Postmate(options).then(child => {
    .then(height => child.frame.style.height = `${height}px`);

The iFrame Element that the parent is communicating with



Why use Promises for an evented API?

Promises provide a clear API for fetching data. Using an evented approach often starts backwards. if the parent wants to know the childs height, the child would need to alert the parent, whereas with Postmate, the Parent will request that information from the child in a synchronous-like manner. The child can emit events to the parent as well, for those other use-cases that still need to be handled.

Silent Parent/Child

I've enabled logging but the parent or child is not logging everything.

Postmate.debug needs to be set in both the parent and child for each of them to log their respective information

The child does not respond to communication from the Parent

Make sure that you have initialized Postmate.Model in your child page.

Restrictive Communication

I want to retrieve information from the parent by the child

Postmate (by design) is restrictive in its modes of communication. This enforces a simplistic approach: The parent is responsible for logic contained within the parent, and the child is responsible for logic contained within the child. If you need to retrieve information from parent -> child, consider setting a default model in the parent that the child may extend.

I want to send messages to the child from the parent

This is specifically what the call function is for.


What is the Handshake and why do I need one?

By default, all message events received by any (parent) page can come from any (child) location. This means that the Parent must always enforce security within its message event, ensuring that the child (origin) is who we expect them to be, that the message is a response from an original request, and that our message is valid. The handshake routine solves this by saving the identities of the child and parent and ensuring that no changes are made to either.

How are messages validated?

The origin of the request, the message type, the postMessage mime-type, and in some cases the message response, are all verified against the original data made when the handshake was completed.



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