event-target-shim

An implementation of WHATWG EventTarget interface, plus few extensions.

• This provides EventTarget constructor that can inherit for your custom object.
• This provides an utility that defines properties of attribute listeners (e.g. obj.onclick).

import {EventTarget, defineEventAttribute} from "event-target-shim"

class Foo extends EventTarget {
    // ...
}

// Define `foo.onhello` property.
defineEventAttribute(Foo.prototype, "hello")

// Use
const foo = new Foo()
foo.addEventListener("hello", e => console.log("hello", e))
foo.onhello = e => console.log("onhello:", e)
foo.dispatchEvent(new CustomEvent("hello"))

Installation

Use npm to install then use a bundler.

npm install event-target-shim

Or download from dist directory.

• dist/event-target-shim.mjs ... ES modules version.
• dist/event-target-shim.js ... Common JS version.
• dist/event-target-shim.umd.js ... UMD (Universal Module Definition) version. This is transpiled by Babel for IE 11.

Usage

import {EventTarget, defineEventAttribute} from "event-target-shim"
// or
const {EventTarget, defineEventAttribute} = require("event-target-shim")

// or UMD version defines a global variable:
const {EventTarget, defineEventAttribute} = window.EventTargetShim

EventTarget

https://dom.spec.whatwg.org/#interface-eventtarget

eventTarget.addEventListener(type, callback, options)

Register an event listener.

• type is a string. This is the event name to register.
• callback is a function. This is the event listener to register.
• options is a boolean or an object { capture?: boolean, passive?: boolean, once?: boolean }. If this is a boolean, it's same meaning as { capture: options }.
  • capture is the flag to register the event listener for capture phase.
  • passive is the flag to ignore event.preventDefault() method in the event listener.
  • once is the flag to remove the event listener automatically after the first call.

eventTarget.removeEventListener(type, callback, options)

Unregister an event listener.

• type is a string. This is the event name to unregister.
• callback is a function. This is the event listener to unregister.
• options is a boolean or an object { capture?: boolean }. If this is a boolean, it's same meaning as { capture: options }.
  • capture is the flag to register the event listener for capture phase.

eventTarget.dispatchEvent(event)

Dispatch an event.

• event is a Event object or an object { type: string, [key: string]: any }. The latter is non-standard but useful. In both cases, listeners receive the event as implementing Event interface.

defineEventAttribute(proto, type)

Define an event attribute (e.g. onclick) to proto. This is non-standard.

• proto is an object (assuming it's a prototype object). This function defines a getter/setter pair for the event attribute.
• type is a string. This is the event name to define.

For example:

class AbortSignal extends EventTarget {
    constructor() {
        this.aborted = false
    }
}
// Define `onabort` property.
defineEventAttribute(AbortSignal.prototype, "abort")

EventTarget(types)

Define a custom EventTarget class with event attributes. This is non-standard.

• types is a string or an array of strings. This is the event name to define.

For example:

// This has `onabort` property.
class AbortSignal extends EventTarget("abort") {
    constructor() {
        this.aborted = false
    }
}

Examples

ES2015 and later

https://jsfiddle.net/636vea92/

const {EventTarget, defineEventAttribute} = EventTargetShim

// Define a derived class.
class Foo extends EventTarget {
    // ...
}

// Define `foo.onhello` property.
defineEventAttribute(Foo.prototype, "hello")

// Register event listeners.
const foo = new Foo()
foo.addEventListener("hello", (e) => {
    console.log("hello", e)
})
foo.onhello = (e) => {
    console.log("onhello", e)
}

// Dispatching events
foo.dispatchEvent(new CustomEvent("hello", { detail: "detail" }))

Typescript

import { EventTarget, defineEventAttribute } from "event-target-shim";

// Define events
type FooEvents = {
    hello: CustomEvent
}
type FooEventAttributes = {
    onhello: CustomEvent
}

// Define a derived class.
class Foo extends EventTarget<FooEvents, FooEventAttributes> {
    // ...
}
// Define `foo.onhello` property's implementation.
defineEventAttribute(Foo.prototype, "hello")

// Register event listeners.
const foo = new Foo()
foo.addEventListener("hello", (e) => {
    console.log("hello", e.detail)
})
foo.onhello = (e) => {
    console.log("onhello", e.detail)
}

// Dispatching events
foo.dispatchEvent(new CustomEvent("hello", { detail: "detail" }))

Unfortunately, both FooEvents and FooEventAttributes are needed because TypeScript doesn't allow the mutation of string literal types. If TypeScript allowed us to compute "onhello" from "hello" in types, FooEventAttributes will be optional.

This EventTarget type is compatible with EventTarget interface of lib.dom.d.ts.

To disallow unknown events

By default, methods such as addEventListener accept unknown events. You can disallow unknown events by the third type parameter "strict".

type FooEvents = {
    hello: CustomEvent
}
class Foo extends EventTarget<FooEvents, {}, "strict"> {
    // ...
}

// OK because `hello` is defined in FooEvents.
foo.addEventListener("hello", (e) => {
})
// Error because `unknown` is not defined in FooEvents.
foo.addEventListener("unknown", (e) => {
})

However, if you use "strict" parameter, it loses compatibility with EventTarget interface of lib.dom.d.ts.

To infer the type of dispatchEvent() method

TypeScript cannot infer the event type of dispatchEvent() method properly from the argument in most cases. You can improve this behavior with the following steps:

1. Use the third type parameter "strict". This prevents inferring to dispatchEvent<string>().
2. Make the type property of event definitions stricter.

type FooEvents = {
    hello: CustomEvent & { type: "hello" }
    hey: Event & { type: "hey" }
}
class Foo extends EventTarget<FooEvents, {}, "strict"> {
    // ...
}

// Error because `detail` property is lacking.
foo.dispatchEvent({ type: "hello" })

ES5

https://jsfiddle.net/522zc9de/

// Define a derived class.
function Foo() {
    EventTarget.call(this)
}
Foo.prototype = Object.create(EventTarget.prototype, {
    constructor: { value: Foo, configurable: true, writable: true }
    // ...
})

// Define `foo.onhello` property.
defineEventAttribute(Foo.prototype, "hello")

// Register event listeners.
var foo = new Foo()
foo.addEventListener("hello", function(e) {
    console.log("hello", e)
})
foo.onhello = function(e) {
    console.log("onhello", e)
}

// Dispatching events
function isSupportEventConstrucor() { // IE does not support.
    try {
        new CusomEvent("hello")
        return true
    } catch (_err) {
        return false
    }
}
if (isSupportEventConstrucor()) {
    foo.dispatchEvent(new CustomEvent("hello", { detail: "detail" }))
} else {
    var e = document.createEvent("CustomEvent")
    e.initCustomEvent("hello", false, false, "detail")
    foo.dispatchEvent(e)
}

Changelog

• See GitHub releases.

Contributing

Contributing is welcome

Please use GitHub issues/PRs.

Development tools

• npm install installs dependencies for development.
• npm test runs tests and measures code coverage.
• npm run clean removes temporary files of tests.
• npm run coverage opens code coverage of the previous test with your default browser.
• npm run lint runs ESLint.
• npm run build generates dist codes.
• npm run watch runs tests on each file change.