hook-emitter

Promised-based chained event emitter with ability to create hooks around functions.

Installation

npm i hook-emitter --save

Examples

Async listener example:

import HookEmitter from 'hook-emitter';

const emitter = new HookEmitter();

emitter.on('sum', (x, y) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log('the sum of ' + x + ' + ' + y + ' = ' + (x + y));
            resolve();
        }, 100);
    });
});

// emit and wait for all listeners to be called
await emitter.emit('sum', 3, 7);

Hook example:

const emitter = new HookEmitter();

const hookedSum = emitter.hook('sum', (x, y) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            // x = 6, y = 14
            resolve(x + y);
        }, 100);
    });
});

emitter.on('sum', function (x, y) {
    console.log('doubling x and y');
    this.args[0] *= 2;
    this.args[1] *= 2;
});

emitter.on('sum', async (x, y, next) => {
	console.log('doubling result after sum function has been called');
	const r = await next();
	r.result *= 2;
	return r;
});

const result = await hookedSum(3, 7);
console.log('The sum of (6 + 14) * 2 = ' + result);

Chaining multiple hooked functions example:

const emitter = new HookEmitter();

await emitter.hook('step1', () => {
    console.log('step 1');
})();

await emitter.hook('step2', () => {
    console.log('step 2');
})();

await emitter.hook('step3', () => {
    console.log('step 3');
})();

API

Constructor

The HookEmitter constructor takes no arguments.

Properties

events

A Map object of event names to arrays of listener functions. This can be iterated over using a for-of loop.

Methods

on(event, listener)

on(event, priority=0, listener)

Adds an event listener. Returns this.

• event String - One or more space-separated event names to add the listener to.
• priority Number (optional) - Defaults to 0. The higher the priority, the sooner the listener is called. Value may be negative.
• listener Function - A function to call when the event is emitted.

once(event, listener)

once(event, priority=0, listener)

Adds an event listener that will only be called once. Returns this.

• event String - One or more space-separated event names to add the listener to.
• priority Number (optional) - Defaults to 0. The higher the priority, the sooner the listener is called. Value may be negative.
• listener Function - A function to call when the event is emitted.

off(event, listener)

Removes an event listener. Returns this.

• event String - One or more space-separated event names to remove the listener from.
• listener Function (optional) - The listener function. If not specified, then all listeners for the specified event are removed.

emit(event, ...args)

Emits one or more events. Returns a Promise.

• event String - The name of the event to emit.
• args * (optional) - One or more additional arguments to be emitted with the event.

hook(event, ctx, fn)

Creates a function hook. Returns a Function which when called returns a Promise.

• event String - The name of the hook's event.
• ctx Object (optional) - The context to run the function in. Useful if fn is going to be overwritten.
• fn Function - The function being hooked up.

Hook listeners are passed the same input arguments plus a next() callback. For example, if the hooked function accepts two arguments x and y, then the listeners will be called with x, y, and next. A listener only needs to call next() if it wishes be invoked after the hooked function has been called.

Listener functions are called in the context of the hook event meaning they can access:

• this.type String - The name of the event.
• this.args Array - The same arguments that the listener is invoked with. This is useful if you want to modify the arguments being passed to the hooked function.
• this.fn Function - The hooked function. You can use this to completely replace the hooked function.
• this.result * - The result from the hooked function. If the hooked function is async, then this will be undefined and the actual result will be returned by the promise chain.

emitter.on('foo', function (x, y, next) {
	console.log('event type:', this.type);
	console.log('args:', this.args);
	console.log('fn:', this.fn);

	// you can modify the args like this:
	this.args = [y, x];
});

link(emitter, prefix)

Links another HookEmitter to this instance. Useful if you have a class that extends a HookEmitter, then another HookEmitter that you want to receive the exact same events.

• emitter HookEmitter - The hook emitter to link to this instance.
• prefix String (optional) - An optional prefix to prepend to the event name being emitted from all linked emitters.

unlink(emitter)

Unlinks another HookEmitter from this instance. It does the opposite of link().

• emitter HookEmitter - The hook emitter to unlink.

License

MIT