The BoundEmitter Pattern

TL;DR: emilis/bound-emitter.

I’ve been using EventEmitters quite much lately. Usually I mix them into modules that need to allow subscription to some internal events. However I am not content with the style of code they require.

Consider this small example:

some-storage.js

var emitter =   new EventEmitter;

exports.on =    emitter.on.bind( emitter );
exports.once =  emitter.once.bind( emitter );
exports.off =   emitter.off.bind( emitter );
...
exports.write = function write( id, value ){
   ...
   emitter.emit( "write", id, value );
   ...
};
...

some-logger.js

var storage = require( "./storage" );

storage.on( "write", console.log.bind( console, "storage wrote:" ));

I don’t like that the code in logger.js has a magical value ("write"). Typos could happen anywhere, event names can change in the future. As a consequence the logger would not receive notifications about the event. More than this: this error would happen silently and you would probably notice it only by it’s confusing effects.

Creating some object for holding the constant values:

some-storage.js

...
exports.EVENTS = { write: "write" };
...

…would not really solve the problem: you can still mistype it or change it. All that would happen is that instead of "wirte" or some other String, emitter.on() or emitter.emit() would get an undefined as it’s first parameter.

So I thought what about this:

some-storage.js

...
emitter.emit.write = emitter.emit.bind( emitter, "write" );
emitter.on.write =   emitter.on.bind( emitter, "write" );
...
exports.on =         emitter.on;
...
exports.write =      function write( id, value ){
    ...
    emitter.emit.write( id, value );
    ...
};
...

some-logger.js

...
storage.on.write( console.log.bind( console, "Storage wrote:" ));
...

This way, if any typo occurs, or the event name is changed we would get instant errors in the style of “TypeError: storage.on.write is not a function”. Easier to catch and debug.

An added bonus: piping events becomes trivial:

some-cache.js

...
storage.on.write( emitter.emit.clear_cache_now );
...

Now, I don’t like typing the ...bind( emitter, ... code all the time, so I wrote myself a small function. It takes an Array of event names:

storage.js

...
var emitter =   BoundEmitter([ "write", "remove" ]);

exports.on =    emitter.on;
exports.once =  emitter.once;
exports.off =   emitter.off;
...
exports.write = function write( id, value ){
    ...
    emitter.emit.write( id, value );
    ...
};
...

The BoundEmitter() returns an EventEmitter with the proper on.event / emit.event / ... functions created and bound to the context and the first argument.

You can examine the code for it at emilis/bound-emitter, or just install it and use via npm:

npm install bound-emitter

var BoundEmitter = require( "bound-emitter" );
...
var your_emitter = BoundEmitter([ "create", "update", "remove" ]);
...
your_emitter.emit.create( some_value );