Messaging is a powerful development pattern that makes decoupling components and providing internal APIs incredibly easy. The primary goal of msngr is to provide a high quality, asynchronous method of consuming and emitting messages that work in both node and the web browser.

Install via npm by running npm install msngr

Grab a copy of the msngr.js or msngr.min.js files directly from this repo

Reference msngr directly from cdnjs

// I can call a function when I receive a specific message!
msngr("Output").on((words) => {
    console.log(words); // Outputs 'Hello, World!', 'I'm chainable!' and 'Weee'
    return 42;
});

// I can emit messages of any type and I can chain them together.
msngr("Output")
  .emit("Hello, World!")
  .emit("I'm chainable!")
  .emit("Weee");
  
// I can notify and even return values when a message is finished being processed
msngr("Output")
    .emit("Processing", (results) => {
        console.log(results); // Prints [42]
  });
  
// Even better, I can return a result asynchronously!
msngr("More")
    .on((input, message, async) => {
        const done = async();
      done(42);
  })
  .emit("Processing", (results) => {
        console.log(results); // Also prints [42]
  });

// I can emit and persist a message so future handlers can receive it
msngr("Important", "Message").persist("I am persisted!");

// See?
msngr("Important", "Message").on((text) => {
    console.log(text); // Outputs 'I am persisted!'
  
  // Then I can remove the persisted data once I'm done with it
    msngr("Important", "Message").cease();
});

// I can even guarantee a handler is called only once
msngr("Once").once((str) => {
    console.log(str); // Only outputs 'Just once'
});

msngr("Once").emit("Just once").emit("asdljadlka").emit("sadjkhada");

// Did I mention I'm completely asynchronous?
msngr("Async")
    .on((text) => {
        console.log(text);
  })
  .emit("This is async text");
  
console.log("This is sync text"); // This will print before everything in this example

Messages can be comprised of a topic, category and subcategory with topic being the only required piece all of which are specified in the main msngr function msngr(topic, category, subcategory). This allows for a large range of message consumption from the very general to the highly specific.

When consuming or emitting a message you can choose to match just the topic, the topic and category or the topic, category and subcategory. This allows for a great amount of specificity when choosing what your handler can take care of (is it generic in that it can handle anything? should it only handle specific categories? subcategories?). Take a look at this example:

// Handler that receives all profile save attempts
msngr("Profile", "Save").on((payload) => {
    console.log(payload);
});

// Handler that only received profile saves in json
msngr("Profile", "Save", "application/json").on((payload) => {
    console.log("Save the json payload");
});

// Handler that only received profile saves in xml
msngr("Profile", "Save", "application/xml").on((payload) => {
    console.log("Save the xml payload");
});

msngr("Profile", "Save", "application/json").emit({ name: "myname!" });
msngr("Profile", "Save", "application/xml").emit("<xml><name>myname!</name></xml>");

The example above demonstrates that the handler will only fire when the specified criteria of a registered on handler is satisfied.

A mechanism for middleware exists that creates a global or localized affect. This allows for payload transformation to occur prior to any messages and their payloads being delivered to handlers. A middleware needs to be registered via msngr.middleware(key, fn, force) where the key is the name of the middleware, the fn is the middleware itself and force is a boolean that specifies whether the middleware should execute on all messages or not (if false then msngr().use(key) needs to be called on a message to have it applied).

msngr.middleware("uppercase", (payload, message) => {
    if (msngr.is(payload).string) {
        return payload.toUpperCase();
    }
    return payload;
}, true);

msngr("SayIt").on((payload) => {
    console.log(payload); // Prints 'HELLO, WORLD!'
});

msngr("SayIt").emit("hello, world!");

Messages can also have a payload that is persistent. This is incredibly handy when doing initialization. You can specify a message when something is initialized and persist the message (even with an empty payload) so that handlers registered before or after said initialization are still called. This is done through msngr().persist() and can be ended through msngr().cease().

There is also a msngr().once() that works just like msngr().on() except it is guaranteed to execute only once. In a similar vein you can drop msngr().on() registered handles either one at a time through msngr().drop(fn) or all at once for a specific message via msngr().dropAll().

Validation is a very important part of dealing with a dynamic language such as JavaScript. The msngr.is(obj) function is an attempt to make it as intuitive and straight-forward as possible. Simply call msngr.is(obj) providing whatever you want to check as the parameter (you can also pass in multiple parameters to check them all in an AND fashion) then call one of the following properties on the returned object:

msngr.is(obj).arguments; // checks for arguments object
msngr.is(obj).boolean; // checks for boolean
msngr.is(obj).string; // check for string
msngr.is(obj).date; // checks for date
msngr.is(obj).array; // checks for array
msngr.is(obj).number; // checks for number
msngr.is(obj).object; // checks for object
msngr.is(obj).function; // checks for function
msngr.is(obj).symbol; // checks for symbol

msngr.is(obj).there; // checks to see if the obj exists
msngr.is(obj).empty; // checks to see if an object, array or string is empty (including null and undefined)

msngr.is(obj).htmlElement; // checks to see if it's an HTML Element
msngr.is(obj).nodeList; // checks to see if it's an HTML NodeList

So if you want to create an asynchronous process many developers turn to setTimeout(fn, 0); but that's actually rather slow to execute in web browsers (typically values less than 10ms or so are ignored and even 0ms isn't guaranteed to execute immediately). Using setImmediate(fn) is a far better choice but it's almost exclusively available in node and IE. This is where msngr.immediate(fn) comes in. There is a way to simulate setImmediate(fn) when it doesn't exist in a web browser so msngr.immediate(fn) will use whatever the fastest solution that exists on the platform it is running on to execute something asynchronously.

msngr.immediate(() => {
    console.log("Hey there!");
});

Handling asynchronous behaviors can be difficult and there are many libraries (async.js) and techniques (promises, async & await) for handling such issues. But sometimes you just need a simple way to take multiple methods and combine them. Msngr uses this capability internally for handling message delegation and processing of middleware so these two methods are the public version of the very same API.

const props = { };
msngr.parallel([
    () => { props.value1 = 42; },
    () => { props.value2 = "forty two"; },
    () => { props.value3 = true; },
], () => {
    console.log(props.value1); // Prints 42
    console.log(props.value2); // Prints 'forty two'
    console.log(props.value3); // Prints true
});

msngr.series([
    () => { return "s" },
    () => { return "e" },
    (async) => { async()("r") },
    (async) => { async()("i") },
    (async) => { async()("e") },
    () => { return "s" }
], (results) => {
    console.log(results.join("")); // Prints "series"
});

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