# Typescript simple observable state and web component

I've been experimenting with wiring up a vanilla web component in a reactive manner to a state box. The goal is that when something changes on the state object, the web component reacts. I realize that there are frameworks and libraries for this, but I want to understand fundamentally what goes on underneath. After two decades on server stuff, I want to branch out. I also wonder if we really need more dependencies if this is simple and straight-forward enough.

My approach is this:

• A base class that extends HTMLElement that can declaratively (via attribute in the markup) specify what data to watch.
• A state base class that is used to facilitate subscription to changes and notifications back to the subscribers.
• An attribute to decorate properties that you want to watch on those classes derived from the state base class.

First, the state base class:

class StateBase {
constructor() {
this._subs = new Map<string, Array<Function>>();
}

private _subs: Map<string, Array<Function>>;

subscribe(propertyName: string, eventHandler: any) {
if (!this._subs.has(propertyName))
this._subs.set(propertyName, new Array<Function>());
var callbacks = this._subs.get(propertyName);
callbacks.push(eventHandler);
}

notify(propertyName: string) {
var callbacks = this._subs.get(propertyName);
if (callbacks)
for (let i of callbacks) {
i();
}
}
}


The subscribe method is called by the web component base with the name of the property to monitor and a reference to its update mechanism. Of course, you could arbitrarily subscribe to any method on an instance. I'm using a Map as a dictionary of callback method arrays, one item for each property. notify is called by the decorator when a property value changes. Instances of this class have to be in the global (window) scope so the web components can find it.

Here's the attribute/decorator function:

const WatchProperty = (target: any, memberName: string) => {
let currentValue: any = target[memberName];
Object.defineProperty(target, memberName, {
set(this: any, newValue: any) {
console.log("watchProperty called on " + memberName + " with value " + newValue);
currentValue = newValue;
this.notify(memberName);
},
get() {return currentValue;}
});
};


The part I don't understand well, but learned about on SO, is Object.defineProperty taking the target, which is a prototype, to get to the actual instance of the state box. It works though. The important part is that it calls notify on the aforementioned base state class.

Here's the web component base:

abstract class ElementBase extends HTMLElement {
// Derived class constructor must call super("IDofTemplateHTML") first.
constructor(templateID: string) {
super();
var el = document.getElementById(templateID) as HTMLTemplateElement;
if (!el)
throw Error(No template found for ID '{templateID}'. Must pass the ID of the template in constructor to base class, like super('myID');)
const template = el.content;
}

connectedCallback() {
var attr = this.getAttribute('caller');
if (!attr)
throw Error("There is no 'caller' attribute on the component.");
var varAndProp = this.parseCallerString(attr);
var state = window[varAndProp[0]];
var delegate = this.update.bind(this);
state.subscribe(varAndProp[1], delegate);
}

update() {
console.log("update called start");
var attr = this.getAttribute('caller');
if (!attr)
throw Error("There is no 'caller' attribute on the component.");
var varAndProp = this.parseCallerString(attr);
var externalValue = window[varAndProp[0]][varAndProp[1]];
this.updateUI(externalValue);
console.log("update called end - " + externalValue);
}

private parseCallerString(caller: string): [string, string] {
var segments = caller.split(".");
if (segments.length != 2)
throw Error("caller attribute must follow 'globalVariable.property' format.");
return [segments[0], segments[1]];
}

// Use this.shadowRoot in the implementation to manipulate the DOM as needed in response to the new data.
abstract updateUI(data: any);
}


Importantly, your constructor has to take in the ID of the template. I imagine I could also include the template in the derived class, but it depends on how you like to organize and encapsulate things. I'd rather have the markup not in the code. The constructor sets up the shadowRoot from the template. connectedCallback takes the declarative caller attribute on the component instance and uses it to register the update method with the base state class. update again parses the caller attribute to find the state and its property value. Finally it calls the abstract updateUI method that the derived web component has to implement.

OK, so here are a concrete implementation examples deriving from the base classes:

class TestState extends StateBase {
constructor() {
super();
this.texty = "";
this.numbery = 0;
}

@WatchProperty
texty: string;

@WatchProperty
numbery: number;
}

class TestElement extends ElementBase {
constructor() {
super("pf-test");
}
updateUI(data) {
}
}


And here's a page where it all comes together:

<!DOCTYPE html>
<html>
<title>TypeScript Hello Web</title>
<body>
<script src="dist/test.js"></script> <!-- has all of the above stuff -->
<script>
var state = new TestState();

customElements.define('pf-test', TestElement);

state.subscribe("texty", () => {
console.log("Call me for texty!");
});
state.subscribe("numbery", () => {
console.log("anon call for numbery");
});
state.texty = "first change";
state.numbery = 42;
state.texty = "second change!";
state.numbery = 123;
});
</script>

<template id="pf-test">
<div>
<h1>original</h1>
</div>
</template>

<pf-test caller="state.texty">
</pf-test>
<pf-test caller="state.numbery">
</pf-test>
</body>

</html>


As you can guess, when the page is all done, there's a line that says "second change!" and another that says "123."

I'm not sure what specific feedback I seek, because I've spent so little time in TypeScript and front-end code in general. I'm interested in:

• Style problems.
• Syntax around member modifiers (public/private), appropriate use of var/let/const which I'm still learning.
• General portability and reuse.
• Potential gotchas or performance issues.
• Sensible error checking.
• Accusations that this entire approach sucks. :)

EDIT: You can follow along with my changes on the Github: https://github.com/jeffputz/ts-observable-web-component

It looks like you have learned a lot from it. I want to concentrate on your points you've listed on what you're looking for in terms of this review.

### Style

Probably the two biggest things that grabs my attention are, that you've decided to leave out parenthesis on most of the if-statements and that most of the time you're using the var keyword. So leaving out the curly braces is a potential gotcha, but I think you are aware of this - linters like ESLint or Prettier will help you with that, if desired. In general, this looks good to me (more about the var stuff in the syntax section, where you explicitly asked for it).

### Syntax

Mainly the var keyword has been used - it can be used but should be avoided. let and const are the better alternatives. As a quick recap, when using the var keyword, the interpreter moves all variables declared that way to the top of their scopes - regardless where they are placed. Each variable is mutable and can be redeclared (var), whereas variables using the let keyword are mutable but cannot be redeclared and const is not mutable at all once assigned:

var x = "foo"; // ✅
var x = "15"; // ✅

let y = "10";
let y = "15"; // ❌
y = "15"; // ✅

const z = "10";
z = "5"; // ❌


This means, avoid var, use const wherever possible unless you need to mutate a variable - then there is let!

A few things I'd to differently codewise:

Adding the event handler to the callback array (the first code block is taken from your posting, below how this code could be transformed).

// Using discouraged var keyword
var callbacks = this._subs.get(propertyName);
callbacks.push(eventHandler);

// Using const keyword
const callbacks = this._subs.get(propertyName);
callbacks.push(eventHandler);

// The variable can be safely inlined as its not used afterwards
this._subs.get(propertyName).push(eventHandler);


// Original code
var segments = caller.split(".");
if (segments.length != 2)
throw Error("caller attribute must follow 'globalVariable.property' format.");
return [segments[0], segments[1]];

const segments = caller.split(".");
if (segments.length != 2)
throw Error("caller attribute must follow 'globalVariable.property' format.");
return [...segments];


Note: I should be perfectly fine to just return segments here, there is no need to create a whole new array!

### Portability and reuse

As is, the portability seems to be just fine. Reusing this code is also perfectly doable. I'm wondering though, there are checks in the code, that the caller is of length 2 - this implies a limitation of the state. For example, using this as a global application state, where many different domains come together, it all has to be on the top-level. This does not feel right and might be a thing to be considered.

### Misc

Since this is TypeScript code, I'd like to mention, that there are a few any usages. Yes, it's tempting to use them, but try to be precise as possible, because most of the time it's the root of all evil. When there is no specific type I can come up with, I use unknown first. For example looking at the function signature of subscribe

subscribe(propertyName: string, eventHandler: any)


I am allowed to pass in anything I'd like. So let's assume I put in the number 5. Later in the code, the "callbacks" are retrieved and the assumption is, that all of them are a function.

notify(propertyName: string) {
var callbacks = this._subs.get(propertyName);
if (callbacks)
for (let i of callbacks) {
i();
}
}


During runtime, this will break and TypeScript didn't warn us about it. This can easily be fixed by adjusting the type to: eventHandler: () => void.

• Great stuff, thank you. I need to get used to how variables are declared instead of where. I don't like to chain a bunch of methods if I can avoid it, because it's harder to debug. My assumption is that if I declare it right, the interpreter will ditch the ref when it exits. Re: the array, I'm using a Typescript tuple there. Would the eventHandler type be Function? May 6 at 14:09
• Regarding the method chaining, just wanted to point it out - there is nothing wrong with your approach - there are valid pros and cons for each of them. Indeed you could also type the eventHandler as eventHandler: Function - this would then allow to pass stuff like: (someParams) => return "fooBar". Depends whether or not you want to allow that, but from what I understood, I would be as strict as possible. May 7 at 16:02