Concepts behind modern frameworks

Many beginners ask "which framework should I learn?" and "How much JS or TS do I need to learn before a framework?" - countless opinionated articles go into promoting the advantages of the author's preferred framework or library, instead of demonstrating the readers the concepts behind them to allow for an informed decision. So let us get the second question out of the way first:

"How much JS/TS to learn before a framework?"

As much as allows you to understand the concepts they are based on. You will need knowledge of the basic data types, functions, basic operators and of the document object model (DOM), which is the representation of your HTML and CSS inside your JS. While everything beyond that will not hurt, it is not strictly required to become proficient with a framework or library.

If you are a complete beginner, JS for cats might be a good resource for your first steps. Keep going until you feel confident, then continue until you stop feeling confident again. That's when you know enough JS/TS and can move on to a framework. The rest you can learn on the go.

"Which concepts do you mean?"

• State
• Effects
• Memoization
• Templating and rendering

All modern frameworks derive their functionality from these concepts.

State

State is just the data powering your application. It may be on a global level, for a larger part of the application, or for a single component. Let us take a simple counter as an example. The count it keeps is the state. We can read the state and write to it to increase the count.

The simplest representation is usually a variable containing the data that our state consists of:

let count = 0;const increment = () => { count++; };const button = document.createElement('button');button.textContent = count;button.addEventListener('click', increment);document.body.appendChild(button);

But this code has a problem: changes to count, like those made by increment, are not updating the text content of the button. We could manually update everything, but that doesn't scale well for more complex use cases.

The ability for count to update its users is called reactivity. This works by subscribing and re-running the subscribed parts of your application to updates.

Almost every modern front-end framework and library has a way to manage state reactively. There are three parts to the solution, and at least one of them or a mix is employed:

• Observables / Signals
• Reconciliation of immutable updates
• Transpilation

Observables / Signals

Observables are basically structures that allow to read via a function that subscribes the readers. The subscribers are then re-run on update:

const state = (initialValue) => ({  _value: initialValue,  get: function() {    /* subscribe */;    return this._value;   },  set: function(value) {    this._value = value;    /* re-run subscribers */;  }});

One of the first uses of this concept was in knockout, which used the same function with and without arguments for write/read access.

This pattern is currently seeing a revival as signals, for example in Solid.js and preact signals, but the same pattern is used under the hood of Vue and Svelte. RxJS, which powers the reactive layer of Angular, is an extension of this principle beyond simple state, but one could argue that its ability to model complexity is a whole arsenal of guns aimed at your feet. Solid.js also comes with further abstractions of these signals in the form of stores (objects that can be manipulated through a setter) and mutables (objects that can be used like normal JS objects or the state in Vue to handle nested state objects.

Reconciliation of immutable states

Immutability means that if the property of an object changes, the whole object reference must change, so a simple comparison of references can easily detect if there are changes, which is what the reconciler does.

const state1 = {  todos: [{ text: 'understand immutability', complete: false }],  currentText: ''};// updating the current text:const state2 = {  todos: state1.todos,  currentText: 'understand reconciliation'};// adding a to-do:const state3 = {  todos: [    state.todos[0],    { text: 'understand reconciliation', complete: true }  ],  currentText: ''};// this breaks immutability:state3.currentText = 'I am not immutable!';

As you can see, references of unchanged items are re-used. If the reconciler detects different object references, it runs all components using the state (props, memos, effects, context) again. Since the read access is passive, this requires the manual specification of dependencies to reactive values.

Obviously, you are not defining state that way. You either construct it from existing properties or use a so-called reducer. A reducer is a function that takes one state and returns another one.

This pattern is used by react and preact. It lends itself to being used with a vDOM, which we will explore later when templating is described.

Not every framework uses its vDOM to make the state fully reactive. Mithril.JS, for example, updates from state changes after the events set in the component; otherwise you have to trigger m.redraw() manually.

Transpilation

Transpilation is a build step that rewrites our code to make it run on older browsers or give it extra abilities; in this case, the technique is employed to change a simple variable into a part of a reactive system.

Svelte is based on a transpiler that also powers their reactive system from seemingly simple variable declaration and access.

As an aside, Solid.js uses transpilation, but not for its state, only for the templating.

Effects

In most cases, we need to do more with our reactive state than deriving from it and render it into the DOM. We have to manage side effects, which are all things that happen due to state changes beyond updates to the view (though some frameworks like Solid.js treat view changes as effects as well).

Remember the first example from state where the subscription handling was intentionally left out? Let us fill this in to handle effects as a reaction to updates:

const context = [];const state = (initialValue) => ({  _subscribers: new Set(),  _value: initialValue,  get: function() {    const current = context.at(-1);    if (current) { this._subscribers.add(current); }    return this._value;  },  set: function(value) {    if (this._value === value) { return; }    this._value = value;    this._subscribers.forEach(sub => sub());  }});const effect = (fn) => {  const execute = () => {    context.push(execute);    try { fn(); } finally { context.pop(); }  };  execute();};

This is basically a simplification of the reactive state in preact signals or Solid.js without error handling and state mutation pattern (using a function that receives the previous value and returns the next), but that would be easy to add.

It allows us to make the previous example reactive:

const count = state(0);const increment = () => count.set(count.get() + 1);const button = document.createElement('button');effect(() => {  button.textContent = count.get();});button.addEventListener('click', increment);document.body.appendChild(button);

Try out the above two code blocks in an empty page using your developer tools.

In most cases, the frameworks allow for different timings to let the effects run before, during or after rendering the DOM.

Memoization

Memoization means caching of values computed from state, to be updated when the state it is derived from changes. It is basically an effect that returns a derived state.

In frameworks that re-run their component functions, like react and preact, this allows to opt out parts of the components again when the state it depends on does not change.

For other frameworks, it is the opposite: it allows you to opt in parts of the component to reactive updates, while caching the previous computation.

For our simple reactive system, memo looks like this:

const memo = (fn) => {  let memoized;  effect(() => {    if (memoized) {      memoized.set(fn());    } else {      memoized = state(fn());    }  });  return memoized.get;};

Templating and rendering

Now that we have state in pure, derived and cached form, we want to show it to the user. In our example, we used the DOM directly to add a button and update its text content.

To be more developer-friendly, almost all modern frameworks support some domain-specific language to write something similar to the desired output inside your code. Even though there are different flavors, like .jsx, .vue or .svelte files, it all comes down to a representation of the DOM in code that resembles HTML, so that basically

<div>Hello, World</div>// in your JS// becomes in your HTML:<div>Hello, World</div>

"Where do I put my state?" you may ask. Excellent question. In most cases, {} are used to express dynamic content, both in attributes and around nodes.

The most used templating language extension to JS is undoubtedly JSX. For react, it is compiled to plain JavaScript in a way that allows it to create a virtual representation of the DOM, an internal view state called virtual document object model or vDOM for short.

This is based on the premise that creating objects is much, much faster than accessing the DOM, so if you can replace the latter with the current, you save time. However, if you either have numerous DOM changes in any case or create countless objects for no changes, the benefits of this solution are easily turned into a disadvantage that has to be circumvented through memoization.

// original code<div>Hello, {name}</div>// transpiled to jscreateElement("div", null, "Hello, ", name);// executed js{  "$$typeof": Symbol(react.element),  "type": "div",  "key": null,  "ref": null,  "props": {    "children": "Hello, World"  },  "_owner": null}// rendered vdom/* HTMLDivElement */<div>Hello, World</div>

JSX is not limited to react, though. Solid, for example, uses its transpiler to change the code more drastically:

// 1. original code<div>Hello, {name()}</div>// 2. transpiled to jsconst _tmpl$ = /*#__PURE__*/_$template(`<div>Hello, </div>`, 2);(() => {  const _el$ = _tmpl$.cloneNode(true),    _el$2 = _el$.firstChild;  _$insert(_el$, name, null);  return _el$;})();// 3. executed js code/* HTMLDivElement */<div>Hello, World</div>

While the transpiled code may look daunting at first, it is rather simple to explain what happens here. First, the template with all the static parts is created, then it is cloned to create a new instance of its contents and the dynamic parts are added and wired to update on state changes.

Svelte goes even further and transpiles not only the templates, but also the state.

// 1. original code<script>let name = 'World';setTimeout(() => { name = 'you'; }, 1000);</script><div>Hello, {name}</div>// 2. transpiled to js/* generated by Svelte v3.55.0 */import {        SvelteComponent,        append,        detach,        element,        init,        insert,        noop,        safe_not_equal,        set_data,        text} from "svelte/internal";function create_fragment(ctx) {        let div;        let t0;        let t1;        return {                c() {                        div = element("div");                        t0 = text("Hello, ");                        t1 = text(/*name*/ ctx[0]);                },                m(target, anchor) {                        insert(target, div, anchor);                        append(div, t0);                        append(div, t1);                },                p(ctx, [dirty]) {                        if (dirty & /*name*/ 1) set_data(t1, /*name*/ ctx[0]);                },                i: noop,                o: noop,                d(detaching) {                        if (detaching) detach(div);                }        };}function instance($$self, $$props, $$invalidate) {        let name = 'World';        setTimeout(                () => {                        $$invalidate(0, name = 'you');                },                1000        );        return [name];}class Component extends SvelteComponent {        constructor(options) {                super();                init(this, options, instance, create_fragment, safe_not_equal, {});        }}export default Component;// 3. executed JS code/* HTMLDivElement */<div>Hello, World</div>

There are exceptions. In Mithril.js, for example, while it is possible to use JSX, you are encouraged to write JS:

// 1. original JS codeconst Hello = {  name: 'World',  oninit: () => setTimeout(() => {    Hello.name = 'you';    m.redraw();  }, 1000),  view: () => m('div', 'Hello, ' + Hello.name + '!')};// 2. executed JS code/* HTMLDivElement */<div>Hello, World</div>

While most people will find the developer experience lacking, others prefer full control over their code. Depending on which issue they aim to solve, the lack of a transpilation step might even be beneficial.

Many other frameworks allow for usage without transpilation, though it is rarely recommended like that.

"And what framework or library should I learn now?"

I have some good news and some bad news for you.

The bad news is: there is no silver bullet. No framework will be much better than all others in every single aspect. Each one of them has their advantages and compromises. React has its hook rules, Angular a lack of simple signals, Vue and Svelte don't scale too well, Solid.js forbids destructuring and Mithril.js is not really reactive, just to name a few.

The good news is: there is no wrong choice at least, unless the requirements of a project are really limited, be it in terms of bundle size or performance. Every framework will do its job. Some may require working around their design decisions, which might slow you down a bit, but you should be able to get a working result in any case.

That being said, going without a framework might be a viable choice, too. Many projects are spoiled by overuse of JavaScript, when static pages with a sprinkle of interactivity would have done the job as well.

Now that you know the concepts that are applied by these frameworks and libraries, choose those which are the best fit for your current task. Do not be afraid to switch frameworks in your next project. It is not necessary to learn all of them.

If you try a new framework, one of the things I found most helpful is to connect to its community, be it on social media, discord, github or elsewhere. They can tell you which approaches are idiomatic for their framework, which will help you to get better solutions faster.

"Come on, you must have a personal preference!"

If your main goal is to become employed, I would suggest learning react. If you want a great experience of effortless performance and control, try Solid.js; you might meet me on Solid's Discord.

But please bear in mind that all other choices are equally valid. You should not choose a framework because I say so, instead use one that works best for you.

If you got through the whole text, thank you for your patience. I hope it was helpful for you. Leave a comment while you're here and have a nice day!