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A Straightforward Model View Controller library for the frontend.

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# smvc

[![NPM Version](https://img.shields.io/npm/v/smvc.svg?style=flat)](https://www.npmjs.com/package/smvc)
[![GitHub License](https://img.shields.io/github/license/lazamar/smvc)](https://github.com/lazamar/smvc/blob/main/LICENSE.md)
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**Straightforward Model View Controller**

This is a simple-as-it-gets library for using a virtual DOM to separate the handling of state and view.

The entire API consists of 4 functions.

It uses simple JavaScript, has no dependencies, requires no transpilation, and has fewer than 300 lines of code.

Read the [blog post](https://lazamar.github.io/virtual-dom/)

### Example

Visit the [demos](https://lazamar.github.io/smvc/demos/) page.

A complete example. See it in action [here](https://lazamar.github.io/smvc/demos/minimal.html).

```html
<html>
  <body>
    <div id="container"></div>
    <script src="./smvc.js"></script>
    <script>
      const { init, h, text } = SMVC;
      const root = document.querySelector("#container");
      const initialState = 0;

const update = (state, msg, enqueue) => state + msg;

const view = (state) => [
        h("div", { style: "color: red", onClick: () => 2 }, [
          h("p", {}, [
            text(`The count is ${state}. Click here to increment.`)
          ])
        ])
      ];

const { enqueue } = init(root, initialState, update, view);
      enqueue(1);
    </script>
  </body>
</html>
```

## Quickstart

After importing `smvc.js` start your application by calling `smvc.init` with the appropriate arguments.

You can use the library as follows:

```javascript
// `SMVC` is the only identifier the library adds to the global
// scope. It is an object with a function to start the
// application and two functions to build html elements
const { init, h, text } = SMVC;

// Where the application will be rendered
const root = document.querySelector("#container");

// An initial state for the application.
const initialState = { ... };

// How to update that state
function update(state, msg, enqueue) { ... }

// A function to build the view using the `h` and `text`
// functions from in the SMVC object.
function view(state) { ... }

// Start the application. `init` returns an object with the
// enqueue function, which is is used to scheduled messages
// to be used to update the state.
const { enqueue } = init(root, initialState, update, view);
```

### Creating HTML

This library creates elements in the page based on a description of how the page should look like.

The user-defined `view` function should return this description in the form of an
array of element descriptions.

There are two functions to create HTML element descriptions.

* `text(str: string) : Element` - Describe a text node.
* `h(tag: string, properties: Object, children: Array<Element>) : Element` - Describe an HTML element.

The `properties` object should contain HTML properties and attributes as keys.

Keys that start with the `on` prefix in the `properties` object are treated as
event listeners and its values should be functions.

Event listener functions will be given the event they are handling as an argument.

If an event listener function returns a value different from `undefined` this
value will be used as a message and queued to be sent to the update function.

Examples:

```javascript
function view(state) {
  return [
    // A `div` with a class.
    h("div", { class: "container" }, [ ... ]),

// A disabled button.
    h("button", { disabled: "true" }, [ ... ]),

// A button that will emit the string `"toggle"` as a message when clicked.
    h("button", { onClick: () => "toggle" }, [ ... ]),

// A button that will emit a message containing the HTML event handled by the onClick listener.
    h("button", { onClick: e => e }, [ ... ])
  ]
}
```

### Emitting messages

The state is updated by handling messages in the user-defined `update` function.

There are two ways to emit messages.

**Returning from an event listener**

Any value that is not `undefined` returned by an event listener will be emitted as a message.

```javascript
// A button which when clicked will emit as a message
// the object { tag: "clicked, event: <event> }
// where <event> is the HTML event being handled.
h(
    "button",
    {
        onClick : function (e) {
            return { tag: "clicked", event: e }
        }
    },
    [ text("click") ]
)
```

**Using the `enqueue` function**

This function is in the object returned by `init` and will schedule a message to be handled.

``` javascript
const { enqueue } = init(root, initialState, update, view);

// Emit a message.
enqueue({ tag: "setCounter", value: 2 });
```

It is safe to call `enqueue` inside the `update` function.

Messages are put in a queue and the `update` function is called for each of them in order.

### Updating the state

The user-defined `update` function is the part of the program where the state
should be changed. It must return the new state.

It takes three arguments:

* The current state.
* The message to be handled.
* The `enqueue` function.

It is safe to call `enqueue` from inside `update`.

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