# Subscript

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Subscript is a reactivity runtime for JavaScript. It takes any valid JavaScript code, reads its dependency graph, and offers a mechanism to run it both in whole and in *reactive* selections, called *dependency threads*.

+ [What's A Dependency Thread?](#whats-a-dependency-thread)
+ [What Is Subscript?](#what-is-subscript)
+ [Concepts](#concepts)
+ [API](#api)
+ [Installation](#installation)
+ [A Custom Element Example](#a-custom-element-example)
+ [Motivation](#motivation)
+ [Getting Involved](#getting-involved)
+ [Issues](#issues)

## What's A Dependency Thread?

Think of it as the dependency chain involving two or more JavaScript expressions. 👇

```js
let count = 10, doubleCount = count * 2, quadCount = doubleCount * 2;
```

We just expressed that `doubleCount` should be two times the value of `count`, and that `quadCount` should be two times the value of `doubleCount`; each subsequent expression being a *dependent* of the previous.

```js
console.log( count, doubleCount, quadCount );
< 10, 20, 40
```

😉 Can you spot that same dependency chain in the following hypothetical UI render function…?

```js
let count = 10;
```

```js
let render = function() {
    let countElement = document.querySelector( '#count' );
    countElement.innerHTML = count;
    
    let doubleCount = count * 2;
    let doubleCountElement = document.querySelector( '#double-count' );
    doubleCountElement.innerHTML = doubleCount;
    
    let quadCount = doubleCount * 2;
    let quadCountElement = document.querySelector( '#quad-count' );
    quadCountElement.innerHTML = quadCount;
}
```

You'll also notice one additional *dependent* at each level of the chain. That brings the *dependency thread* for `count` to being the following sequence: `statement 2` -> `statement 3` -> `statement 5` -> `statement 6` -> `statement 8`; excluding statements `1`, `4`, `7`.

🤝 Good analysis! But what's the deal?

Programs are generally expected to run **in whole**, **not in dependency threads**! It would take some magic to have the latter. But... well, that's what's for dinner with Subscript! 😁

Problem is: the mathematical relationship above only holds for as long as nothing changes. Should the value of `count` change, then its dependents are sure out of sync.

```js
count ++;
```

```js
console.log( count, doubleCount, quadCount );
< 11, 20, 40
```

This is that reminder that expressions in JavaScript aren't automatically bound to their dependencies. (Something we'd expect of any programming language.) The `render()` function must be called again each time the value of `count` changes.

An important worry is that we end up running overheads on sebsequent calls to `render()`, as those `document.querySelector()` calls traverse the DOM again, just to return the same elements as in previous runs. (In real life, there could be even more expensive operations up there.)

Enter dependency threads; suddenly, we can get statements to run in isolation in response to a change! **Here comes a new way to think about reactivity and performance in JavaScript**! 👇

\> Obtain `SubscriptFunction` and use as a drop-in replacement for `Function`! 👇

```js
let render = new SubscriptFunction(`
    let countElement = document.querySelector( '#count' );
    countElement.innerHTML = count;
    
    let doubleCount = count * 2;
    let doubleCountElement = document.querySelector( '#double-count' );
    doubleCountElement.innerHTML = doubleCount;
    
    let quadCount = doubleCount * 2;
    let quadCountElement = document.querySelector( '#quad-count' );
    quadCountElement.innerHTML = quadCount;`
);
```

> More about the syntatic rhyme between `SubscriptFunction` and `Function` [ahead](#api).

\> Use `render` as a normal function…

```js
render();
```

*The above executes the function body in whole as we'd expect. Elements are selected and assigned content. And we can see the counters in the console.*

```js
console.log( count, doubleCount, quadCount );
< 10, 20, 40
```

\> Run just the `count` dependency thread…

```js
count ++;
render.thread( [ 'count' ] );
```

*This time, only statements `2` -> `3` -> `5` -> `6` -> `8` are run - *the "count" dependency thread*; and the previously selected UI elements in those local variables are only now updated.*

```js
console.log( count, doubleCount, quadCount );
< 11, 22, 44
```

\> See `SubscriptFunction` in real life.

*A Custom Element Example [ahead](#a-custom-element-example)*

*Or click on the demo below to **visualize** the "count" dependency thread in realtime.*


[![Demo Count Button](https://unpkg.com/@webqit/subscript/dist/dependency-thread-demo-3-counts-button.webqit.repl.co_%20(1).png)](https://replit.com/@WebQit/Dependency-Thread-Demo-3-Counts-Button)

## What Is Subscript?

A general-purpose reactivity runtime for JavaScript, with an overarching philosophy of *reactivity that is based on the dependency graph of your own code, and nothing of its own syntax*!

It takes any piece of code and compiles it into an ordinary JavaScript function that can also run expressions in *dependency threads* via a `.thread()` method!

Being function-based let's us have all of Subscript as a building block… to fit anywhere!

## Concepts

+ [Thread Events](#thread-events)
+ [References And Bindings](#references-and-bindings)
+ [Conditionals And Logic](#conditionals-and-logic)
+ [Loops](#loops)
+ [Functions](#functions)

### Thread Events

Subscript is not concerned with how changes happen or are detected on the outer scope of the function. It simply gives us a way to announce that something has changed. That announcement is called a *thread event*.

A Subscript function has a `thread()` method that lets us trigger a thread for the list of outside variables or properties that have changed.

```js
let a = 'Apple', b = 'Banana', c = { prop: 'Fruits' };
```

```js
let fn = new SubscriptFunction(`
    console.log( \`The value of 'a' is: \${ a }\` );
    console.log( \`The value of 'b' is: \${ b }\` );
    console.log( \`The value of 'c.prop' is: \${ c.prop }\` );
`);
```

```js
// Initial run
fn();
```

```js
// Updates and threads
b = 'Breadfruit';
fn.thread( [ 'b' ] );
```

The array syntax allows us to represent properties as paths.

```js
fn.thread( [ 'c', 'prop' ] );
```

And we can run one thread for multiple changes.

```js
fn.thread( [ 'a' ], [ 'b' ] );
```

Variable declarations within the function belong in their own scope and do not respond to outside events. But their containing expression may also maintain a binding to those variables from the outside scope.

```js
let fn = new SubscriptFunction(`
    let a = 'Apple', b = 'Banana' + ' ' + c.prop;
    console.log( \`The value of 'a' is: \${ a }\` );
    console.log( \`The value of 'b' is: \${ b }\` );
    console.log( \`The value of 'c.prop' is: \${ c.prop }\` );
`);
```

```js
// Initial run
fn();
```

```js
// The following events will have no effect since "a" and "b" are local variables.
fn.thread( [ 'a' ], [ 'b' ] );
```

```js
// The local variable "b" will be part of the dependency thread of "c.prop"
// (The console will therefore show the result of the last two statements in the function)
fn.thread( [ 'c', 'prop' ] );
```

### References And Bindings

Expressions and statements in Subscript maintain a binding to their references. And that's the basis for reactivity in Subscript.

For example, variable declarations, with `let` and `var`, and assignment expressions, are bound to any references that may be in their argument. (`const` declarations are an exception as they're always *const* in nature.)

```js
var tense = score > 50 ? 'passed' : 'failed';
```

*Above, the assignment expression is bound to the reference `score`; and thus responds to a thread event for `score`.*

The thread continues with any susequent bindings to the `tense` variable itself...

```js
let message = `Hi ${ candidate.firstName }, you ${ tense } this test!`;
```

*Above, the assignment expression is bound to the references `candidate`, `candidate.firstName`, and `tense`; and thus responds to a thread event for each.*

And the thread continues with any susequent bindings to the `message` variable itself... and any bindings of those bindings...

```js
let fullMessage = [ message, ' ', 'Thank you!' ].join( '' );
```

```js
let broadcast = { [ candidate.username ]: fullMessage };
```

```js
console.log( broadcast );
```

```js
let broadcastInstance = new BroadcastMessage( broadcast );
```

And ES6 syntax niceties can come in anywhere.

```js
let { username, profile: { avatar: avatarUrl } } = candidate;
```

*And that's essentially two variables declared up there: `username` and `avatarUrl`! And while `username` is bound to `candidate.username`, `avatarUrl` is bound to `candidate.profile.avatar`. Each gets updated independent of the other; in sync with their own binding. (But a thread event for their common root object - `candidate` - gets both variables updated.)*

### Conditionals And Logic

When the *test expression* of an "If/Else" statement, "Switch" statement, or other logical expressions contains references, the statement or logical expression is bound to those references. This lets us have *reactive conditionals and logic*.

#### "If/Else" Statements

An "If/Else" statement is bound to references in its "test" expression.

```js
if ( score > 80 && passesSomeOtherTest() ) {
    addBadge( candidate );
    candidate.remark = 'You\'ve got a badge';
} else {
}
```

*Above, the "If/Else" construct is bound to the references `score` and `passesSomeOtherTest` - yes, should that also change. A thread event for any of these gets the construct re-evaluated; first, the "test" expression (`score > 80 && passesSomeOtherTest()`), then, the body of the appropriate branch of the construct.*

Statements in the body of the "consequent" and "alternate" branches form a binding to references of their own, independent of their containing "If" construct. But they only respond to thread events for as long as the "state" of all *conditions in context* allows.

*Above, the `addBadge()` expression is bound to the reference `candidate`, and joins alone in the dependency thread, independent of the "If" construct, but for as long as the condition in context (`score > 80 && passesSomeOtherTest()`) holds true.*

> The "state" of all *conditions in context* are determined via *memoization*, and no re-evaluation ever takes place.

An "Else/If" block is taken for just an "If" statement in the "Else" block of a parent "If" statement...

```js
if ( score > 80 && passesSomeOtherTest() ) {
    addBadge( candidate );
    candidate.remark = 'You\'ve got a badge';
} else if ( someOtherCondition ) {
} else {
}
```

...and is bound to references in its own "test" expression, independent of its parent. But it only responds to thread events for as long as the "state" of all *conditions in context* allows.

*Above, the nested "If" statement is bound to the reference `someOtherCondition`, and joins alone in the dependency thread, independent of the parent "If" construct, but for as long as the parent condition (`score > 80 && passesSomeOtherTest()`) holds false.*

#### "Switch" Statements

A "Switch" statement is bound to references in its "test" expressions - the "switch/case" expressions.

```js
switch( score ) {
    case 0:
        candidate.remark = 'You got nothing at all';
        break;
    case maxScore:
        candidate.remark = 'You got the most';
        break;
    default:
        candidate.remark = defaultRemark;
}
```

*Above, the "Switch" construct is bound to the references `score` and `maxScore`. A thread event for any of these gets the construct re-evaluated; first, the "switch/case" expressions (`score === 0` | `score === maxScore` | `score === null`), then, the body of the appropriate branch of the construct.*

Statements in the body of the branches form a binding to references of their own, independent of the "Switch" construct. But they only respond to thread events for as long as the "state" of all *conditions in context* allows.

*Above, the assignment to `candidate.remark` (in the "default" case) is bound to the reference `defaultRemark`, and joins alone in the dependency thread, independent of the "Switch" construct, but for as long as the conditions in context (`score === null`) hold true.*

> The "state" of all *conditions in context* are determined via *memoization*, and no re-evaluation ever takes place.

#### Logical And Ternary Expressions

Subscript observes the state of logical (`a && b || c`) and ternary (`a ? b : c`) expressions when running dependency threads.

```js
let a = () => 1;
let b = 2;
let c = 3;
let d, e;
```

A logical expression...

```js
e = a() && b || c;
```

A ternary expression...

```js
d = a() ? b : c;
```

*Above, each of the two expressions is bound to the references `a`, `b` and `c`. A thread event for any of `a` and `b` - or `a` and `c`, as determined by the "logical state" of the expressions<sup>*</sup> - gets the expressions re-evaluated; first, the "test" expression (`a()`), then, the expression on the appropriate side of the construct.*

<sup>*</sup>Since expressions in the "consequent" and "alternate" sides of a conditional or logical expression are mutually exclusive (`b` and `c` above), as determined by the "test" expression (`a()` above), only the thread events for the references in the currently active side (`b` above) are honoured by the expression at any given point in time.

### Loops

When the parameters of a loop ("For" loops, "While" and "Do … While" loops) contain references, the loop is bound to those references. This lets us have reactive loops.

#### A `for` Loop, `while` And `do … while` Loop

A "For" loop is bound to references in its 3-part definition.

```js
let start = 0;
let items = [ 'one', 'two', 'three', 'four', 'five' ];
let targetItems = [];
```

```js
for ( let index = start; index < items.length; index ++ ) {
    targetItems[ index ] = items[ index ];
}
```

*The loop above is bound to the references `start`, `items`, and `items.length`. A thread event for any of these gets the loop to run again.*

```js
// Say, "start" were a global variable
start = 2;
fn.thread( [ 'start' ] );
```

```js
// Say, "items" were a global variable
items.unshift( 'zero' );
fn.thread( [ 'items', 'length' ] );
```

As with a "For" loop, a "While" and "Do ... While" loop are bound to references in their "test" expression.

```js
let index = 0;
let items = [ 'one', 'two', 'three', 'four', 'five' ];
let targetItems = [];
```

```js
while ( index < items.length ) {
    targetItems[ index ] = items[ index ];
    index ++;
}
```

*The loop above is bound to the references `items` and `items.length`. A thread event for any of these gets the loop to run again.*

```js
// Say, items were global variables
items.unshift( 'zero' );
fn.thread( [ 'items', 'length' ] );
```

#### A `for … of` Loop

A "For ... Of" loop is bound to references in its *iteratee*.

```js
let  entries = [ 'one', 'two', 'three', 'four', 'five' ];
let targetEntries = [];
```

```js
for ( let entry of entries ) {
    let index = entries.indexOf( entry );
    console.log( `Current iteration index is: ${ index }, and entry is: '${ entry }'` );
    targetEntries[ index ] = entries[ index ];
}
```

*The loop above is bound to the reference `entries`. A thread event for `entries` gets the loop to run again.*

```js
// Say, entries were a global variable
entries = [ 'six', 'seven', 'eight', 'nine', 'ten' ];
fn.thread( [ 'entries' ] );
```

As an added advantage of this form of loop, updating a specific entry in `entries` moves the loop's pointer to the specific iteration involving that entry, and the body of that iteration is run again.

```js
entries[ 7 ] = 'This is new eight';
fn.thread( [ 'entries', 7 ] );
```

Now, the console reports…

```js
Current iteration index is: 7, and entry is: 'This is new eight'
```

…and index `7` of `targetEntries` is updated.

#### A `for … in` Loop

A "For ... In" loop is bound to references in its *iteratee*.

```js
let  entries = { one: 'one', two: 'two', three: 'three', four: 'four', five: 'five' };
let targetEntries = {};
```

```js
for ( let propertyName in entries ) {
    console.log( `Current property name is: ${ propertyName }, and associated value is: '${ entries[ propertyName ] }'` );
    targetEntries[ propertyName ] = entries[ propertyName ];
}
```

*The loop above is bound to the reference `entries`. A thread event for `entries` gets the loop to run again.*

```js
// Say, entries were a global variable
entries = { six: 'six', seven: 'seven', eight: 'eight', nine: 'nine', ten: 'ten' };
fn.thread( [ 'entries' ] );
```

As an added advantage of this form of loop, updating a specific property in `entries` moves the loop's pointer to the specific iteration involving that property, and the body of that iteration is run again.

```js
entries[ 'eight' ] = 'This is new eight';
fn.thread( [ 'entries', 'eight' ] );
```

Now, the console reports…

```js
Current property name is: eight, and property value is: 'This is new eight'
```

…and the property `eight` of `targetEntries` is updated.

#### Iteration States

Conceptually, each round of iteration in a loop is an instance that Subscript can access directly when running a thread. A round of iteration is thus updatable in isolation, in response to a directed event. This is what happens when the *iteratee* of a "For ... Of" and "For ... In" loop has any of its properties updated, as seen above.

Below is a similar case.

```js
let  entries = { one: { name: 'one' }, two: { name: 'two' } };
let targetEntries = {};
```

```js
for ( let propertyName in entries ) {
    console.log( `Current property name is: ${ propertyName }, and its alias name is: '${ entries[ propertyName ].name }'` );
    targetEntries[ propertyName ] = entries[ propertyName ];
}
```

On updating the first entry, only the first round of iteration is executed again.

```js
entries[ 'one' ] = { name: 'New one' };
fn.thread( [ 'entries', 'one' ] );
```

For even more granularity, individual expressions inside a round of iteration are also responsive to thread events of their own. So, if we updated just `entries.one.name`…

```js
entries.one.name = 'New one';
fn.thread( [ 'entries', 'one', 'name' ] );
```

…we would have skipped the iteration instance itself, to target just the first statement within it.

This granular reactivity makes it often pointless to trigger a full rerun of a loop, offering multiple opportunities to deliver unmatched performance.

#### Breakouts

Subscript observes `break` and `continue` statements even when running a thread. And any of these statements may employ *labels*.

```js
let  entries = { one: { name: 'one' }, two: { name: 'two' } };
```

```js
parentLoop: for ( let propertyName in entries ) {
    childLoop: for ( let subPropertyName in entries[ propertyName ] ) {
        If ( propertyName === 'one' ) {
            break parentLoop;
        }
        console.log( propertyName, subPropertyName );
    }
}
```

### Functions

Functions are *static* definitions...

```js
function sum( a, b ) {
}
```

```js
let sum = function( a, b ) {
}
```

```js
let sum = ( a, b ) => {
}
```

...and nothing about their parameters is reactive!

They are really only significant at *call-time*; and call-time arguments are rightly *reactive*!

```js
let result = sum( score, 100 );
```

*The expression above is bound to the reference `score`. A thread event for `score` gets the `sum()` function called again with its current value.*

#### Side Effects

When a function modifies anything outside of its scope, it is said to have *side effects*.

```js
let callCount = 0;
function sum( a, b ) {
    callCount ++;
    return a + b;
}
```

When it does not, it is said to be a *pure function*.

```js
function sum( a, b ) {
    return a + b;
}
```

Regardless, Subscript's dependency threads are fully able to pick up changes made via a side effect. (Side effects made by class methods are currently not being detected.)


```js
function sum( a, b ) {
    callCount ++;
    return a + b;
}
let callCount = 0;
let result = sum( score, 100 );
console.log( 'Number of times we\'ve summed:', callCount );
```

*Above, each time the thread event for `score` gets the `sum()` expression to run again, `callCount` is incremented as a side effect; and the dependent `console.log()` expression joins in the thread to pick that up!*

Since statements in a dependency thread are executed in normal program execution order, side effects only trigger dependent expressions that appear *after the point of call*, *not before*.


```js
function sum( a, b ) {
    callCount ++;
    return a + b;
}
let callCount = 0;
console.log( 'BEFORE POINT OF CALL: Number of times we\'ve summed:', callCount );
let result = sum( score, 100 );
console.log( 'AFTER POINT OF CALL: Number of times we\'ve summed:', callCount );
```

*Above, on the thread event for `score`, the first `console.log()` expression doesn't run because at that point `sum()` hasn't been called to make the side effect!*

Also, since Subscript does not change runtime expection in any way, side effects made by function calls outside of a running thread do not get to start a thread in a bid to engage its dependent expressions!

```js
function sum( a, b ) {
    callCount ++;
    return a + b;
}
let callCount = 0;
document.body.addEventListener( 'click', () => {
    let result = sum( score, 100 );
} );
console.log( 'Number of times we\'ve summed:', callCount );
```

*This time, `sum()` is triggerred from a click event handler, not via a dependency thread, and we do not expect the `console.log()` expression to run!*

#### Subscript Function Syntax (`**`)

Subscript explores the possibility of defining functions outright as *reactive* functions using regular *Function Declaration* and *Function Expression* syntaxes!

```js
function** sum( a, b ) {
}
```

```js
let sum = function**( a, b ) {
}
```

*Notice the double star `**` symbol above; it's just one star extra to the standard syntax for [Generator Functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator) (`function* gen() {}`) - one more thing in the same classification of a special-purpose function in JavaScript! 😎*

Functions defined this way are compiled as `SubscriptFunction`, exposing a `.thread()` method for running dependency threads, and offering everything else as in when we use the `SubscriptFunction` constructor.

The following syntaxes produce a reactive function...

```js
function** sum( a, b ) {
    return a + b;
}
```

```js
let sum = function**( a, b ) {
    return a + b;
}
```

```js
let sum = new SubscriptFunction( `a`, `b`, `return a + b;` );
```

...but the first two (proposed) syntaxes are only currently supported from within Subscript Function itself!

```js
let score = 10;
let program = new SubscriptFunction(`
    function** sum( a, b ) {
        callCount ++;
        return a + b;
    }

    let callCount = 0;

    // The following call results in a side effect
    let result = sum( score, 100 );
    
    // and "callCount" is logged as "1" to the console 
    console.log( 'Number of times we\'ve summed:', callCount );

    // The following call runs a dependency thread that excludes the side effect
    // while return the sum of the previous values of "a" and "b"
    let result = sum.thread( [ 'a' ] );

    // and "callCount" is still logged as "1", not "2", to the console 
    console.log( 'Number of times we\'ve summed:', callCount );
`);
program();
```

Objects and classes have an equivalent syntax for a Subscript method...

```js
let myObject = {
    sum: function**( a, b ) {
        return a + b;
    }
}
```

```js
let myObject = {
    **sum( a, b ) {
        return a + b;
    }
}
```

```js
class MyClass {
    **sum( a, b ) {
        return a + b;
    }
}
```

...but these (proposed) syntaxes are only currently supported from within Subscript Function itself! (Also, class methods only currently support the double-star syntax at face value; they do not yet compile as Subscript methods.)

However, Subscript offers a *[Class Mixin](#subscriptclass)* that automatically redefines class methods as Subscript methods.

```js
class MyClass extends SubscriptClass() {

    static get subscriptMethods() {
        return [ 'sum' ];
    }

    sum( a, b ) {
        return a + b;
    }
}
```

```js
class MyClass extends SubscriptClass( HTMLElement ) {

    static get subscriptMethods() {
        return [ 'render' ];
    }

    render() {
    }
}
```

## API

### SubscriptFunction

`SubscriptFunction` is a one-to-one equivalent of the [JavaScript Function constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/Function). They work interchangeably 😎.

#### Syntax

```js
// Statically
let subscrFunction = SubscriptFunction( functionBody );
let subscrFunction = SubscriptFunction( arg1, functionBody );
let subscrFunction = SubscriptFunction( arg1, ... argN, functionBody );

// With the new operator
let subscrFunction = new SubscriptFunction( functionBody );
let subscrFunction = new SubscriptFunction( arg1, functionBody );
let subscrFunction = new SubscriptFunction( arg1, ... argN, functionBody );
```

#### Parameters

##### `arg1, ... argN`

Names to be used by the function as formal argument names. Each must be a string that corresponds to a valid JavaScript parameter (any of plain identifier, rest parameter, or destructured parameter, optionally with a default), or a list of such strings separated with commas.

##### `functionBody`

A string that represents the function body.

#### Return Value

A regular `Function` object, or an `async function` object where the `await` keyword is used within `functionBody`.

```js
// Create a regular function - sum
let sum = SubscriptFunction( 'a', 'b', 'return a + b;' );

// Call the returned sum function and log the result
console.log( sum( 10, 2 ) );
< 12
```

```js
// Create an async function - sum
let sum = SubscriptFunction( 'a', 'b', 'return a + await b;' );

// Call the returned sum function and log the result
sum( 10, 2 ).then( result => {
    console.log( result );
} );
< 12
```

#### The `this` Binding

Functions returned by `SubscriptFunction` are standard functions that can have their own [`this`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this) binding at *call time*.

```js
// Create a function - colorSwitch - that sets a DOM element's color
let colorSwitch = SubscriptFunction( 'color', 'this.style.color = color;' );

// Call colorSwitch, with document.body as it's this binding
let element = document.body;
colorSwitch.call( element, 'red' );
```

But, where the `this` binding is `undefined` at call time, the `this` binding of the `SubscriptFunction` itself is used. This lets us have a default `this` binding at *creation time*.

```js
// Create the same colorSwitch, this time, with a this binding that can be used at call time
let element = document.body;
let colorSwitch = SubscriptFunction.call( element, 'color', 'this.style.color = color;' );

// Call colorSwitch, without a this binding
colorSwitch( 'red' );
colorSwitch.call( undefined, 'red' );

// Call colorSwitch, with a different this binding
let h1Element = document.getElementById( 'h1' );
colorSwitch.call( h1Element, 'red' );
```

#### The `subscrFunction.thread()` Method

The `.thread()` method is the *reactivity* API in Subscript functions that lets us send *thread events* into the *reactivity runtime*. It constitues one clear interaction point and enables a one-liner approach to reactivity.

It takes a list of the outside variables or properties that have changed; each as an array path.

##### Syntax

```js
let returnValue = subscrFunction.thread( path1, ... pathN );
```

##### Parameters

##### `path1, ... pathN`

An array path representing each variable, or object property, that has changed. *See [Thread Events](#thread-events) for concepts and usage.*

##### Return Value

The return value of this method depends on the return value of the *dependency thread* it initiates within the function body.

```js
// Global variables to use
a = 10;
b = 2;

// Create a function with two possible values
let sum = SubscriptFunction(`
    if ( a > 10 ) {
        return a + await b;
    }
    return a + b;
`);

// Run normally
console.log( sum() );
< 12

// Run a thread with a different return value
a = 20;
console.log( sum.thread( [ 'a' ] ) );
< Promise { 22 }
```

### SubscriptClass

`SubscriptClass` is a *convenience* base class *Mixin* that automatically transforms regular class methods as Subscript methods.

#### Syntax

```js
class MyClass extends SubscriptClass( [ BaseClass = null ] ) {

    static get subscriptMethods() {
        return [ methodName, ... methodNameN ];
    }

    method() {
    }
}
```

#### Parameters

##### `BaseClass`

An optional base class that should be extended.

##### `methodName, ... methodNameN`

Names of the methods that should be transformed to Subscript methods.

#### Return Value

A *class* object.

*See [below](#custom-element-example) for usage examples*

## Installation

\> Install via npm

```cmd
npm i @webqit/subscript
```
```js
import { SubscriptFunction, SubscriptClass } from '@webqit/subscript';
```

\> Include from a CDN

```html
<script src="https://unpkg.com/@webqit/subscript/dist/main.js"></script>
```
```js
const { SubscriptFunction, SubscriptClass } = WebQit.Subscript;
```

## A Custom Element Example

As trivial as our hypothetical [`render()`](#whats-a-dependency-thread) function above is, we can see it applicable in real life places! Consider a neat reactive *Custom Element* example based on [`SubscriptClass`](#subscriptclass).

```js
// We'll still keep count as a global variable for now
let count = 10;
```

```js
// This custom element extends Subscript as a base class… more on this later
customElements.define( 'click-counter', class extends SubscriptClass( HTMLElement ) {
    
    // This is how we designate methods as reactive methods
    static get subscriptMethods() {
        return [ 'render' ];
    }
        
    connectedCallback() {
        // Full execution at connected time
        this.render();
        // Granularly-selective execution at click time
        this.addEventListener( 'click', () => {
            count ++;
            this.render.thread( [ 'count' ] );
        } );
    }

    render() {
        let countElement = document.querySelector( '#count' );
        countElement.innerHTML = count;
        
        let doubleCount = count * 2;
        let doubleCountElement = document.querySelector( '#double-count' );
        doubleCountElement.innerHTML = doubleCount;
        
        let quadCount = doubleCount * 2;
        let quadCountElement = document.querySelector( '#quad-count' );
        quadCountElement.innerHTML = quadCount;
    }
} );
```

*See also [SubscriptElement](https://webqit.io/tooling/oohtml/docs/spec/subscript#subscript-element-mixin) - the [OOHTML](https://webqit.io/tooling/oohtml) extension of [`SubscriptClass`](#subscriptclass)*

## Motivation

### The Best Syntax Is No Syntax At All!

*Frontend has a syntax problem*! Every framework has come contributing something *JavaScript-like*, *HTML-like*, or even *JavaScript/HTML-like<sup>2</sup>* to the plague! And for many of us, that bit is a non-starter! 😡

So, we're rethinking reactivity, again! This, time, to lay its very principles on nothing at all but plain JavaScript!

### Performant JavaScript

With an insane focus on pure JavaScript syntax, Subscript is able to keep its footprint, and your footprint, ridiculously small. This *less clutter*, is *more performance*!

Subscript follows a compiler-aided approach that translates to a tiny, highly-optimized piece at runtime - no diffing; no *callback wizardry*!

### Developer "Joooy" 😎

Much work goes into learning and using today's slew of reactivity primitives - those `on____` and `use____` hooks! But to explicitly construct reactive relationships is to slave over something that is *implicit* in a program's dependency graph!

Subscript lets you write your code, not the hooks! Graph-based reactivity just kicks in! 🤩

Offering the full range of modern JavaScript, with zero additional clutter, none of a complex toolchain and no required build step is a new dimension to developer productivity!

### Compasable Reactivity

Subscript comes as *reactivity in a function* - the smallest possible unit of composition, and this is new! But that is to say: composition is king!

Thinking of reactive JavaScript classes? Make one... with Subscript Function as a method! Building the next reactive system? Put Subscript Functions under the hood!

### Progressive Development

What's the possibility of turning reactivity *on* and *off* on an existing code base, in an afterthought? Oh that's a nobrainer with Subscript Functions!

+ Using the [Function Constructor](#api) approach? Just toggle between the function type, while everything else stays intact:

    ```js
    let sum = new Function( `a`, `b`, `return a + b` );
    ```

    ```js
    let sum = new SubscriptFunction( `a`, `b`, `return a + b` );
    ```

+ Using the [Function Synctax](#subscript-function-syntax-new) approach? Just toggle the *double star*, while everything else stays intact:

    ```js
    function sum( a, b, ) {
        return a + b;
    }
    ```

    ```js
    function** sum( a, b, ) {
        return a + b;
    }
    ```

This togglability is new!

## Getting Involved

We'd be super excited to have you raise an issue, make a PR, or join in the discussion at [Subscript's Github Discussions](https://github.com/webqit/subscript/discussions).

**And, please comrade, your star counts up there 🤨**

## Issues

To report bugs or request features, please submit an [issue](https://github.com/webqit/subscript/issues).

## License

MIT.
