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index.js
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/* (c) 2023 - by Markus Walther, MIT License.
*
* sig-html is a micro framework for web apps based on plain or computed signals + lit-html template notation.
*
* Supported lit-html notation (cf. https://lit.dev):
*
* <div attribute="${1}" .property="${2}" ?booleanAttribute="${false}" @event="${e => alert(e)}">${5}</div>
*
* Known restrictions:
* - Signals are the only mechanism provided for reactivity
* - any literal variables must span the entire attribute value or text content in which it occurs, i.e. no 'some ${...} other text or template variable' is allowed (for now)
* - no HTML validation or sanitizing whatsoever
* - no careful memory or performance optimizations (yet)
*/
// constants
let SUBSCRIBE = "subscribe";
let TEXTCONTENT = "textContent";
let ATTRIBUTE = "Attribute";
let GET = "get";
let SET = "set";
let REMOVE = "remove";
let GETATTRIBUTE = GET + ATTRIBUTE;
let GETATTRIBUTENAMES = GETATTRIBUTE + "Names";
let APPENDCHILD = "appendChild";
let DEFAULT_EFFECT = (domNode, value) => (domNode[TEXTCONTENT] = value); // set the text content of an assumed Text node to a value
let DEFAULT_REACTION = identity => identity; // the identity value transform
let LIT_HTML_SPECIALS = ".?@";
// module globals
let plugs2Values = {};
let plugCounter = 0;
let parser = new DOMParser();
let d = document;
// helper functions
let parse = htmlText => {
return parser.parseFromString(htmlText, "text/html"); // returns document tree corresponding to <html><head></head><body>child nodes</body></html>
};
let extractNodes = dom => dom?.all[2].childNodes; // extract the <html>, then the <body>, then the children under it
let isText = domNode => domNode.nodeType === 3; // is it a Text node?
let isFunction = f => typeof f === "function";
// determine an attribute's type; returns a pair of [sanitizedAttribute, indexOfSpecialCharacterOrZero]
// Note: missing/undefined attribute initialized to 'impossible' string "\0" with which search for special character will fail with -1 index
let attributeType = (attribute = "\0") => {
let firstCharacter = attribute[0];
// lit-html-style prefixed attribute?
let litHTMLSpecialIndex = LIT_HTML_SPECIALS.indexOf(firstCharacter) + 1;
return [
litHTMLSpecialIndex ? /* yes, strip first character */ attribute.slice(1) : /* no, return as-is */ attribute,
litHTMLSpecialIndex
];
};
let deriveEffect =
([attribute, litHTMLSpecialIndex]) =>
// given a DOM node and a value:
(domNode, value) =>
[
/* a Text node or a plain, undecorated attribute */
() =>
isText(domNode)
? DEFAULT_EFFECT(domNode, value) // effect: set text content to value
: domNode[SET + ATTRIBUTE](attribute, value), // effect: set attribute to value
/* a .property */
() => (domNode[attribute] = value), // effect: set property to value
/* a ?Boolean attribute */
() => domNode[(value ? SET : REMOVE) + ATTRIBUTE](attribute, ""), // effect: add or remove Boolean attribute
/* an @event handler */
() => {
let { listener, options } = isFunction(value) ? { listener: value, options: false } : value;
domNode.addEventListener(attribute, listener, options);
} // effect: attach event listener for event named in value
][litHTMLSpecialIndex]();
let handleEffect = (attribute, domNode, value) => {
// try to get initial value from attribute value
if (value === undefined && GETATTRIBUTE in domNode) {
value = domNode[GETATTRIBUTE](attribute);
}
// determine attribute type according to lit-html's notational conventions
let type = attributeType(attribute);
// is it a lit-html-annotated attribute?
if (type[1]) {
// yes, remove it (it will replaced by the effect it induces on the DOM node)
domNode[REMOVE + ATTRIBUTE](attribute);
}
// is the value indicative of a filled string-template variable a.k.a. 'plug'?
if (value in plugs2Values) {
// yes, get its associated real value
value = plugs2Values[value];
}
// return the tuple of [effect, value], with side effects of handling any signal values
return [deriveEffect(type), handleSignal(domNode, value, attribute)];
};
let isSignal = signal => signal instanceof Signal;
let handleSignal = (domNode, value, attribute) => {
// value represents a straightforward signal?
if (isSignal(value)) {
// yes, subscribe DOM node and get initial value of the signal
value = value[SUBSCRIBE](domNode, attribute);
// value represents a computed signal?
} else if (Array.isArray(value) && isSignal(value[0])) {
// get its effect
let effect = value[1];
// install effect for DOM node
let reaction = effect(domNode, attribute);
// compute initial value as reaction to initial signal value
value = reaction(value);
}
// return the - possibly signal-transformed - value
return value;
};
let handleVariables = domNode => {
// Text node?
if (isText(domNode)) {
// yes, try to see whether it contains a plug
let value = plugs2Values[domNode[TEXTCONTENT]];
// it does?
if (value != undefined) {
// yes, see if it is a signal, too
let parent = domNode.parentNode;
let signal = isSignal(value) && value;
value = handleSignal(domNode, value);
// and - for text - assign its initial text content
if (/string|number|boolean/.test(typeof value)) DEFAULT_EFFECT(domNode, value);
else if (parent && signal && "value" in signal && value instanceof Node) {
// for signals containing a piece of DOM...
// remove text node
domNode[REMOVE]();
// set its initial DOM content on the parent of the text node
parent[APPENDCHILD](value);
// and update the signal to point to the parent, so later render(signal,...) works as expected
signal.value = parent;
}
}
}
// HTMLElement node?
if (!isFunction(domNode[GETATTRIBUTENAMES])) return; // no (also exit for Text nodes)
// yes, loop through its attributes:
for (let attribute of domNode[GETATTRIBUTENAMES]()) {
// getting the appropriate effect and initial value for each
let [effect, value] = handleEffect(attribute, domNode);
// and executing that effect initially once
effect(domNode, value);
}
};
// API
// replace the string-template variables with unique 'plug' strings s.t. the resulting fully instantiated string can be DOM-parsed.
// Along the way, remember the association of plugs with their corresponding variable values
export let html = (fragments, ...values) => {
let n = fragments.length;
let m = values.length;
// otherwise initialize a result strings array (we need to copy, because fragments originating from tagged string template literals are read-only,
// i.e. can not be re-assigned in-place)
let strings = [];
// for all string fragments, in order:
for (let i = 0, plug; i < n; i++) {
// calculate unique string-valued variable filler a.k.a. 'plug' that's unlikely to be confused with a real attribute value or text content
plug = i < m ? `_${++plugCounter}${Math.random() * 1e18}_` : "";
// the fragment with right-adjacent variable is replaced by the fragment with right-adjacent 'plug'
strings[i] = fragments[i] + plug;
// and we remember the association between 'plug' and original variable value
plugs2Values[plug] = values[i];
}
return strings.join("");
};
// render plugged string-template HTML under a root node
export let render = (rootNode, pluggedHTML = "", domTree) => {
let aSignal = isSignal(rootNode);
domTree = domTree ?? aSignal ? rootNode.value : d.createDocumentFragment();
// extract a list of child nodes from parsed, string-template-variables-plugged HTML text
let domHTML = extractNodes(parse(pluggedHTML));
// create a corresponding DOM tree, reparenting the child nodes under a new document fragment
for (let domNode of domHTML) {
domTree[APPENDCHILD](domNode);
}
// walk the entire DOM tree under the document fragment...
for (let treeWalker = d.createTreeWalker(domTree, 5); treeWalker.nextNode(); ) {
// ... handling its string template variables-turned-unique-plugs
handleVariables(treeWalker.currentNode);
}
// finally, append the DOM tree under the root node provided
return aSignal ? domTree : rootNode ? rootNode[APPENDCHILD](domTree) : domTree;
};
// provide plain and computed signals
export class Signal {
// private instance variables
#value;
#subscribers;
#effects;
constructor(initialValue) {
this.#value = initialValue;
this.#subscribers = new Set();
this.#effects = new WeakMap();
}
subscribe(domNode, attribute, reaction = DEFAULT_REACTION) {
this.#subscribers.add(domNode);
if (isSignal(domNode)) return;
let effect = deriveEffect(attributeType(attribute));
let effects = this.#effects.get(domNode) || new Set();
effects.add((newValue, node) => effect(node, reaction(newValue)));
this.#effects[SET](domNode, effects);
return this.#value;
}
get value() {
return this.#value;
}
set value(newValue) {
this.#value = newValue;
for (let domNode of this.#subscribers) {
if (isSignal(domNode)) {
domNode.value = undefined;
continue;
}
for (let effect of this.#effects[GET](domNode)) {
effect(newValue, domNode);
}
}
}
// define a derived signal, which applies a value transform a.k.a. reaction whenever a signal value changes
computed(reaction, when = []) {
when.forEach(signal => signal[SUBSCRIBE](this));
return [
this,
(domNode, attribute) => {
this[SUBSCRIBE](domNode, attribute, reaction);
return reaction;
}
];
}
}