ivi HTML Template Language

@ivi/htm module provides an interface for creating ivi templates with template literals:

• htm creates a template with HTMLElement nodes.
• svg creates a template with SVGElement nodes.

htm`
  <div id="app">
    <h1>Template Example</h1>
    <div class="content">
      ${condition ? "true" : "false"}
    </div>
  </div>
`;

Multiple Root Nodes

htm`
  <div></div>
  ${expr}
  text
  <div></div>
`

Childless elements can be self closed with a /> syntax

htm`
  <div
    class="a"
  />
`;

Whitespaces

1. Whitespaces around newlines are removed:

<div>
  <p></p>
  ab
  <p></p>
</div>

<div><p></p>ab<p></p></div>

2. Inline whitespaces are collapsed into one whitespace:

<div>  <span>  a  b  </span>  </div>

<div> <span> a b </span> </div>

3. Whitespaces around newlines in text nodes are collapsed into one whitespace:

<div>
  ab
  cd
</div>

<div>ab cd</div>

4. Vertical tab \v character prevents from removing all whitespaces around newlines:

<div>
  <b>1</b>
  \v item left
<div>

<div><b>1</b> item left</div>

Element Attributes Syntax

HTML Template Language supports additional syntax to work with DOM properties, events, etc.

• <div name="value" /> - Static attribute.
• <div name /> - Static attribute.
• <div name=${expr} /> - Dynamic attribute element.setAttribute(name, expr).
• <div .name=${expr} /> - Property element[name] = expr.
• <div *name=${expr} /> - Property element[name] = expr, diffs against a DOM value.
• <div ~name="value" /> - Static style <div style="name:value;">.
• <div ~name=${expr} /> - Dynamic style element.style.setProperty(name, expr).
• <div @name=${expr} /> - Event element.addEventListener(name, expr).
• <div ${directive} /> - Client-Side Element Directive directive(element).
• <div &=${directive} /> - Client-Side Element Directive directive(element).
• <div &:ssr=${directive} /> - Element Directive that works during Client-Side and Server-Side Rendering directive(element, hydrate).
• <div .textContent=${expr} /> - Text content.

Attributes

• <div name="value" /> - Static attribute with a value <div name="value">.
• <div name /> - Static attribute without a value <div name>.
• <div name=${expr} /> - Dynamic attribute element.setAttribute(name, expr).

DOM attributes are assigned with Element.setAttribute(..).

When dynamic attribute has an undefined, null or false value, it will be removed from the DOM element with Element.removeAttribute(..) method.

Properties

• <div .name=${expr} /> - Property element[name] = expr.
• <div *name=${expr} /> - Property element[name] = expr, diffs against a DOM value.

Properties are assigned with an assignment operator Element.name = value.

Diffing with a DOM value is useful in use cases when we use <input> values to avoid triggering unnecessary input events.

Styles

• <div ~name="value" /> - Static style <div style="value">.
• <div ~name=${expr} /> - Dynamic style element.style.setProperty(name, expr).

Static styles are automatically merged with :style="value" attribute.

Dynamic styles are assigned with a CSSStyleDeclaration.setProperty(..) method.

When style has an undefined, null or false value, it will be removed with CSSStyleDeclaration.removeProperty(..) method.

Events

• <div @name=${expr} /> - Event element.addEventListener(name, expr).

Events are assigned with an EventTarget.addEventListener(..) method.

When event has an undefined, null or false value, it will be removed with EventTarget.removeEventListener(..) method.

Directives

• <div ${directive} /> - Client-Side Element Directive directive(element).
• <div &=${directive} /> - Client-Side Element Directive directive(element).
• <div &:ssr=${directive} /> - Element Directive that works during Client-Side and Server-Side Rendering directive(element, hydrate).

Directive is a function that is invoked each time template is updated and receives a DOM element associated with a directive:

type ElementDirective = <E extends Element>(
  element: E,
  hydrate?: boolean,
) => void | string | { a?: string, c?: string; };

Directive function is invoked only when template is created with a different function, so if we are going to reuse the same function, it can be used as a DOM element created callback:

const Example = component((c) => {
  const onCreated = (innerElement) => {
    // ..
  };
  return () => htm`
    <div>
      <div class="Inner" ${onCreated} />
    </div>
  `;
});

Directives can be used not just as a simple DOM created callbacks, but also as stateful directives. E.g.

function createStatefulDirective() {
  // Internal state that stores previous value.
  let prev;
  // Returns a factory that creates directive functions.
  return (next) => (element) => {
    // Check if previous value has been changed.
    if (prev !== next) {
      prev = next;
      // Updates textContent only when input value is changed.
      element.textContent = next;
    }
  };
}

const Example = component((c) => {
  const directive = createStatefulDirective();
  return (i) => htm`
    <div ${directive(i)} />
  `;
});

Text Content

• <div .textContent=${expr} /> - Text Content element.textContent = expr.

Text content property can be used as an optimization that slightly reduces memory consumption for elements with a text child. It will create a text node with a Node.textContent property and won't have any stateful nodes associated with a text node.

Text content value should have an undefined, null, false, string or a number type.