ivi Template Language

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

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

htm`
div :id='app'
  h1 'Template Example'
  div.content
    ${condition ? "true" : "false"}
`

In this example we are creating a root element <div id="app"> with two children: <h1>TemplateExample</h1> and <div class="content">{..}</div>. In an HTML it could look something like that:

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

As we can see from this example, indentation level is used for children nesting. Also, children node can be nested by declaring them on the same line as their parent. E.g.

htm`div a ${expr}`

HTML:

<div><a>{expr}</a></div>

Or in a mixed form:

htm`
div a ${0}
  ${1}
`

HTML:

<div><a>{0}</a>{1}</div>

Inline nodes that can't have any children will be rendered as siblings:

htm`div 'prefix' ${expr} 'suffix'`

HTML:

<div>prefix{expr}suffix</div>

Text Nodes

Static text nodes are declared either with 'text', "text" or #'text'# syntax.

Text nodes are escaped automatically:

htm`div 'escape & <'`

HTML:

<div>escape &amp; &lt;</div>

Multiple Root Nodes

htm`
  div 'a'
  ${expr}
  text'
  div 'b'
`

Element Properties Syntax

• div.classA.classB - Static class names <div class="classA classB">
• div${expr} - Dynamic class names element.className = expr
• div :name='value' - Static attribute with a value <div name="value">.
• div :name - Static attribute without a value <div name>.
• div :name=${expr} - Dynamic attribute element.setAttribute(name, expr).
• 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="value">.
• div ~name=${expr} - Dynamic style element.style.setProperty(name, expr).
• div @name=${expr} - Event element.addEventListener(name, expr).
• div =${expr} - Text Content element.textContent = expr.
• div &=${directive} - Client-Side Element Directive directive(element).
• div &:ssr=${directive} - Element Directive that works during Client-Side and Server-Side Rendering directive(element, hydrate).

Element properties can be declared on the same line as element or with an indentation level.

htm`
div :inline-attr1 :inline-attr2
  :indented-attr
    :can-be-indented-with-any-amount-of-spaces
  child-element
`

Class Names

Static class names are declared with a . character immediately after a tag name:

htm`div.class-one.class-two ${expr}`

HTML:

<div class="class-one class-two"></div>

Dynamic class names are declared with an expression immediately after a tag name:

htm`div${condition ? "class-one" : "class-two"}`

HTML:

<div class={condition ? "class-one" : "class-two"}></div>

Static and dynamic class names cannot be mixed together.

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.

Attribute values are escaped automatically:

htm`div :name='escape & "'`

HTML:

<div name="escape &amp; &quot;"></div>

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.

Text Content

• div =${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.

Directives

• 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.outer
      div.inner &=${onCreated}
  `;
});

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)}
  `;
});