Compile-time optimization analysis

Compile-Time Optimization Analysis

Vue.js's compile-time optimizations are a key aspect of the framework's performance improvements. By statically analyzing templates, the compiler can identify dynamic and static content, thereby generating more efficient render function code. These optimizations are particularly noticeable in large-scale applications, significantly reducing runtime overhead.

Static Node Hoisting

The compiler detects static nodes in templates (nodes without any dynamic bindings) and hoists these nodes outside the render function. This means these nodes only need to be created once during the initial render and can be reused directly in subsequent updates.

<div>
  <h1>Static Title</h1>  <!-- Static node -->
  <p>{{ dynamicText }}</p>  <!-- Dynamic node -->
</div>

The compiled render function would look something like this:

const _hoisted_1 = /*#__PURE__*/_createVNode("h1", null, "Static Title", -1 /* HOISTED */)

function render() {
  return (_openBlock(), _createBlock("div", null, [
    _hoisted_1,
    _createVNode("p", null, _toDisplayString(_ctx.dynamicText), 1 /* TEXT */)
  ]))
}

Static Attribute Hoisting

Not only entire nodes but also individual static attributes are hoisted. When an element has multiple attributes with some being static, the compiler lifts these static attributes into constants.

<div 
  class="static-class" 
  :style="dynamicStyle"
  data-test="static-data"
></div>

The compilation result separates static and dynamic attributes:

const _hoisted_1 = {
  class: "static-class",
  "data-test": "static-data"
}

function render() {
  return (_openBlock(), _createBlock("div", {
    ..._hoisted_1,
    style: _normalizeStyle(_ctx.dynamicStyle)
  }, null, 4 /* STYLE */))
}

Pre-Stringification

When encountering consecutive static nodes, the compiler serializes them into strings and sets them all at once during runtime using innerHTML. This reduces the overhead of creating numerous virtual DOM nodes.

<div>
  <p>Paragraph 1</p>
  <p>Paragraph 2</p>
  <p>Paragraph 3</p>
  <!-- More static p tags -->
</div>

The compiled output might look like:

const _hoisted_1 = /*#__PURE__*/_createStaticVNode("<p>Paragraph 1</p><p>Paragraph 2</p><p>Paragraph 3</p>", 3)

function render() {
  return (_openBlock(), _createBlock("div", null, [
    _hoisted_1
  ]))
}

Event Handler Caching

For inline event handlers, the compiler automatically generates caches to avoid creating new function instances on every render.

<button @click="count++">Increment</button>

This would be compiled as:

function render() {
  return (_openBlock(), _createBlock("button", {
    onClick: _cache[0] || (_cache[0] = $event => (_ctx.count++))
  }, "Increment"))
}

Patch Flag Optimization

The compiler analyzes the dynamic parts of nodes and generates optimization hints for the virtual DOM patching process. These flags inform the runtime about which parts need comparison and which can be skipped.

<div :class="dynamicClass">
  <span :title="dynamicTitle">{{ dynamicText }}</span>
  <p>Static content</p>
</div>

The compilation result includes PatchFlags:

function render() {
  return (_openBlock(), _createBlock("div", {
    class: _normalizeClass(_ctx.dynamicClass)
  }, [
    _createVNode("span", {
      title: _ctx.dynamicTitle
    }, _toDisplayString(_ctx.dynamicText), 9 /* TEXT, PROPS */, ["title"]),
    _createVNode("p", null, "Static content")
  ], 2 /* CLASS */))
}

Block Tree Optimization

Through the coordination of openBlock and createBlock, the compiler establishes a block tree structure. This structure allows static subtrees to be skipped during updates, comparing only the dynamic parts.

<div>
  <div v-if="show">
    <p>{{ text }}</p>
  </div>
  <ul>
    <li v-for="item in items" :key="item.id">{{ item.name }}</li>
  </ul>
</div>

The compiled block structure:

function render() {
  return (_openBlock(), _createBlock("div", null, [
    (_ctx.show)
      ? (_openBlock(), _createBlock("div", { key: 0 }, [
          _createVNode("p", null, _toDisplayString(_ctx.text), 1 /* TEXT */)
        ]))
      : _createCommentVNode("v-if", true),
    (_openBlock(), _createBlock("ul", null, [
      (_openBlock(true), _createBlock(_Fragment, null, _renderList(_ctx.items, (item) => {
        return (_openBlock(), _createBlock("li", { key: item.id }, _toDisplayString(item.name), 1 /* TEXT */))
      }), 128 /* KEYED_FRAGMENT */))
    ]))
  ]))
}

Dynamic Component Optimization

For dynamic components, the compiler generates more efficient code to handle component switching.

<component :is="currentComponent" />

Optimized compilation result:

function render() {
  return (_openBlock(), _createBlock(_resolveDynamicComponent(_ctx.currentComponent)))
}

Slot Content Optimization

The compiler distinguishes between static and dynamic slot content, hoisting static slot content.

<MyComponent>
  <template #header>
    <h1>Static Header</h1>
  </template>
  <template #default>
    <p>{{ dynamicContent }}</p>
  </template>
</MyComponent>

Compiled output:

const _hoisted_1 = [_createVNode("h1", null, "Static Header")]

function render() {
  return (_openBlock(), _createBlock(MyComponent, null, {
    header: _hoisted_1,
    default: () => [
      _createVNode("p", null, _toDisplayString(_ctx.dynamicContent), 1 /* TEXT */)
    ],
    _: 1 /* STABLE */
  }))
}

Static Root Node Handling

When the entire template consists of static content, the compiler handles it specially, generating simpler rendering logic.

<div>
  <h1>Static Page</h1>
  <p>This page never changes</p>
</div>

Might be compiled as:

const _hoisted_1 = /*#__PURE__*/_createStaticVNode("<div><h1>Static Page</h1><p>This page never changes</p></div>", 1)

function render() {
  return _hoisted_1
}

Custom Directive Optimization

For custom directives, the compiler performs different processing based on the directive's characteristics. Purely functional directives are statically hoisted.

<div v-my-static-directive></div>

Might generate:

const _hoisted_1 = /*#__PURE__*/_createVNode("div", {
  "directive": "my-static-directive"
}, null, 512 /* NEED_PATCH */)

function render() {
  return _hoisted_1
}

Conditional Branch Optimization

In conditional rendering, the compiler extracts static parts as much as possible to reduce redundant code in conditional branches.

<div>
  <template v-if="condition">
    <h1>Title</h1>
    <p>{{ dynamicText }}</p>
  </template>
  <template v-else>
    <h1>Title</h1>
    <p>Fallback text</p>
  </template>
</div>

Optimized compilation result:

const _hoisted_1 = _createVNode("h1", null, "Title")

function render() {
  return (_openBlock(), _createBlock("div", null, [
    (_ctx.condition)
      ? (_openBlock(), _createBlock(_Fragment, { key: 0 }, [
          _hoisted_1,
          _createVNode("p", null, _toDisplayString(_ctx.dynamicText), 1 /* TEXT */)
        ], 64 /* STABLE_FRAGMENT */))
      : (_openBlock(), _createBlock(_Fragment, { key: 1 }, [
          _hoisted_1,
          _createVNode("p", null, "Fallback text")
        ], 64 /* STABLE_FRAGMENT */))
  ]))
}

Static Class Name Merging

For mixed static and dynamic class names, the compiler intelligently merges them.

<div class="static-class" :class="dynamicClass"></div>

Compilation optimization:

function render() {
  return (_openBlock(), _createBlock("div", {
    class: _normalizeClass(["static-class", _ctx.dynamicClass])
  }, null, 2 /* CLASS */))
}

Dynamic Attribute Merging

Similarly, for mixed static and dynamic attributes, the compiler generates optimal merging logic.

<div id="static-id" :[dynamicAttr]="value"></div>

Compilation handling:

function render() {
  return (_openBlock(), _createBlock("div", _mergeProps({
    id: "static-id"
  }, _ctx.dynamicAttr ? { [_ctx.dynamicAttr]: _ctx.value } : null), null, 16 /* FULL_PROPS */))
}

Static Style Handling

For static style objects, the compiler performs constant hoisting.

<div style="color: red; font-size: 16px" :style="dynamicStyle"></div>

Compilation result:

const _hoisted_1 = {
  style: {
    color: 'red',
    'font-size': '16px'
  }
}

function render() {
  return (_openBlock(), _createBlock("div", {
    style: _normalizeStyle([_hoisted_1.style, _normalizeStyle(_ctx.dynamicStyle)])
  }, null, 4 /* STYLE */))
}