Static node hoisting

Static node hoisting is one of Vue.js's compiler optimization strategies. It identifies static nodes in templates that never change and hoists them outside the render function to avoid performance overhead during repeated rendering.

Principle of Static Node Hoisting

Vue's template compiler analyzes the template structure during the compilation phase and marks static nodes (i.e., nodes without any dynamic bindings or directives). These nodes do not change during component updates, so they can be extracted outside the render function and created only once during initialization.

<div>
  <h1>Static Title</h1>
  <p>{{ dynamicContent }}</p>
</div>

In this example, the <h1> tag is a static node, while the <p> tag is a dynamic node because it contains an interpolation expression. The compiled code will hoist the static node:

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

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

Advantages of Static Node Hoisting

1. Reduces Virtual DOM Creation Overhead: Static nodes only need to create a virtual DOM node once and can be reused in subsequent updates.
2. Optimizes the Patch Process: The diff algorithm can completely skip comparisons for static nodes.
3. Improves Memory Efficiency: Multiple component instances can share references to the same static node.

Conditions for Identifying Static Nodes

The Vue compiler considers the following nodes as static:

• Pure text nodes
• Element nodes without any dynamic bindings
• Nodes without directives like v-if or v-for
• Nodes without interpolation expressions {{ }}
• Nodes without dynamic attribute bindings

<!-- Static node -->
<div class="container">
  <p>Hello World</p>
</div>

<!-- Dynamic node -->
<div :class="containerClass">
  <p>{{ message }}</p>
</div>

Hoisting Multi-Level Static Trees

Vue can identify and hoist entire static subtrees, not just individual nodes:

<div>
  <header>
    <h1>App Title</h1>
    <nav>
      <ul>
        <li>Home</li>
        <li>About</li>
      </ul>
    </nav>
  </header>
  <main>{{ content }}</main>
</div>

In this example, the entire <header> and its child nodes will be hoisted as a static tree:

const _hoisted_1 = /*#__PURE__*/_createVNode("header", null, [
  /*#__PURE__*/_createVNode("h1", null, "App Title"),
  /*#__PURE__*/_createVNode("nav", null, [
    /*#__PURE__*/_createVNode("ul", null, [
      /*#__PURE__*/_createVNode("li", null, "Home"),
      /*#__PURE__*/_createVNode("li", null, "About")
    ])
  ])
], -1 /* HOISTED */)

Static Hoisting and Server-Side Rendering

Static node hoisting is particularly effective in SSR because:

1. Static content can be cached as strings during server-side rendering.
2. The client can directly reuse this static content during hydration.
3. Reduces the overhead of virtual DOM creation during server-side rendering.

// In SSR, static nodes are serialized as strings
const staticNode = '<h1>Static Title</h1>'

Techniques to Force Static Hoisting

Sometimes, we want to ensure certain nodes are statically hoisted. Here are some methods:

1. Use the v-once directive to force a node to render only once.
2. Extract static content into child components.
3. Use computed properties to cache dynamic content.

<template>
  <div v-once>
    <h1>This will only render once</h1>
  </div>
</template>

Edge Cases of Static Hoisting

In some cases, static hoisting may not work as expected:

1. Nodes with the v-pre directive will not be hoisted.
2. Nodes with v-bind binding static values will not be hoisted.
3. Slot content is not statically hoisted by default.

<!-- Will not be hoisted -->
<div v-pre>
  <p>This won't be hoisted</p>
</div>

<!-- Will not be hoisted -->
<div :class="'static-class'">
  <p>This looks static but won't be hoisted</p>
</div>

Performance Impact of Static Hoisting

A simple performance test can demonstrate the effect of static hoisting:

// Component without static hoisting
const NonHoistedComponent = {
  template: `
    <div>
      <h1>Same Title</h1>
      <p>{{ count }}</p>
    </div>
  `,
  data() {
    return { count: 0 }
  }
}

// Component with static hoisting
const HoistedComponent = {
  template: `
    <div>
      <h1>Same Title</h1>
      <p>{{ count }}</p>
    </div>
  `,
  data() {
    return { count: 0 }
  }
}

// Performance test
console.time('non-hoisted')
for (let i = 0; i < 10000; i++) {
  render(h(NonHoistedComponent))
}
console.timeEnd('non-hoisted')

console.time('hoisted')
for (let i = 0; i < 10000; i++) {
  render(h(HoistedComponent))
}
console.timeEnd('hoisted')

The test results typically show a significant performance advantage for the statically hoisted version.

Compiler Configuration and Static Hoisting

In the Vue compiler, the following configurations can affect static hoisting behavior:

const { compile } = require('@vue/compiler-dom')

const { code } = compile(template, {
  hoistStatic: true, // Enable static hoisting
  cacheHandlers: true, // Cache event handlers
  prefixIdentifiers: true // Prefix identifiers
})

Static Hoisting and TypeScript

When using TypeScript, the types of hoisted nodes are correctly inferred:

const _hoisted_1 = /*#__PURE__*/_createVNode("div", null, "static", -1 /* HOISTED */)

// TypeScript correctly infers _hoisted_1 as type VNode

Debugging Static Hoisting

You can check if static hoisting is working by:

1. Inspecting the compiled render function code.
2. Using Vue Devtools to examine the virtual DOM structure.
3. Adding comments: true to the compiler options to view hoisting annotations.

const { code } = compile(template, {
  comments: true // Preserve compilation comments
})

Static Hoisting and the Composition API

In the Composition API, static hoisting behaves the same as in the Options API:

<script setup>
import { ref } from 'vue'

const count = ref(0)
</script>

<template>
  <div>
    <h1>Static Title</h1>
    <p>{{ count }}</p>
  </div>
</template>

After compilation, the <h1> tag will still be statically hoisted.

Limitations of Static Hoisting

Static hoisting is not a silver bullet and has the following limitations:

1. For large amounts of static content, memory usage may increase.
2. In extreme cases, excessive hoisting may slow down initialization.
3. Static content inside dynamic components will not be hoisted.

<component :is="dynamicComponent">
  <p>This static content won't be hoisted</p>
</component>

Best Practices for Static Hoisting

To maximize the benefits of static hoisting:

1. Split large static content into separate components.
2. Avoid wrapping large static content inside dynamic nodes.
3. Use v-once judiciously for nodes that rarely change.
4. Regularly inspect compiled output to confirm hoisting effectiveness.

<!-- Good practice -->
<StaticHeader/>
<DynamicContent :data="data"/>

<!-- Less ideal practice -->
<div v-for="item in list">
  <div class="static-content">...</div>
</div>

Comparison with Other Frameworks

Comparing with similar optimizations in other frameworks:

1. React achieves similar effects with React.memo.
2. Svelte automatically optimizes static content during compilation.
3. Angular's change detection strategy naturally skips static content.

// React's similar optimization
const MemoizedComponent = React.memo(() => (
  <div>
    <h1>Static Title</h1>
    <p>{dynamicContent}</p>
  </div>
))

Future Developments in Static Hoisting

Vue 3's static hoisting is more aggressive than Vue 2's:

1. Supports static hoisting in more scenarios.
2. The improved compiler can identify more static patterns.
3. Better integration with new features like Suspense.

<Suspense>
  <template #default>
    <AsyncComponent/>
  </template>
  <template #fallback>
    <div>Loading...</div> <!-- This static content will be hoisted -->
  </template>
</Suspense>