core/packages/runtime-core/src/patchFlags.ts

// Patch flags are optimization hints generated by the compiler.
// when a block with dynamicChildren is encountered during diff, the algorithm
// enters "optimized mode". In this mode, we know that the vdom is produced by
// a render function generated by the compiler, so the algorithm only needs to
// handle updates explicitly marked by these patch flags.

// Patch flags can be combined using the | bitwise operator and can be checked
// using the & operator, e.g.
//
//   const flag = TEXT | CLASS
//   if (flag & TEXT) { ... }
//
// Check the `patchElement` function in './createRednerer.ts' to see how the
// flags are handled during diff.

// Indicates an element with dynamic textContent (children fast path)
export const TEXT = 1

// Indicates an element with dynamic class.
// The compiler also pre-normalizes the :class binding:
// - b -> normalize(b)
// - ['foo', b] -> 'foo' + normalize(b)
// - { a, b: c } -> (a ? a : '') + (b ? c : '')
// - ['a', b, { c }] -> 'a' + normalize(b) + (c ? c : '')
export const CLASS = 1 << 1

// Indicates an element with dynamic style
// The compiler pre-compiles static string styles into static objects
// + detects and hoists inline static objects
// e.g. style="color: red" and :style="{ color: 'red' }" both get hoisted as
//   const style = { color: 'red' }
//   render() { return e('div', { style }) }
export const STYLE = 1 << 2

// Indicates an element that has non-class/style dynamic props.
// Can also be on a component that has any dynamic props (includes class/style).
// when this flag is present, the vnode also has a dynamicProps array that
// contains the keys of the props that may change so the runtime can diff
// them faster (without having to worry about removed props)
export const PROPS = 1 << 3

// Indicates an element with props with dynamic keys. When keys change, a full
// diff is always needed to remove the old key. This flag is mutually exclusive
// with CLASS, STYLE and PROPS.
export const FULL_PROPS = 1 << 4

// Indicates a fragment or element with keyed or partially-keyed v-for children
export const KEYED = 1 << 5

// Indicates a fragment or element that contains unkeyed v-for children
export const UNKEYED = 1 << 6

// Indicates a component with dynamic slots (e.g. slot that references a v-for
// iterated value, or dynamic slot names).
// Components with this flag are always force updated.
export const DYNAMIC_SLOTS = 1 << 7

// Indicates an element with ref. This includes static string refs because the
// refs object is refreshed on each update and all refs need to set again.
export const REF = 1 << 8