# Behavior traces — sveltejs/svelte > Canonical page: https://codevetter.com/unpack/svelte/behavior-traces # Behavior traces — sveltejs/svelte Scanned commit 636eaaaa6f064b55072e7d192bb76dc9d8c4516e (2026-09-18). Generated by codevetter codevetter 1.12.0 on 2026-09-22. Three flows dominate. (1) Compile: `compile()` resets global state, validates options, parses, strips TS, analyzes, transforms, and re-attaches a public AST. (2) Client startup: `mount()` initialises DOM operation caches, opens a root effect and an error boundary, pushes a component context, invokes the compiled component function, and returns its exports. (3) Update: a `$state` write sets a source, which schedules its reactions into a `Batch`, which is flushed on the microtask or synchronously via `flushSync`. A fourth, the SSR→hydrate round trip, is what the test harness itself exercises on every fixture. - Compile trace: `remove_bom` → `state.reset` → `validate_component_options` → `_parse(source)` → destructure `parsed.options` into `combined_options` → conditional `remove_typescript_nodes` → `analyze_component(parsed, source, combined_options)` → `transform_component(analysis, source, combined_options)` → `to_public_ast`. — [packages/svelte/src/compiler/index.js#L23-60](https://github.com/sveltejs/svelte/blob/636eaaaa6f064b55072e7d192bb76dc9d8c4516e/packages/svelte/src/compiler/index.js#L23-L60) - Transform trace (client): `client_component` seeds state with `hoisted: [b.import_all('$', 'svelte/internal/client')]` and an empty `templates` Map, walks the template AST with the visitor table, appends `template.body` into `component_block.body`, pushes a `$$css` const when styles are injected, then re-orders `module.body` so imports stay on top followed by hoisted statements. — [packages/svelte/src/compiler/phases/3-transform/client/transform-client.js#L135-200](https://github.com/sveltejs/svelte/blob/636eaaaa6f064b55072e7d192bb76dc9d8c4516e/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js#L135-L200) [packages/svelte/src/compiler/phases/3-transform/client/transform-client.js#L377-500](https://github.com/sveltejs/svelte/blob/636eaaaa6f064b55072e7d192bb76dc9d8c4516e/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js#L377-L500) - Mount trace: `mount()` → `_mount()` → `init_operations()` → `component_root(...)` → create an anchor text node via `target.appendChild(create_text())` → `boundary(anchor, { pending }, cb)` → `push({})` to open a component context → assign `props.$$events` if `events` given → set `should_intro = intro` → call `Component(anchor_node, props)`. — [packages/svelte/src/internal/client/render.js#L156-200](https://github.com/sveltejs/svelte/blob/636eaaaa6f064b55072e7d192bb76dc9d8c4516e/packages/svelte/src/internal/client/render.js#L156-L200) - Hydrate trace: `hydrate()` forces `intro ?? false`, scans forward from `get_first_child(target)` for a comment node whose `data === HYDRATION_START`, throws `HYDRATION_ERROR` if absent, calls `set_hydrating(true)` + `set_hydrate_node(anchor)`, then delegates to the same `_mount` with the discovered anchor. — [packages/svelte/src/internal/client/render.js#L95-145](https://github.com/sveltejs/svelte/blob/636eaaaa6f064b55072e7d192bb76dc9d8c4516e/packages/svelte/src/internal/client/render.js#L95-L145) - Hydration failure recovery: non-Svelte errors are `console.warn`ed, `options.recover === false` escalates to `e.hydration_failed()`, otherwise `init_operations()` + `clear_text_content(target)` runs and the component falls back to a fresh `mount(component, options)` — with `hydrating`/`hydrate_node` restored in `finally`. — [packages/svelte/src/internal/client/render.js#L118-150](https://github.com/sveltejs/svelte/blob/636eaaaa6f064b55072e7d192bb76dc9d8c4516e/packages/svelte/src/internal/client/render.js#L118-L150) - Read trace: `get(signal)` adds the signal to `captured_signals`, registers the dependency on `active_reaction`, and takes one of two registration paths depending on whether `REACTION_IS_UPDATING` is set — the post-`await` path deduplicates with `includes.call` because `remove_reactions` would otherwise disconnect still-live deps. — [packages/svelte/src/internal/client/runtime.js#L548-605](https://github.com/sveltejs/svelte/blob/636eaaaa6f064b55072e7d192bb76dc9d8c4516e/packages/svelte/src/internal/client/runtime.js#L548-L605) - Synchronous flush trace: `flushSync(fn)` sets `is_flushing_sync`, flushes any non-fork `current_batch` before running `fn`, then loops `flush_tasks()` + `current_batch.flush()` until `current_batch === null`, restoring the previous flag in `finally`. — [packages/svelte/src/internal/client/reactivity/batch.js#L1033-1058](https://github.com/sveltejs/svelte/blob/636eaaaa6f064b55072e7d192bb76dc9d8c4516e/packages/svelte/src/internal/client/reactivity/batch.js#L1033-L1058) - Batch flush trace: `Batch.flush()` clears DEV `source_stacks`, sets `is_processing = true` and `current_batch = this`, runs the private `#process()`, then in `finally` resets `flush_count`, `last_scheduled_effect`, `collected_effects`, `legacy_updates`, `current_batch`, `batch_values`, and `old_values`. — [packages/svelte/src/internal/client/reactivity/batch.js#L668-698](https://github.com/sveltejs/svelte/blob/636eaaaa6f064b55072e7d192bb76dc9d8c4516e/packages/svelte/src/internal/client/reactivity/batch.js#L668-L698) - `{@html}` trace: the `html()` block runs inside a `template_effect`, short-circuits on unchanged value, uses `parent_node.innerHTML = value` when it is the only child, otherwise builds a `