performance/sequential-awaits · Sequential independent awaits

Awaits that don't use each other's results still run one after another; start them together.

Severity: info · Category: performance

What it checks

Flags an await in a load function (universal or server) that uses no result from any await before it. The requests serialize for no data-flow reason.

Detection uses the same conservative straight-line scan as performance/load-waterfall: forward taint through bindings and intermediate constants, callback-parameter shadowing respected, await parent() exempt. A container a callback over an earlier result fills (lists.forEach((l) => ids.add(l.id))) carries that result on. Awaiting an already-created promise (await somePromise, await store.ready) starts no request and is never flagged: only an await of a call, tagged template, new, or import() counts as starting work. A method of an object the load created itself, awaited for its effect alone (const auth = getAuth(); await auth.initialize()), is read as filling that object, so a later await that reads it depends on it.

Why it matters

Two independent requests awaited sequentially cost the sum of their latencies; started together they cost only the slowest. In a load function this is pure waste on every page visit. Promise.all gives the same data with no behavior change when the requests are truly independent.

How to fix

const [a, b] = await Promise.all([fetchA(), fetchB()]);

Limitations

Static data flow cannot see side-effect ordering. If an earlier await performs setup a later request relies on (sessions, locale, cache warming), the sequence is intentional. That is why this rule reports at info severity. Suppress a deliberate sequence per line with // svelte-vitals-disable-next-line performance/sequential-awaits, or raise/lower the severity in your config.

Mode differences

None. This rule reads source, the same .svelte and .ts files, everywhere it runs. The CLI, the Vite plugin’s build pass, and the live dashboard’s static baseline all report it identically, and the rendered-HTML pass never re-evaluates it. Scoping a run with --route skips it: component-scoped rules have no route to attribute a finding to.

Disabling

export default {
  rules: {
    'performance/sequential-awaits': 'off'
  }
};