Phoenix TypeScript Conventions

These conventions apply to all TypeScript in the Phoenix monorepo — the app/ frontend, the js/packages/ libraries (phoenix-client, phoenix-cli, phoenix-evals, phoenix-mcp, phoenix-otel, phoenix-config), examples, and benchmarks.

Before writing new code, explore the directory you're working in to understand existing patterns — then follow these rules.

Naming

Self-documenting names eliminate mental parsing for the next reader.

• Variables must not use single letters — even loop counters benefit from index, row, char.
• Complex conditions should be extracted into named booleans so code reads as prose.
• Booleans must use verb prefixes: isAllowed, hasError, canSubmit — not allowed, error.
• Function names must start with an action verb that describes what the function does: getUser, normalizeTimestamp, logEvent, parseResponse, buildQuery — not user(), timestamp(), event().

// Bad — single letters and ambiguous names
for (let i = 0; i < s.length; i++) {
  const d = s[i].ts - s[i - 1]?.ts;
  const r = fn(s[i].v);
}

// Good — self-documenting
for (let index = 0; index < spans.length; index++) {
  const elapsed = spans[index].timestamp - spans[index - 1]?.timestamp;
  const result = normalizeValue(spans[index].value);
}

// Bad — boolean without verb prefix, condition inline
<Button isDisabled={!permission || submitting}>

// Good — named boolean with verb prefix
const isDisabled = !hasPermission || isSubmitting;
<Button isDisabled={isDisabled}>

Functions

• Functions with 2+ parameters should use object destructuring over positional args — this makes call sites readable and resilient to reordering.
• Object parameters should be documented with JSDoc using @param dot notation so editors surface descriptions on hover and during autocomplete.
• Behavior should be built from composition (functions and hooks), not inheritance.
• Transforms should prefer functional purity over mutation — use map not reduce for element-wise transforms, return new objects instead of mutating.

/**
 * Fetch spans matching the given filters.
 * @param params - query parameters
 * @param params.projectId - project to query
 * @param params.timeRange - optional time window to restrict results
 * @param params.limit - max rows to return (default 100)
 */
function fetchSpans({
  projectId,
  timeRange,
  limit = 100,
}: {
  projectId: string;
  timeRange?: TimeRange;
  limit?: number;
}) {

Type Safety

TypeScript's type system is most valuable when it catches bugs at compile time rather than runtime.

• Type guards must be used to narrow complex union types; edge cases where discriminants might be missing must be tested.
• any must not be used; prefer unknown and narrow explicitly. If any is genuinely necessary (e.g., interfacing with an untyped external API), add a comment explaining why.
• Record<K, V> used as a lookup map (where keys may be absent) must include undefined in the value type — the repo does not enable noUncheckedIndexedAccess, so missing-key lookups silently return undefined while the type says V. Use Partial<Record<K, V>> for sparse maps or Record<K, V | undefined> when the key set is known but values are nullable.

// Bad — lookup returns string at compile time, undefined at runtime
const map: Record<string, string> = {};
const value = map["missing"]; // typed as string, actually undefined

// Good — forces a null check at every access site
const map: Partial<Record<string, string>> = {};
const value = map["missing"]; // typed as string | undefined

Reuse

Existing shared utilities must be checked before writing inline helpers. Duplicated logic should be extracted to a shared module. When working in js/packages/, check sibling packages for existing utilities before adding new dependencies or reimplementing.