phoenix-typescript

name	phoenix-typescript
description	TypeScript conventions and patterns for any TypeScript code in the Phoenix monorepo — including js/packages/, app/, and any other TS directories. Use this skill whenever writing, reviewing, or modifying TypeScript code — new functions, types, exports, tests, or refactors. Also trigger when the user asks about TS patterns, naming conventions, or best practices for this project.
metadata	{"internal":true}

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.
• Pure utilities must not reach for hardcoded module constants or ambient state (Date.now(), new Date(), config singletons) deep inside their bodies. Expose every such value as an optional parameter whose default is the constant, so callers — and especially tests — can override it and the function's output depends only on its inputs. The module constant becomes the default, prefixed DEFAULT_.

const DEFAULT_ZOOM_FACTOR = 2;
const DEFAULT_MIN_WINDOW_MS = 60_000;

// Bad — reads a module constant and the clock internally; output isn't
// determined by inputs, so tests can't pin the window or the factor.
function zoomOut(value: TimeRange): TimeRange {
  const now = new Date();
  return widen(value, ZOOM_FACTOR, now);
}

// Good — constants are overridable defaults; pass `now` to make it pure.
function zoomOut({
  value,
  now = new Date(),
  zoomFactor = DEFAULT_ZOOM_FACTOR,
  minWindowMs = DEFAULT_MIN_WINDOW_MS,
}: {
  value: TimeRange;
  /** Reference "now". Defaults to the current time. */
  now?: Date;
  /** Multiplier applied to the window duration. */
  zoomFactor?: number;
  /** Smallest window the zoom will produce. */
  minWindowMs?: number;
}): TimeRange {
  return widen(value, zoomFactor, now, minWindowMs);
}

/**
 * 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.