Unreachable Code

ID

javascript.unreachable_code

Severity

low

Remediation Complexity

auto_fix

Remediation Risk

low

Remediation Effort

low

Resource

Dead Code

Language

JavaScript

Tags

dead-code, reliability

Description

Reports a statement that can never execute because control flow always leaves the enclosing region before reaching it. The analysis walks the function’s control-flow graph: code becomes unreachable after an unconditional jump (return, throw, break, continue), after a never-returning call (process.exit(…​)), or inside an always-taken constant branch. Only the first statement of each dead region is flagged.

Hoisted forms (function, async function, function* declarations and empty statements ;) are not reported, because the engine moves them to the start of the enclosing scope, so they remain reachable regardless of where they appear textually.

Rationale

  • Unreachable code is dead weight — it ships in the bundle, costs reviewer attention, and very often hides a logic mistake (the early return was meant to be inside an if, the break was meant to be after the work, etc.).

  • Unlike many style rules, this one nearly always points at a real bug. The author either expected the early exit to be conditional, or expected the trailing code to run before exiting.

// Bad
function fetchUser(id) {
  return cache.get(id);
  cache.set(id, build(id));   // never runs
}

Remediation

Either remove the unreachable statement, or move it before the terminator if it was supposed to run.

// Good
function fetchUser(id) {
  if (cache.has(id)) {
    return cache.get(id);
  }
  const user = build(id);
  cache.set(id, user);
  return user;
}

References