Duplicate Else-If Conditions

ID

javascript.duplicate_else_if_conditions

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

JavaScript

Tags

dead-code, reliability, suspicious-comparison

Description

Reports if / else if / …​ chains in which two branches share the same condition. The second occurrence is unreachable — anything matching the duplicated condition fires the first branch — and almost always a copy-paste error.

Rationale

Duplicate conditions are dead code that masks a bug:

  • The developer probably intended to change the operator or comparison value in one of the branches.

  • Tests typically don’t catch the duplicate because the data falls into the first branch as designed.

  • The bug stays latent until a different code path or a refactor exposes it.

// Bad - second branch is unreachable
if (x === 1) {
  return "one";
} else if (x === 2) {
  return "two";
} else if (x === 1) {
  return "unreachable";
}

Remediation

Either fix the duplicated condition (likely the second was meant to be a different value) or remove it.

// Good
if (x === 1) {
  return "one";
} else if (x === 2) {
  return "two";
} else if (x === 3) {
  return "three";
}

References