elseIfDuplicates

Reports duplicate conditions in if-else-if chains that make code unreachable.

✅ This rule is included in the ts logical preset.

When an if-else-if chain contains duplicate conditions, the branch with the duplicate condition will never execute. This is because the earlier identical condition will always be evaluated first and match when true, preventing the duplicate condition from ever being reached. Duplicate conditions typically indicate a copy-paste error or a logic mistake in the code.

Examples

❌ Incorrect

if (status === "pending") {
  handlePending();
} else if (status === "active") {
  handleActive();
} else if (status === "pending") {
  handlePendingAgain();
}

if (isValid && isActive) {
  processValid();
} else if (isPending) {
  processPending();
} else if (isValid && isActive) {
  processValidAgain();
}

if (count === 1) {
  handleOne();
} else if (count === 2) {
  handleTwo();
} else if (count === 1) {
  handleOneAgain();
}

✅ Correct

if (status === "pending") {
  handlePending();
} else if (status === "active") {
  handleActive();
} else if (status === "completed") {
  handleCompleted();
}

if (isValid && isActive) {
  processValid();
} else if (isPending) {
  processPending();
} else if (isValid && !isActive) {
  processInactive();
}

if (count === 1) {
  handleOne();
} else if (count === 2) {
  handleTwo();
} else if (count === 3) {
  handleThree();
}

Options

This rule is not configurable.

When Not To Use It

This rule should always be enabled. Duplicate conditions in if-else-if chains represent unreachable code and are almost always a mistake.

Further Reading

• MDN: if…else
• MDN: Conditional statements

Equivalents in Other Linters

• Biome: noDuplicateElseIf
• ESLint: no-dupe-else-if
• Oxlint: eslint/no-dupe-else-if