consistent-existence-index-check.md

consistent-existence-index-check

📝 Enforce consistent style for element existence checks with indexOf(), lastIndexOf(), findIndex(), and findLastIndex().

💼 This rule is enabled in the following configs: ✅ recommended, ☑️ unopinionated.

🔧 This rule is automatically fixable by the --fix CLI option.

Enforce consistent style for element existence checks with indexOf(), lastIndexOf(), findIndex(), and findLastIndex().

Prefer using index === -1 to check if an element does not exist and index !== -1 to check if an element does exist.

The value -1 is the sentinel returned by these methods when no match is found, so comparing it directly with === or !== makes the check read as a not-found sentinel check instead of a numeric range check.

Similar to the explicit-length-check rule.

Examples

const index = foo.indexOf('bar');

// ❌
if (index < 0) {}

// ✅
if (index === -1) {}

const index = foo.indexOf('bar');

// ❌
if (index >= 0) {}

// ✅
if (index !== -1) {}

const index = foo.indexOf('bar');

// ❌
if (index > -1) {}

// ✅
if (index !== -1) {}

const index = foo.lastIndexOf('bar');

// ❌
if (index >= 0) {}

// ✅
if (index !== -1) {}

const index = array.findIndex(element => element > 10);

// ❌
if (index < 0) {}

// ✅
if (index === -1) {}

const index = array.findLastIndex(element => element > 10);

// ❌
if (index < 0) {}

// ✅
if (index === -1) {}