arrayExistenceChecksConsistency

Reports inconsistent styles for checking element existence using index methods.

✅ This rule is included in the ts stylistic and stylisticStrict presets.

Enforces 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. These methods return -1 when an element is not found, so checking against -1 is more explicit and precise than using broad comparisons like < 0 or >= 0.

Examples

❌ Incorrect

const index = values.indexOf("test");
if (index < 0) {
  // not found
}

const index = values.indexOf("test");
if (index >= 0) {
  // found
}

const index = values.indexOf("test");
if (index > -1) {
  // found
}

✅ Correct

const index = values.indexOf("test");
if (index === -1) {
  // not found
}

const index = values.indexOf("test");
if (index !== -1) {
  // found
}

if (values.findIndex((value) => value > 10) === -1) {
  // not found
}

Options

This rule is not configurable.

When Not To Use It

If your codebase has an established convention using < 0 or >= 0 for index checks, you may disable this rule. Some developers find the comparison style more readable in certain contexts.

Further Reading

• MDN: Array.prototype.indexOf()
• MDN: Array.prototype.findIndex()

Equivalents in Other Linters

• ESLint: unicorn/consistent-existence-index-check
• Oxlint: unicorn/consistent-existence-index-check