arrayIndexOfMethods

Reports using .findIndex() or .findLastIndex() with simple equality checks that can be replaced with .indexOf() or .lastIndexOf().

✅ This rule is included in the ts stylistic presets.

Array.prototype.findIndex() and Array.prototype.findLastIndex() are intended for more complex predicate checks. When the callback only performs a simple strict equality check (x === value), .indexOf() or .lastIndexOf() are more readable and expressive.

This rule reports when .findIndex() or .findLastIndex() can be simplified.

Examples

❌ Incorrect

declare const array: string[];
array.findIndex((item) => item === "value");

declare const array: string[];
array.findIndex((item) => "value" === item);

declare const array: string[];
array.findLastIndex((item) => item === "value");

declare const array: string[];
array.findIndex(function (item) {
  return item === "value";
});

✅ Correct

declare const array: string[];
array.indexOf("value");

declare const array: string[];
array.lastIndexOf("value");

declare const array: string[];
array.findIndex((item) => item.startsWith("v"));

declare const array: number[];
array.findIndex((item) => item > 0);

declare const array: object[];
array.findIndex((item) => item.id === 1);

Options

This rule is not configurable.

When Not To Use It

If you prefer the functional style of .findIndex() for consistency, or if your codebase has a large number of existing uses, you may disable this rule.

Further Reading

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

Equivalents in Other Linters

• Biome: useIndexOf
• ESLint: unicorn/prefer-array-index-of
• Oxlint: unicorn/prefer-array-index-of