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prefer-includes

Enforce includes method over indexOf method.

🎨

Extending "plugin:@typescript-eslint/stylistic-type-checked" in an ESLint configuration enables this rule.

🔧

Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.

💭

This rule requires type information to run, which comes with performance tradeoffs.

Prior to ES2015, Array#indexOf and String#indexOf comparisons against -1 were the standard ways to check whether a value exists in an array or string, respectively. Alternatives that are easier to read and write now exist: ES2015 added String#includes and ES2016 added Array#includes.

This rule reports when an .indexOf call can be replaced with an .includes. Additionally, this rule reports the tests of simple regular expressions in favor of String#includes.

This rule will report on any receiver object of an indexOf method call that has an includes method where the two methods have the same parameters. Matching types include: String, Array, ReadonlyArray, and typed arrays.

eslint.config.mjs
export default defineConfig({
  rules: {
    "@typescript-eslint/prefer-includes": "error"
  }
});

Try this rule in the playground ↗

Examples​

const str: string;
const array: any[];
const readonlyArray: ReadonlyArray<any>;
const typedArray: UInt8Array;
const maybe: string;
const userDefined: {
  indexOf(x: any): number;
  includes(x: any): boolean;
};

str.indexOf(value) !== -1;
array.indexOf(value) !== -1;
readonlyArray.indexOf(value) === -1;
typedArray.indexOf(value) > -1;
maybe?.indexOf('') !== -1;
userDefined.indexOf(value) >= 0;

/example/.test(str);
Open in Playground

Options​

This rule is not configurable.

When Not To Use It​

Type checked lint rules are more powerful than traditional lint rules, but also require configuring type checked linting.

See Troubleshooting > Linting with Type Information > Performance if you experience performance degradations after enabling type checked rules.

Resources​