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prefer-includes

Prefer .includes() over index-based existence checks.

Targeted pattern scope​

This rule targets array and string existence checks based on index comparison.

What this rule reports​

This rule reports patterns such as indexOf(...) !== -1 and equivalent index-based presence checks.

Why this rule exists​

.includes(...) is clearer and less error-prone than manual index comparisons.

❌ Incorrect​

const hasValue =
 [
  1,
  2,
  3,
 ].indexOf(2) !== -1;

✅ Correct​

const hasValue = [
 1,
 2,
 3,
].includes(2);

Behavior and migration notes​

This rule forwards options and behavior to unicorn/prefer-includes.

  • Lifecycle: Deprecated and frozen.
  • Deprecated since: v1.0.0
  • Available until: v2.0.0
  • Use instead: unicorn/prefer-includes

Additional examples​

const hasName = names.indexOf("alice") > -1;
// ❌ reported

const hasName2 = names.includes("alice");
// ✅ valid

ESLint flat config example​

import etcMisc from "eslint-plugin-etc-misc";

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/prefer-includes": "error",
  },
 },
];

When not to use it​

Disable this rule if you target runtimes where .includes() is unavailable and cannot be polyfilled safely.

Package documentation​

Rule catalog ID: R057

Further reading​

Adoption resources​

  • Start at warning level in CI, then move to error after cleanup.
  • Use focused codemods/autofix batches per package or directory.