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📖 Guide DocumentsLint & FormatRulesUnicorn

Unicorn

Modernization and style rules from eslint-plugin-unicorn.

They span array iteration, string and regex idioms, Node.js APIs, error handling, module syntax, DOM APIs, and code shape.

They rewrite legacy patterns into modern counterparts, forbid known anti-patterns, and pin a consistent style for things Core and TypeScript leave underspecified.

Pure-AST; no checker dependencies.

Source: eslint-plugin-unicorn (MIT).

Rule index

Each rule name links to the detailed section below.

Examples come from the checked lint corpus  or package-level rule tests when project layout matters.

Rules whose native port is registered but intentionally diagnostic-free are marked as illustrative in their detailed section.

Disallow

Prefer

Require

Consistency

Other checks

Rules

unicorn/better-regex

Rewrite regex literals into shorter, consistent, and safer form (character-class shorthands, redundant ranges).

Illustrative shape:

const digits = /[0-9]+/;

unicorn/catch-error-name

Enforce a canonical parameter name (error) in catch clauses.

Example:

// reports: unicorn/catch-error-name (error)
try {
} catch (err) {
  void err;
}

unicorn/consistent-assert

Enforce consistent assertion style when using node:assert.

Example:

import assert from "node:assert";
 
// reports: unicorn/consistent-assert (error)
assert.equal(1, 1);

unicorn/consistent-date-clone

Prefer passing a Date directly to the Date constructor when cloning, not +date or date.getTime().

Example:

const original = new Date();
// reports: unicorn/consistent-date-clone (error)
const clone = new Date(original.getTime());

unicorn/consistent-destructuring

Once a property is destructured from an object, require subsequent reads to use the destructured binding.

Example:

const user = { profile: { name: "Ada" } };
const { profile } = user;
 
// reports: unicorn/consistent-destructuring (error)
console.log(user.profile.name);

unicorn/consistent-empty-array-spread

Require both branches of a ternary spread inside an array literal to be array-typed.

Example:

const cond = true;
const x = 1;
// reports: unicorn/consistent-empty-array-spread (error)
const a = [1, ...(cond ? [x] : 2)];

unicorn/consistent-existence-index-check

Enforce a consistent comparison form (< 0 vs === -1, >= 0 vs !== -1) for indexOf / findIndex existence checks.

Example:

const arr = [1, 2, 3];
// reports: unicorn/consistent-existence-index-check (error)
const found = arr.indexOf(2) >= 0;

unicorn/consistent-function-scoping

Hoist function declarations to the highest scope that does not capture any outer variables.

Example:

function formatNames(names: string[]) {
  // reports: unicorn/consistent-function-scoping (error)
  function normalize(name: string) {
    return name.trim().toLowerCase();
  }
 
  return names.map(normalize);
}

unicorn/consistent-template-literal-escape

Enforce a consistent style (always \${ or always $\{) when escaping ${ in template literals.

Example:

// reports: unicorn/consistent-template-literal-escape (error)
const template = `Use \${name}, not $\{name}.`;

unicorn/custom-error-definition

Require user-defined Error subclasses to set name, call super(message), and assign their stack correctly.

Example:

// reports: unicorn/custom-error-definition (error)
class MyError extends Error {
  constructor() {
    void 0;
  }
}

unicorn/empty-brace-spaces

Reject whitespace inside empty {} braces.

Example:

// reports: unicorn/empty-brace-spaces (error)
const o = {};

unicorn/error-message

Require a non-empty message argument when constructing a built-in Error.

Example:

// reports: unicorn/error-message (error)
throw new Error();

unicorn/escape-case

Require consistent case for escape sequences (\xA9 over \xa9, \u00B5 over \u00b5).

Example:

// reports: unicorn/escape-case (error)
const s = "\xa9";

unicorn/expiring-todo-comments

Require every TODO/FIXME/XXX comment to declare an expiration date or package version.

Example:

// reports: unicorn/expiring-todo-comments (error)
// TODO: remove the legacy branch after migration.
export const legacyMode = true;

unicorn/explicit-length-check

Require explicit comparison of .length / .size instead of relying on truthy coercion.

Example:

const xs = [1, 2, 3];
if (
  // reports: unicorn/explicit-length-check (error)
  xs.length
) {
  void 0;
}

unicorn/filename-case

Enforce a single case style (kebab / camel / snake / pascal) for source filenames.

Illustrative shape:

// File: UserProfile.ts
export function renderUserProfile() {}

unicorn/import-style

Restrict each module’s allowed import styles (named only, default only, namespace only).

Illustrative shape:

// With Node.js builtins configured for namespace imports.
import { readFile } from "node:fs/promises";

unicorn/isolated-functions

Reject references to outer-scope variables inside functions marked as isolated (e.g., the body of a web worker).

Illustrative shape:

const prefix = "user:";
const isolated = (task: () => string) => task;
 
const worker = isolated(() => `${prefix}ready`);

unicorn/new-for-builtins

Require new when calling builtin constructors like Error, Map, Set, Date, and forbid new on primitive wrappers like String, Number, Boolean.

Example:

// reports: unicorn/new-for-builtins (error)
const xs = Array(3);

unicorn/no-abusive-eslint-disable

Require every eslint-disable* directive to name the rules it disables.

Example:

// reports: unicorn/no-abusive-eslint-disable (error)
// eslint-disable-next-line
const _x = 0;

unicorn/no-accessor-recursion

Reject recursive reads on this.<prop> inside the getter / setter for <prop>.

Example:

class C {
  get value() {
    // reports: unicorn/no-accessor-recursion (error)
    return this.value;
  }
}

unicorn/no-anonymous-default-export

Require a name on every default-exported function, class, or object.

Example:

// reports: unicorn/no-anonymous-default-export (error)
export default function () {
  return 1;
}

unicorn/no-array-callback-reference

Reject passing a function reference directly as the callback to map / filter / forEach / etc., which silently leaks extra index/array arguments to the callee.

Example:

function isEven(n: number) {
  return n % 2 === 0;
}
// reports: unicorn/no-array-callback-reference (error)
const evens = [1, 2, 3].filter(isEven);

unicorn/no-array-for-each

Prefer for...of over Array.prototype.forEach.

Example:

// reports: unicorn/no-array-for-each (error)
[1, 2, 3].forEach((x) => {
  console.log(x);
});

unicorn/no-array-method-this-argument

Reject the second thisArg argument to array methods; use an explicit closure instead.

Example:

// reports: unicorn/no-array-method-this-argument (error)
[1, 2].forEach(
  function (x) {
    console.log(this, x);
  },
  { tag: "ctx" },
);

unicorn/no-array-reduce

Reject Array#reduce / Array#reduceRight in favor of explicit loops or other helpers.

Example:

// reports: unicorn/no-array-reduce (error)
const total = [1, 2, 3].reduce((a, b) => a + b, 0);

unicorn/no-array-reverse

Prefer Array#toReversed over the mutating Array#reverse.

Example:

// reports: unicorn/no-array-reverse (error)
const r = [1, 2, 3].reverse();

unicorn/no-array-sort

Prefer Array#toSorted over the mutating Array#sort.

Example:

// reports: unicorn/no-array-sort (error)
const s = [3, 1, 2].sort();

unicorn/no-await-expression-member

Reject member access on an await expression without parens; require (await x).y.

Example:

async function f() {
  // reports: unicorn/no-await-expression-member (error)
  return (await Promise.resolve({ a: 1 })).a;
}

unicorn/no-await-in-promise-methods

Reject await inside arrays passed to Promise.all / Promise.allSettled / Promise.race / Promise.any, the awaits serialize the calls.

Example:

async function f() {
  // reports: unicorn/no-await-in-promise-methods (error)
  await Promise.all([await Promise.resolve(1), Promise.resolve(2)]);
}

unicorn/no-console-spaces

Reject leading or trailing spaces in arguments to console.log and friends, console already inserts spaces between args.

Example:

// reports: unicorn/no-console-spaces (error)
console.log("hello ", "world");

Reject direct reads or assignments to document.cookie; use the Cookie Store API or a wrapper.

Example:

// reports: unicorn/no-document-cookie (error)
document.cookie = "name=value";

unicorn/no-empty-file

Reject source files whose only content is whitespace and/or comments.

Example:

// reports: unicorn/no-empty-file (error)
;

unicorn/no-for-loop

Prefer for...of over index-based for loops over arrays.

Example:

const xs = [1, 2, 3];
// reports: unicorn/no-for-loop (error)
for (let i = 0; i < xs.length; i++) {
  void xs[i];
}

unicorn/no-hex-escape

Prefer Unicode escape (\u00A9) over hexadecimal escape (\xA9).

Example:

// reports: unicorn/no-hex-escape (error)
const s = "\xA9";

unicorn/no-immediate-mutation

Reject mutating a value on the same expression that produces it ([...x].push(y)); separate the construction and the mutation.

Example:

// reports: unicorn/no-immediate-mutation (error)
const last = [1, 2, 3].push(4);

unicorn/no-instanceof-builtins

Reject instanceof Array, instanceof Error, instanceof Map, etc., they fail across realms and for subclasses.

Example:

const x: unknown = [];
// reports: unicorn/no-instanceof-builtins (error)
if (x instanceof Array) {
  void x;
}

unicorn/no-invalid-fetch-options

Reject GET / HEAD fetch() calls that also set a request body, which throws at runtime.

Example:

// reports: unicorn/no-invalid-fetch-options (error)
fetch("https://example.com", { method: "GET", body: "x" });

unicorn/no-invalid-remove-event-listener

Reject removeEventListener calls whose handler argument is a fresh function reference and therefore matches no registered listener.

Example:

const el = new EventTarget();
// reports: unicorn/no-invalid-remove-event-listener (error)
el.removeEventListener("click", () => {});

unicorn/no-keyword-prefix

Reject identifiers that start with a reserved word (newFoo, classBar).

Example:

// reports: unicorn/no-keyword-prefix (error)
const newFoo = 1;

unicorn/no-lonely-if

Reject if as the only statement inside an else block; use else if instead.

Example:

if (1 === 1) {
  void 0;
} else {
  // reports: unicorn/no-lonely-if (error)
  if (2 === 2) {
    void 0;
  }
}

unicorn/no-magic-array-flat-depth

Reject magic-number depth arguments to Array#flat; require Infinity or a named constant.

Example:

// reports: unicorn/no-magic-array-flat-depth (error)
const flat = [1, [2, [3]]].flat(2);

unicorn/no-named-default

Reject re-importing or re-exporting a default binding under a name that differs from the upstream binding.

Example:

// reports: unicorn/no-named-default (error)
import { default as React } from "react";

unicorn/no-negated-condition

Reject negated conditions in if/else and ternaries when the positive form is shorter.

Example:

const x = 1;
// reports: unicorn/no-negated-condition (error)
if (x !== 0) {
  void "nonzero";
} else {
  void "zero";
}

unicorn/no-negation-in-equality-check

Reject !a === b; require a !== b or !(a === b).

Example:

const a = 1;
const b = 2;
// reports: unicorn/no-negation-in-equality-check (error)
const eq = !a === b;

unicorn/no-nested-ternary

Reject ternaries nested inside other ternaries.

Example:

const x = 1;
// reports: unicorn/no-nested-ternary (error)
const r = x === 0 ? "zero" : x > 0 ? "pos" : "neg";

unicorn/no-new-array

Reject the new Array(...) constructor; use array literals or Array.from / Array.of.

Example:

// reports: unicorn/no-new-array (error)
const a = new Array(3);

unicorn/no-new-buffer

Reject the deprecated new Buffer() constructor; use Buffer.from or Buffer.alloc.

Example:

// reports: unicorn/no-new-buffer (error)
const b = new Buffer(10);

unicorn/no-null

Reject the null literal in favor of undefined.

Example:

// reports: unicorn/no-null (error)
const x = null;

unicorn/no-object-as-default-parameter

Reject inline object literals as default values for function parameters.

Example:

// reports: unicorn/no-object-as-default-parameter (error)
function f(opts = { tag: "default" }) {
  void opts;
}

unicorn/no-process-exit

Reject process.exit(); throw or return a non-zero status instead.

Example:

// reports: unicorn/no-process-exit (error)
process.exit(1);

unicorn/no-single-promise-in-promise-methods

Reject Promise.all / Promise.race / etc. called with a single-element array; the wrapper is redundant.

Example:

// reports: unicorn/no-single-promise-in-promise-methods (error)
const p = Promise.all([Promise.resolve(1)]);

unicorn/no-static-only-class

Reject classes whose every member is static; use a plain module-level namespace instead.

Example:

// reports: unicorn/no-static-only-class (error)
class Utility {
  static helper() {
    return 42;
  }
}

unicorn/no-thenable

Reject defining a property named then on objects, modules, or classes, await and Promise resolution accidentally invoke it.

Example:

const o = {
  // reports: unicorn/no-thenable (error)
  then() {
    return 1;
  },
};

unicorn/no-this-assignment

Reject const self = this and similar aliases; capture via arrow functions instead.

Example:

class C {
  m() {
    // reports: unicorn/no-this-assignment (error)
    const self = this;
    return self;
  }
}

unicorn/no-typeof-undefined

Reject typeof x === "undefined"; compare against undefined directly.

Example:

// reports: unicorn/no-typeof-undefined (error)
typeof globalThis === "undefined";

unicorn/no-unnecessary-array-flat-depth

Reject 1 as the explicit depth argument of Array#flat; the default is already 1.

Example:

// reports: unicorn/no-unnecessary-array-flat-depth (error)
const flat = [1, [2]].flat(1);

unicorn/no-unnecessary-array-splice-count

Reject .length / Infinity as the deleteCount argument to splice / toSpliced; omit it to delete to the end.

Example:

const arr = [1, 2, 3];
// reports: unicorn/no-unnecessary-array-splice-count (error)
arr.splice(0, arr.length);

unicorn/no-unnecessary-await

Reject await on non-thenable expressions.

Example:

async function f() {
  // reports: unicorn/no-unnecessary-await (error)
  const x = await 42;
}

unicorn/no-unnecessary-polyfills

Reject polyfill imports for APIs already available in the project’s targeted Node / browser baseline.

Illustrative shape:

import "core-js/features/array/at";
 
const last = ["a", "b"].at(-1);

unicorn/no-unnecessary-slice-end

Reject .length / Infinity as the end argument to slice; omit it to slice to the end.

Example:

const arr = [1, 2, 3];
// reports: unicorn/no-unnecessary-slice-end (error)
const c = arr.slice(0, arr.length);

unicorn/no-unreadable-array-destructuring

Reject destructuring patterns with long hole runs ([,,,,a]); use a named index instead.

Example:

// reports: unicorn/no-unreadable-array-destructuring (error)
const [, , , , a] = [1, 2, 3, 4, 5];

unicorn/no-unreadable-iife

Reject IIFEs whose nesting (multiple parens, arrow IIFE arguments) is hard to read.

Example:

// reports: unicorn/no-unreadable-iife (error)
const r = (() => Math.random())();

unicorn/no-unused-properties

Reject object properties that are never read after definition.

Illustrative shape:

const user = {
  id: "u_1",
  debugLabel: "local-only",
};
 
console.log(user.id);

unicorn/no-useless-collection-argument

Reject useless initializer arguments (new Set(), new Map([]), new Set(undefined)) on collection constructors.

Example:

// reports: unicorn/no-useless-collection-argument (error)
const s = new Set([]);

unicorn/no-useless-error-capture-stack-trace

Reject Error.captureStackTrace(this, constructor) when the surrounding subclass relies on the default Error capture.

Example:

class MyError extends Error {
  constructor(msg: string) {
    super(msg);
    // reports: unicorn/no-useless-error-capture-stack-trace (error)
    Error.captureStackTrace(this, MyError);
  }
}
void new MyError("x");

unicorn/no-useless-fallback-in-spread

Reject ...(x ?? {}) and similar fallbacks when spreading; the spread of null / undefined is already a no-op.

Example:

const x: { a: number } | null = null;
// reports: unicorn/no-useless-fallback-in-spread (error)
const o = { ...(x ?? {}) };

unicorn/no-useless-iterator-to-array

Reject [...iterator] / Array.from(iterator) when the iterator can be consumed directly (e.g., inside for...of).

Example:

const arr = [1, 2];
// reports: unicorn/no-useless-iterator-to-array (error)
for (const e of [...arr.entries()]) {
  void e;
}

unicorn/no-useless-length-check

Reject arr.length checks that the iteration method itself already handles.

Example:

const xs = [1, 2, 3];
// reports: unicorn/no-useless-length-check (error)
const hasPositive = xs.length > 0 && xs.some((x) => x > 0);

unicorn/no-useless-promise-resolve-reject

Reject return Promise.resolve(x) / return Promise.reject(e) inside async functions, return x and throw e work identically.

Example:

async function f() {
  // reports: unicorn/no-useless-promise-resolve-reject (error)
  return Promise.resolve(1);
}

unicorn/no-useless-spread

Reject spreading a single iterable into a new collection of the same kind ([...arr], {...obj}) when the original would suffice.

Example:

// reports: unicorn/no-useless-spread (error)
const a = [...[1, 2, 3]];

unicorn/no-useless-switch-case

Reject case clauses with an empty body that immediately precede a default whose body executes for them.

Example:

const x = 2;
switch (x) {
  case 1:
    void 0;
    break;
  // reports: unicorn/no-useless-switch-case (error)
  case 2:
  default:
    void 0;
}

unicorn/no-useless-undefined

Reject explicit undefined returns, default initializers, and arguments where the omission has the same meaning.

Example:

function f() {
  // reports: unicorn/no-useless-undefined (error)
  return undefined;
}

unicorn/no-zero-fractions

Reject 1.0 / 1. / .5e0 in favor of 1, 1, and 0.5.

Example:

// reports: unicorn/no-zero-fractions (error)
const n = 1.0;

unicorn/number-literal-case

Enforce one consistent case for the prefix and digits of hex / binary / octal literals (0xFF over 0xff).

Example:

// reports: unicorn/number-literal-case (error)
const n = 0xff;

unicorn/numeric-separators-style

Enforce _ separator grouping (every 3 digits for decimal, every 4 for hex) in numeric literals.

Example:

// reports: unicorn/numeric-separators-style (error)
const big = 1_2345;

unicorn/prefer-add-event-listener

Prefer addEventListener / removeEventListener over assigning to on* properties.

Example:

const el = document.querySelector("button")!;
// reports: unicorn/prefer-add-event-listener (error)
el.onclick = () => {};

unicorn/prefer-array-find

Prefer Array#find / Array#findLast over filter(...)[0] / filter(...).at(-1).

Example:

const xs = [1, 2, 3];
// reports: unicorn/prefer-array-find (error)
const first = xs.filter((x) => x > 1)[0];

unicorn/prefer-array-flat

Prefer Array#flat over legacy flattening idioms ([].concat(...arrs), reduce with concat).

Example:

// reports: unicorn/prefer-array-flat (error)
const flat = [].concat([1, 2], [3, 4]);

unicorn/prefer-array-flat-map

Prefer Array#flatMap over map(...).flat().

Example:

// reports: unicorn/prefer-array-flat-map (error)
const result = [1, 2].map((x) => [x, x]).flat();

unicorn/prefer-array-index-of

Prefer indexOf / lastIndexOf over findIndex / findLastIndex when matching by ===.

Example:

const xs = [1, 2, 3];
// reports: unicorn/prefer-array-index-of (error)
const i = xs.findIndex((x) => x === 2);

unicorn/prefer-array-some

Prefer Array#some over filter(...).length > 0, find(...) !== undefined, and similar shapes.

Example:

const xs = [1, 2, 3];
// reports: unicorn/prefer-array-some (error)
const hasLarge = xs.filter((x) => x > 1).length > 0;

unicorn/prefer-at

Prefer Array#at / String#at over index arithmetic and charAt.

Example:

const xs = [1, 2, 3];
// reports: unicorn/prefer-at (error)
const last = xs[xs.length - 1];

unicorn/prefer-bigint-literals

Prefer 1n over BigInt(1) and BigInt("1").

Example:

// reports: unicorn/prefer-bigint-literals (error)
const big = BigInt(1);

unicorn/prefer-blob-reading-methods

Prefer Blob#arrayBuffer() / Blob#text() over FileReader#readAsArrayBuffer / readAsText.

Example:

const reader = new FileReader();
const blob = new Blob(["hello"]);
// reports: unicorn/prefer-blob-reading-methods (error)
reader.readAsArrayBuffer(blob);

unicorn/prefer-class-fields

Prefer class field declarations over constructor assignments to this.field = value.

Example:

class C {
  field: number;
  constructor() {
    // reports: unicorn/prefer-class-fields (error)
    this.field = 1;
  }
}

unicorn/prefer-classlist-toggle

Prefer Element#classList.toggle(name, condition) over manual add / remove branches.

Example:

const el = document.createElement("button");
const cond = true;
// reports: unicorn/prefer-classlist-toggle (error)
if (cond) {
  el.classList.add("active");
} else {
  el.classList.remove("active");
}

unicorn/prefer-code-point

Prefer String#codePointAt / String.fromCodePoint over charCodeAt / fromCharCode.

Example:

// reports: unicorn/prefer-code-point (error)
const code = "a".charCodeAt(0);

unicorn/prefer-date-now

Prefer Date.now() over new Date().getTime() / +new Date().

Example:

// reports: unicorn/prefer-date-now (error)
const t = new Date().getTime();

unicorn/prefer-default-parameters

Prefer default parameter syntax over x = x ?? default reassignments inside the function body.

Example:

function f(name?: string) {
  // reports: unicorn/prefer-default-parameters (error)
  name = name ?? "guest";
  return name;
}

unicorn/prefer-dom-node-append

Prefer Node#append over Node#appendChild.

Example:

const parent = document.createElement("div");
const child = document.createElement("span");
// reports: unicorn/prefer-dom-node-append (error)
parent.appendChild(child);

unicorn/prefer-dom-node-dataset

Prefer Element#dataset over getAttribute / setAttribute for data-* attributes.

Example:

const el = document.createElement("div");
// reports: unicorn/prefer-dom-node-dataset (error)
el.getAttribute("data-user-id");

unicorn/prefer-dom-node-remove

Prefer ChildNode#remove over parent.removeChild(child).

Example:

const parent = document.createElement("div");
const child = document.createElement("span");
// reports: unicorn/prefer-dom-node-remove (error)
parent.removeChild(child);

unicorn/prefer-dom-node-text-content

Prefer Node#textContent over HTMLElement#innerText.

Example:

const el = document.createElement("div");
// reports: unicorn/prefer-dom-node-text-content (error)
el.innerText;

unicorn/prefer-event-target

Prefer EventTarget over Node’s EventEmitter when the code is shared between Node and the browser.

Example:

import { EventEmitter } from "node:events";
 
// reports: unicorn/prefer-event-target (error)
const em = new EventEmitter();

unicorn/prefer-export-from

Prefer export ... from over importing-then-re-exporting in two statements.

Example:

import { useState } from "react";
 
// reports: unicorn/prefer-export-from (error)
export { useState };

unicorn/prefer-global-this

Prefer globalThis over window, self, and global.

Example:

// reports: unicorn/prefer-global-this (error)
const root = window;

unicorn/prefer-import-meta-properties

Prefer import.meta.dirname / import.meta.filename over fileURLToPath workarounds.

Example:

import { fileURLToPath } from "node:url";
 
// reports: unicorn/prefer-import-meta-properties (error)
const filename = fileURLToPath(import.meta.url);

unicorn/prefer-includes

Prefer String#includes / Array#includes over indexOf(...) !== -1 and some(x => x === target).

Example:

const arr = [1, 2, 3];
// reports: unicorn/prefer-includes (error)
const found = arr.indexOf(2) !== -1;

unicorn/prefer-json-parse-buffer

Prefer passing a Buffer directly to JSON.parse (Node 21+) instead of decoding to a string first.

Example:

const buf = Buffer.from('{"ok":true}');
// reports: unicorn/prefer-json-parse-buffer (error)
const data = JSON.parse(buf.toString());

unicorn/prefer-keyboard-event-key

Prefer KeyboardEvent#key over the deprecated KeyboardEvent#keyCode / charCode / which.

Example:

const event = new KeyboardEvent("keydown");
// reports: unicorn/prefer-keyboard-event-key (error)
const code = event.keyCode;

unicorn/prefer-logical-operator-over-ternary

Prefer a || b / a ?? b over the equivalent ternary a ? a : b.

Example:

const x: number | undefined = 1;
// reports: unicorn/prefer-logical-operator-over-ternary (error)
const y = x ? x : 0;

unicorn/prefer-math-min-max

Prefer Math.min / Math.max over ternaries computing the same value.

Example:

const a = 1;
const b = 2;
// reports: unicorn/prefer-math-min-max (error)
const m = a < b ? a : b;

unicorn/prefer-math-trunc

Prefer Math.trunc over ~~x / x | 0 for integer truncation.

Example:

// reports: unicorn/prefer-math-trunc (error)
const i = ~~3.7;

unicorn/prefer-modern-dom-apis

Prefer before / after / replaceWith over insertBefore / replaceChild / insertAdjacentText.

Example:

const parent = document.createElement("div");
const ref = document.createElement("span");
const node = document.createElement("strong");
// reports: unicorn/prefer-modern-dom-apis (error)
parent.insertBefore(node, ref);

unicorn/prefer-modern-math-apis

Prefer Math.log10 / Math.hypot / Math.log2 / Math.cbrt over their legacy approximations.

Example:

const x = 10;
// reports: unicorn/prefer-modern-math-apis (error)
const l = Math.log(x) * Math.LOG10E;

unicorn/prefer-module

Prefer ES modules (import / export) over CommonJS (require / module.exports / __dirname / __filename).

Example:

// reports: unicorn/prefer-module (error)
require("path");

unicorn/prefer-native-coercion-functions

Prefer the bare String / Number / Boolean / BigInt functions over x => String(x) arrow wrappers.

Example:

// reports: unicorn/prefer-native-coercion-functions (error)
const xs = ["1", "2"].map((x) => Number(x));

unicorn/prefer-negative-index

Prefer negative-index lookups (arr.at(-1), arr.slice(-2)) over arr.length - 1 / arr.length - 2 arithmetic.

Example:

const a = [1, 2, 3];
// reports: unicorn/prefer-negative-index (error)
const tail = a.slice(a.length - 1);

unicorn/prefer-node-protocol

Prefer node:fs / node:path / etc. over the bare Node builtin specifier.

Example:

// reports: unicorn/prefer-node-protocol (error)
import * as fs from "fs";

unicorn/prefer-number-properties

Prefer Number.isNaN / Number.parseInt / Number.NaN over their global counterparts.

Example:

// reports: unicorn/prefer-number-properties (error)
void isNaN(0);

unicorn/prefer-object-from-entries

Prefer Object.fromEntries over reduce-into-object patterns.

Example:

const entries: Array<[string, number]> = [["a", 1]];
// reports: unicorn/prefer-object-from-entries (error)
const obj = entries.reduce((acc, [k, v]) => ({ ...acc, [k]: v }), {});

unicorn/prefer-optional-catch-binding

Prefer catch { ... } over catch (e) { ... } when e is unused.

Example:

try {
  throw new Error("x");
  // reports: unicorn/prefer-optional-catch-binding (error)
} catch (e) {
  void 0;
}

unicorn/prefer-prototype-methods

Prefer borrowing prototype methods (Array.prototype.slice.call) over [].slice.call empty-instance lookups.

Example:

// reports: unicorn/prefer-prototype-methods (error)
const slice = [].slice;

unicorn/prefer-query-selector

Prefer Document#querySelector over getElementById, getElementsByClassName, and getElementsByTagName.

Example:

const doc = document;
// reports: unicorn/prefer-query-selector (error)
doc.getElementById("main");

unicorn/prefer-reflect-apply

Prefer Reflect.apply(fn, thisArg, args) over Function.prototype.apply.call(fn, thisArg, args).

Example:

function f(a: number, b: number) {
  return a + b;
}
// reports: unicorn/prefer-reflect-apply (error)
const r = Function.prototype.apply.call(f, null, [1, 2]);

unicorn/prefer-regexp-test

Prefer RegExp#test over String#match / RegExp#exec when only a boolean is needed.

Example:

// reports: unicorn/prefer-regexp-test (error)
if ("abc".match(/a/)) {
  void 0;
}

unicorn/prefer-response-static-json

Prefer Response.json(value) over new Response( JSON.stringify(value), { headers: {"content-type": "application/json"} }).

Example:

// reports: unicorn/prefer-response-static-json (error)
const r = new Response(JSON.stringify({ ok: true }));

unicorn/prefer-set-has

Prefer Set#has over Array#includes for repeated membership lookups against a constant collection.

Example:

const x = 2;
// reports: unicorn/prefer-set-has (error)
const found = [1, 2, 3].includes(x);

unicorn/prefer-set-size

Prefer Set#size over [...set].length and Array.from(set).length.

Example:

const s = new Set([1, 2, 3]);
// reports: unicorn/prefer-set-size (error)
const n = [...s].length;

unicorn/prefer-simple-condition-first

Prefer the simpler operand on the left of && / || so the short-circuit reads in evaluation order.

Illustrative shape:

function isAllowed(user: { role: string }) {
  return user.role === "admin";
}
 
const user = { role: "admin" };
const featureEnabled = true;
 
if (isAllowed(user) && featureEnabled) {
  console.log("enabled");
}

unicorn/prefer-single-call

Prefer a single push / unshift / classList.add / addEventListener with multiple arguments over consecutive single-argument calls.

Example:

const xs: number[] = [];
xs.push(1);
// reports: unicorn/prefer-single-call (error)
xs.push(2);

unicorn/prefer-spread

Prefer spread ([...arr], [...str]) over Array.from, Array.prototype.slice.call, concat([]), and split('').

Example:

const a = [1, 2, 3];
// reports: unicorn/prefer-spread (error)
const b = Array.from(a);

unicorn/prefer-string-raw

Prefer String.raw for path literals and other strings that would otherwise need backslash escapes.

Example:

// reports: unicorn/prefer-string-raw (error)
const p = "C:\\Users\\me";

unicorn/prefer-string-replace-all

Prefer String#replaceAll(literal, replacement) over replace(/literal/g, replacement).

Example:

// reports: unicorn/prefer-string-replace-all (error)
const out = "abc".replace(/a/g, "x");

unicorn/prefer-string-slice

Prefer String#slice over the deprecated substr / substring.

Example:

// reports: unicorn/prefer-string-slice (error)
const s = "hello".substr(0, 3);

unicorn/prefer-string-starts-ends-with

Prefer String#startsWith / String#endsWith over equivalent RegExp#test and slice-then-compare idioms.

Example:

const s = "https://example.com";
// reports: unicorn/prefer-string-starts-ends-with (error)
const b = s.slice(0, 4) === "http";

unicorn/prefer-string-trim-start-end

Prefer String#trimStart / String#trimEnd over the deprecated trimLeft / trimRight.

Example:

// reports: unicorn/prefer-string-trim-start-end (error)
const s = "  hi  ".trimLeft();

unicorn/prefer-structured-clone

Prefer structuredClone(x) over JSON.parse(JSON.stringify(x)) for deep cloning.

Example:

const original = { a: 1 };
// reports: unicorn/prefer-structured-clone (error)
const clone = JSON.parse(JSON.stringify(original));

unicorn/prefer-switch

Prefer switch over chains of three or more else if clauses comparing the same discriminant.

Example:

const k = "a";
// reports: unicorn/prefer-switch (error)
if (k === "a") {
  void 0;
} else if (k === "b") {
  void 0;
} else if (k === "c") {
  void 0;
}

unicorn/prefer-ternary

Prefer a ternary over if / else whose two branches differ only in the right-hand side of a common assignment, return, or throw.

Example:

const cond = true;
function f(): number {
  // reports: unicorn/prefer-ternary (error)
  if (cond) {
    return 1;
  } else {
    return 2;
  }
}

unicorn/prefer-top-level-await

Prefer top-level await over .then / IIFE wrappers in ES modules.

Example:

async function load(): Promise<string> {
  return "ready";
}
 
// reports: unicorn/prefer-top-level-await (error)
load().then((value) => console.log(value));

unicorn/prefer-type-error

Require throwing TypeError (not a bare Error) when the surrounding if is a runtime type check.

Example:

function f(x: unknown) {
  if (typeof x !== "number") {
    // reports: unicorn/prefer-type-error (error)
    throw new Error("must be number");
  }
  return x;
}

unicorn/prevent-abbreviations

Reject common identifier abbreviations (btn, arr, idx) and replace them with their long forms.

Example:

// reports: unicorn/prevent-abbreviations (error)
const btn = document.querySelector("button");

unicorn/relative-url-style

Enforce a single style (always leading ./ vs. never) for relative URLs passed to new URL.

Example:

const base = "https://example.com";
const u = new URL(
  // reports: unicorn/relative-url-style (error)
  "./dashboard",
  base,
);

unicorn/require-array-join-separator

Require an explicit separator argument to Array#join instead of relying on the default ",".

Example:

// reports: unicorn/require-array-join-separator (error)
const s = [1, 2, 3].join();

unicorn/require-module-attributes

Require non-empty with / assert options on import / export statements that use them at all.

Example:

// reports: unicorn/require-module-attributes (error)
import data from "./data.json";

unicorn/require-module-specifiers

Require a non-empty specifier list on every import / export statement.

Example:

// reports: unicorn/require-module-specifiers (error)
import "./side-effect.js";

unicorn/require-number-to-fixed-digits-argument

Require an explicit digits argument to Number#toFixed.

Example:

// reports: unicorn/require-number-to-fixed-digits-argument (error)
const s = (1.234).toFixed();

unicorn/require-post-message-target-origin

Require an explicit targetOrigin argument to window.postMessage.

Example:

const win = window;
// reports: unicorn/require-post-message-target-origin (error)
win.postMessage({ kind: "ping" });

unicorn/string-content

Enforce or replace configured string-content patterns (e.g., curly quotes for straight ones).

Illustrative shape:

// With a configured pattern that replaces "TODO" with "To do".
const label = "TODO";

unicorn/switch-case-braces

Enforce a consistent presence/absence of {} braces around case clauses inside switch.

Example:

const k = "a";
switch (k) {
  // reports: unicorn/switch-case-braces (error)
  case "a":
    void 0;
    break;
}

unicorn/switch-case-break-position

Enforce a consistent position for break (or return / throw) inside case clauses.

Illustrative shape:

const state = "idle";
function reset(): void {}
 
switch (state) {
  case "idle":
    break;
    reset();
}

unicorn/template-indent

Re-indent the body of tagged template literals (html, gql, sql) to the indentation of the opening backtick.

Illustrative shape:

function sql(strings: TemplateStringsArray): string {
  return strings.join("");
}
 
const query = sql`
SELECT *
    FROM users
WHERE active = true
`;

unicorn/text-encoding-identifier-case

Enforce a canonical case for text-encoding identifiers, "utf-8" (not "UTF-8" / "utf8").

Example:

// reports: unicorn/text-encoding-identifier-case (error)
const enc = "UTF-8";

unicorn/throw-new-error

Require throw new Error(...) over throw Error(...).

Example:

// reports: unicorn/throw-new-error (error)
throw Error("oops");
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