Unicorn

Modernization and style rules from eslint-plugin-unicorn, spanning array iteration, string and regex idioms, Node.js APIs, error handling, module syntax, DOM APIs, and code shape. Rewrites legacy patterns into their modern counterparts (for...of over forEach, Array#flatMap over map().flat(), String#replaceAll over a global-regex replace, Math.trunc over | 0), forbids known anti-patterns (null literals, abusive eslint-disable, new Buffer, process.exit, instanceof Array), and pins a consistent style for things ESLint core and TypeScript leave underspecified (filename case, numeric separators, catch-binding names, expiring TODOs, escape-sequence case, switch-case braces). Pure-AST; no checker dependencies.

Source: eslint-plugin-unicorn (MIT).

unicorn/better-regex: Rewrite regex literals into shorter, consistent, and safer form (character-class shorthands, redundant ranges).
unicorn/catch-error-name: Enforce a canonical parameter name (error) in catch clauses.
unicorn/consistent-assert: Enforce consistent assertion style when using node:assert.
unicorn/consistent-date-clone: Prefer passing a Date directly to the Date constructor when cloning, not +date or date.getTime().
unicorn/consistent-destructuring: Once a property is destructured from an object, require subsequent reads to use the destructured binding.
unicorn/consistent-empty-array-spread: Require both branches of a ternary spread inside an array literal to be array-typed.
unicorn/consistent-existence-index-check: Enforce a consistent comparison form (< 0 vs === -1, >= 0 vs !== -1) for indexOf / findIndex existence checks.
unicorn/consistent-function-scoping: Hoist function declarations to the highest scope that does not capture any outer variables.
unicorn/consistent-template-literal-escape: Enforce a consistent style (always \${ or always $\{) when escaping ${ in template literals.
unicorn/custom-error-definition: Require user-defined Error subclasses to set name, call super(message), and assign their stack correctly.
unicorn/empty-brace-spaces: Reject whitespace inside empty {} braces.
unicorn/error-message: Require a non-empty message argument when constructing a built-in Error.
unicorn/escape-case: Require consistent case for escape sequences (\xA9 over \xa9, µ over µ).
unicorn/expiring-todo-comments: Require every TODO/FIXME/XXX comment to declare an expiration date or package version.
unicorn/explicit-length-check: Require explicit comparison of .length / .size instead of relying on truthy coercion.
unicorn/filename-case: Enforce a single case style (kebab / camel / snake / pascal) for source filenames.
unicorn/import-style: Restrict each module’s allowed import styles (named only, default only, namespace only).
unicorn/isolated-functions: Reject references to outer-scope variables inside functions marked as isolated (e.g., the body of a web worker).
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.
unicorn/no-abusive-eslint-disable: Require every eslint-disable* directive to name the rules it disables.
unicorn/no-accessor-recursion: Reject recursive reads on this.<prop> inside the getter / setter for <prop>.
unicorn/no-anonymous-default-export: Require a name on every default-exported function, class, or object.
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.
unicorn/no-array-for-each: Prefer for...of over Array.prototype.forEach.
unicorn/no-array-method-this-argument: Reject the second thisArg argument to array methods; use an explicit closure instead.
unicorn/no-array-reduce: Reject Array#reduce / Array#reduceRight in favor of explicit loops or other helpers.
unicorn/no-array-reverse: Prefer Array#toReversed over the mutating Array#reverse.
unicorn/no-array-sort: Prefer Array#toSorted over the mutating Array#sort.
unicorn/no-await-expression-member: Reject member access on an await expression without parens; require (await x).y.
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.
unicorn/no-console-spaces: Reject leading or trailing spaces in arguments to console.log and friends, console already inserts spaces between arguments.
unicorn/no-document-cookie: Reject direct reads or assignments to document.cookie; use the Cookie Store API or a wrapper.
unicorn/no-empty-file: Reject source files whose only content is whitespace and/or comments.
unicorn/no-for-loop: Prefer for...of over index-based for loops over arrays.
unicorn/no-immediate-mutation: Reject mutating a value on the same expression that produces it ([...x].push(y)); separate the construction and the mutation.
unicorn/no-instanceof-builtins: Reject instanceof Array, instanceof Error, instanceof Map, etc.. They fail across realms and for subclasses.
unicorn/no-invalid-fetch-options: Reject GET / HEAD fetch() calls that also set a request body, which throws at runtime.
unicorn/no-invalid-remove-event-listener: Reject removeEventListener calls whose handler argument is a fresh function reference and therefore matches no registered listener.
unicorn/no-keyword-prefix: Reject identifiers that start with a reserved word (newFoo, classBar).
unicorn/no-lonely-if: Reject if as the only statement inside an else block; use else if instead.
unicorn/no-magic-array-flat-depth: Reject magic-number depth arguments to Array#flat; require Infinity or a named constant.
unicorn/no-named-default: Reject re-importing or re-exporting a default binding under a name that differs from the upstream binding.
unicorn/no-negated-condition: Reject negated conditions in if / else and ternaries when the positive form is shorter.
unicorn/no-negation-in-equality-check: Reject !a === b; require a !== b or !(a === b).
unicorn/no-nested-ternary: Reject ternaries nested inside other ternaries.
unicorn/no-new-array: Reject the new Array(...) constructor; use array literals or Array.from / Array.of.
unicorn/no-new-buffer: Reject the deprecated new Buffer() constructor; use Buffer.from or Buffer.alloc.
unicorn/no-null: Reject the null literal in favor of undefined.
unicorn/no-object-as-default-parameter: Reject inline object literals as default values for function parameters.
unicorn/no-process-exit: Reject process.exit(); throw or return a non-zero status instead.
unicorn/no-single-promise-in-promise-methods: Reject Promise.all / Promise.race / etc. Called with a single-element array; the wrapper is redundant.
unicorn/no-static-only-class: Reject classes whose every member is static; use a plain module-level namespace instead.
unicorn/no-thenable: Reject defining a property named then on objects, modules, or classes, await and Promise resolution accidentally invoke it.
unicorn/no-this-assignment: Reject const self = this and similar aliases; capture via arrow functions instead.
unicorn/no-typeof-undefined: Reject typeof x === "undefined"; compare against undefined directly.
unicorn/no-unnecessary-array-flat-depth: Reject 1 as the explicit depth argument of Array#flat; the default is already 1.
unicorn/no-unnecessary-array-splice-count: Reject .length / Infinity as the deleteCount argument to splice / toSpliced; omit it to delete to the end.
unicorn/no-unnecessary-await: Reject await on non-thenable expressions.
unicorn/no-unnecessary-polyfills: Reject polyfill imports for APIs already available in the project’s targeted Node / browser baseline.
unicorn/no-unnecessary-slice-end: Reject .length / Infinity as the end argument to slice; omit it to slice to the end.
unicorn/no-unreadable-array-destructuring: Reject destructuring patterns with long hole runs ([,,,,a]); use a named index instead.
unicorn/no-unreadable-iife: Reject IIFEs whose nesting (multiple parens, arrow IIFE arguments) is hard to read.
unicorn/no-unused-properties: Reject object properties that are never read after definition.
unicorn/no-useless-collection-argument: Reject useless initializer arguments (new Set(), new Map([]), new Set(undefined)) on collection constructors.
unicorn/no-useless-error-capture-stack-trace: Reject Error.captureStackTrace(this, constructor) when the surrounding subclass relies on the default Error capture.
unicorn/no-useless-fallback-in-spread: Reject ...(x ?? {}) and similar fallbacks when spreading; the spread of null / undefined is already a no-op.
unicorn/no-useless-iterator-to-array: Reject [...iterator] / Array.from(iterator) when the iterator can be consumed directly (e.g., inside for...of).
unicorn/no-useless-length-check: Reject arr.length checks that the iteration method itself already handles.
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.
unicorn/no-useless-spread: Reject spreading a single iterable into a new collection of the same kind ([...arr], {...obj}) when the original would suffice.
unicorn/no-useless-switch-case: Reject case clauses with an empty body that immediately precede a default whose body executes for them.
unicorn/no-useless-undefined: Reject explicit undefined returns, default initializers, and arguments where the omission has the same meaning.
unicorn/no-zero-fractions: Reject 1.0 / 1. / .5e0 in favor of 1, 1, and 0.5.
unicorn/number-literal-case: Enforce one consistent case for the prefix and digits of hex / binary / octal literals (0xFF over 0xff).
unicorn/numeric-separators-style: Enforce _ separator grouping (every 3 digits for decimal, every 4 for hex) in numeric literals.
unicorn/prefer-add-event-listener: Prefer addEventListener / removeEventListener over assigning to on* properties.
unicorn/prefer-array-find: Prefer Array#find / Array#findLast over filter(...)[0] / filter(...).at(-1).
unicorn/prefer-array-flat: Prefer Array#flat over legacy flattening idioms ([].concat(...arrs), reduce with concat).
unicorn/prefer-array-flat-map: Prefer Array#flatMap over map(...).flat().
unicorn/prefer-array-index-of: Prefer indexOf / lastIndexOf over findIndex / findLastIndex when matching by ===.
unicorn/prefer-array-some: Prefer Array#some over filter(...).length > 0, find(...) !== undefined, and similar shapes.
unicorn/prefer-at: Prefer Array#at / String#at over index arithmetic and charAt.
unicorn/prefer-bigint-literals: Prefer 1n over BigInt(1) and BigInt("1").
unicorn/prefer-blob-reading-methods: Prefer Blob#arrayBuffer() / Blob#text() over FileReader#readAsArrayBuffer / readAsText.
unicorn/prefer-class-fields: Prefer class field declarations over constructor assignments to this.field = value.
unicorn/prefer-classlist-toggle: Prefer Element#classList.toggle(name, condition) over manual add / remove branches.
unicorn/prefer-code-point: Prefer String#codePointAt / String.fromCodePoint over charCodeAt / fromCharCode.
unicorn/prefer-date-now: Prefer Date.now() over new Date().getTime() / +new Date().
unicorn/prefer-default-parameters: Prefer default parameter syntax over x = x ?? default reassignments inside the function body.
unicorn/prefer-dom-node-append: Prefer Node#append over Node#appendChild.
unicorn/prefer-dom-node-dataset: Prefer Element#dataset over getAttribute / setAttribute for data-* attributes.
unicorn/prefer-dom-node-remove: Prefer ChildNode#remove over parent.removeChild(child).
unicorn/prefer-dom-node-text-content: Prefer Node#textContent over HTMLElement#innerText.
unicorn/prefer-event-target: Prefer EventTarget over Node’s EventEmitter when the code is shared between Node and the browser.
unicorn/prefer-export-from: Prefer export ... from over importing-then-re-exporting in two statements.
unicorn/prefer-global-this: Prefer globalThis over window, self, and global.
unicorn/prefer-import-meta-properties: Prefer import.meta.dirname / import.meta.filename over fileURLToPath workarounds.
unicorn/prefer-includes: Prefer String#includes / Array#includes over indexOf(...) !== -1 and some(x => x === target).
unicorn/prefer-json-parse-buffer: Prefer passing a Buffer directly to JSON.parse (Node 21+) instead of decoding to a string first.
unicorn/prefer-keyboard-event-key: Prefer KeyboardEvent#key over the deprecated KeyboardEvent#keyCode / charCode / which.
unicorn/prefer-logical-operator-over-ternary: Prefer a || b / a ?? b over the equivalent ternary a ? a : b.
unicorn/prefer-math-min-max: Prefer Math.min / Math.max over ternaries computing the same value.
unicorn/prefer-math-trunc: Prefer Math.trunc over ~~x / x | 0 for integer truncation.
unicorn/prefer-modern-dom-apis: Prefer before / after / replaceWith over insertBefore / replaceChild / insertAdjacentText.
unicorn/prefer-modern-math-apis: Prefer Math.log10 / Math.hypot / Math.log2 / Math.cbrt over their legacy approximations.
unicorn/prefer-module: Prefer ES modules (import / export) over CommonJS (require / module.exports / __dirname / __filename).
unicorn/prefer-native-coercion-functions: Prefer the bare String / Number / Boolean / BigInt functions over x => String(x) arrow wrappers.
unicorn/prefer-negative-index: Prefer negative-index lookups (arr.at(-1), arr.slice(-2)) over arr.length - 1 / arr.length - 2 arithmetic.
unicorn/prefer-node-protocol: Prefer node:fs / node:path / etc. Over the bare Node builtin specifier.
unicorn/prefer-number-properties: Prefer Number.isNaN / Number.parseInt / Number.NaN over their global counterparts.
unicorn/prefer-object-from-entries: Prefer Object.fromEntries over reduce-into-object patterns.
unicorn/prefer-optional-catch-binding: Prefer catch { ... } over catch (e) { ... } when e is unused.
unicorn/prefer-prototype-methods: Prefer borrowing prototype methods (Array.prototype.slice.call) over [].slice.call empty-instance lookups.
unicorn/prefer-query-selector: Prefer Document#querySelector over getElementById, getElementsByClassName, and getElementsByTagName.
unicorn/prefer-reflect-apply: Prefer Reflect.apply(fn, thisArg, args) over Function.prototype.apply.call(fn, thisArg, args).
unicorn/prefer-regexp-test: Prefer RegExp#test over String#match / RegExp#exec when only a boolean is needed.
unicorn/prefer-response-static-json: Prefer Response.json(value) over new Response(JSON.stringify(value), { headers: { "content-type": "application/json" } }).
unicorn/prefer-set-has: Prefer Set#has over Array#includes for repeated membership lookups against a constant collection.
unicorn/prefer-set-size: Prefer Set#size over [...set].length and Array.from(set).length.
unicorn/prefer-simple-condition-first: Prefer the simpler operand on the left of && / || so the short-circuit reads in evaluation order.
unicorn/prefer-single-call: Prefer a single push / unshift / classList.add / addEventListener with multiple arguments over consecutive single-argument calls.
unicorn/prefer-spread: Prefer spread ([...arr], [...str]) over Array.from, Array.prototype.slice.call, concat([]), and split('').
unicorn/prefer-string-raw: Prefer String.raw for path literals and other strings that would otherwise need backslash escapes.
unicorn/prefer-string-replace-all: Prefer String#replaceAll(literal, replacement) over replace(/literal/g, replacement).
unicorn/prefer-string-slice: Prefer String#slice over the deprecated substr / substring.
unicorn/prefer-string-starts-ends-with: Prefer String#startsWith / String#endsWith over equivalent RegExp#test and slice-then-compare idioms.
unicorn/prefer-string-trim-start-end: Prefer String#trimStart / String#trimEnd over the deprecated trimLeft / trimRight.
unicorn/prefer-structured-clone: Prefer structuredClone(x) over JSON.parse(JSON.stringify(x)) for deep cloning.
unicorn/prefer-switch: Prefer switch over chains of three or more else if clauses comparing the same discriminant.
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.
unicorn/prefer-top-level-await: Prefer top-level await over .then / IIFE wrappers in ES modules.
unicorn/prefer-type-error: Require throwing TypeError (not a bare Error) when the surrounding if is a runtime type check.
unicorn/prevent-abbreviations: Reject common identifier abbreviations (btn, arr, idx) and replace them with their long forms.
unicorn/relative-url-style: Enforce a single style (always leading ./ vs. Never) for relative URLs passed to new URL.
unicorn/require-array-join-separator: Require an explicit separator argument to Array#join instead of relying on the default ",".
unicorn/require-module-attributes: Require non-empty with / assert options on import / export statements that use them at all.
unicorn/require-module-specifiers: Require a non-empty specifier list on every import / export statement.
unicorn/require-number-to-fixed-digits-argument: Require an explicit digits argument to Number#toFixed.
unicorn/require-post-message-target-origin: Require an explicit targetOrigin argument to window.postMessage.
unicorn/string-content: Enforce or replace configured string-content patterns (e.g., curly quotes for straight ones).
unicorn/switch-case-braces: Enforce a consistent presence/absence of {} braces around case clauses inside switch.
unicorn/switch-case-break-position: Enforce a consistent position for break (or return / throw) inside case clauses.
unicorn/template-indent: Re-indent the body of tagged template literals (html, gql, sql) to the indentation of the opening backtick.
unicorn/text-encoding-identifier-case: Enforce a canonical case for text-encoding identifiers, "utf-8" (not "UTF-8" / "utf8").
unicorn/throw-new-error: Require throw new Error(...) over throw Error(...).