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eslint/new-cap Style

🚧 An auto-fix is planned for this rule, but not implemented at this time.

What it does

This rule requires constructor names to begin with a capital letter.

Why is this bad?

The new operator in JavaScript creates a new instance of a particular type of object. That type of object is represented by a constructor function. Since constructor functions are just regular functions, the only defining characteristic is that new is being used as part of the call. Native JavaScript functions begin with an uppercase letter to distinguish those functions that are to be used as constructors from functions that are not. Many style guides recommend following this pattern to more easily determine which functions are to be used as constructors.

Warning: The option newIsCapExceptionPattern and capIsNewExceptionPattern are implemented with the rust regex syntax. Many JavaScript features are not supported (Lookahead, Lookbehinds, ...).

Examples

Examples of incorrect code for this rule:

function foo(arg) {
  return Boolean(arg);
}

Examples of incorrect code for this rule with the default { "newIsCap": true } option:

/* new-cap: ["error", { "newIsCap": true }] */

var friend = new person();

Examples of correct code for this rule with the default { "newIsCap": true } option:

/* new-cap: ["error", { "newIsCap": true }] */

var friend = new Person();

Examples of correct code for this rule with the { "newIsCap": false } option:

/* new-cap: ["error", { "newIsCap": false }] */

var friend = new person();

Examples of incorrect code for this rule with the default { "capIsNew": true } option:

/* new-cap: ["error", { "capIsNew": true }] */

var colleague = Person();

Examples of correct code for this rule with the default { "capIsNew": true } option:

/* new-cap: ["error", { "capIsNew": true }] */

var colleague = new Person();

Examples of correct code for this rule with the { "capIsNew": false } option:

/* new-cap: ["error", { "capIsNew": false }] */

var colleague = Person();

Examples of additional correct code for this rule with the { "newIsCapExceptions": ["events"] } option:

/* new-cap: ["error", { "newIsCapExceptions": ["events"] }] */

var events = require("events");

var emitter = new events();

Examples of additional correct code for this rule with the { "newIsCapExceptionPattern": "^person\\.." } option:

/* new-cap: ["error", { "newIsCapExceptionPattern": "^person\\.." }] */

var friend = new person.acquaintance();

var bestFriend = new person.friend();

Examples of additional correct code for this rule with the { "newIsCapExceptionPattern": "\\.bar$" } option:

/* new-cap: ["error", { "newIsCapExceptionPattern": "\\.bar$" }] */

var friend = new person.bar();

Examples of additional correct code for this rule with the { "capIsNewExceptions": ["Person"] } option:

/* new-cap: ["error", { "capIsNewExceptions": ["Person"] }] */

function foo(arg) {
  return Person(arg);
}

Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "^person\\.." } option:

/* new-cap: ["error", { "capIsNewExceptionPattern": "^person\\.." }] */

var friend = person.Acquaintance();
var bestFriend = person.Friend();

Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "\\.Bar$" } option:

/* new-cap: ["error", { "capIsNewExceptionPattern": "\\.Bar$" }] */

foo.Bar();

Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "^Foo" } option:

/* new-cap: ["error", { "capIsNewExceptionPattern": "^Foo" }] */

var x = Foo(42);

var y = Foobar(42);

var z = Foo.Bar(42);

properties

Examples of incorrect code for this rule with the default { "properties": true } option:

/* new-cap: ["error", { "properties": true }] */

var friend = new person.acquaintance();

Examples of correct code for this rule with the default { "properties": true } option:

/* new-cap: ["error", { "properties": true }] */

var friend = new person.Acquaintance();

Examples of correct code for this rule with the { "properties": false } option:

/* new-cap: ["error", { "properties": false }] */

var friend = new person.acquaintance();

Examples of incorrect code for this rule with the default { "newIsCap": true } option:

/* new-cap: ["error", { "newIsCap": true }] */

var friend = new person();

Examples of correct code for this rule with the default { "newIsCap": true } option:

/* new-cap: ["error", { "newIsCap": true }] */

var friend = new Person();

Examples of correct code for this rule with the { "newIsCap": false } option:

/* new-cap: ["error", { "newIsCap": false }] */

var friend = new person();

Examples of incorrect code for this rule with the default { "capIsNew": true } option:

/* new-cap: ["error", { "capIsNew": true }] */

var colleague = Person();

Examples of correct code for this rule with the default { "capIsNew": true } option:

/* new-cap: ["error", { "capIsNew": true }] */

var colleague = new Person();

Examples of correct code for this rule with the { "capIsNew": false } option:

/* new-cap: ["error", { "capIsNew": false }] */

var colleague = Person();

Examples of additional correct code for this rule with the { "newIsCapExceptions": ["events"] } option:

/* new-cap: ["error", { "newIsCapExceptions": ["events"] }] */

var events = require("events");

var emitter = new events();

Examples of additional correct code for this rule with the { "newIsCapExceptionPattern": "^person\\.." } option:

/* new-cap: ["error", { "newIsCapExceptionPattern": "^person\\.." }] */

var friend = new person.acquaintance();

var bestFriend = new person.friend();

Examples of additional correct code for this rule with the { "newIsCapExceptionPattern": "\\.bar$" } option:

/* new-cap: ["error", { "newIsCapExceptionPattern": "\\.bar$" }] */

var friend = new person.bar();

Examples of additional correct code for this rule with the { "capIsNewExceptions": ["Person"] } option:

::: correct

/* new-cap: ["error", { "capIsNewExceptions": ["Person"] }] */

function foo(arg) {
  return Person(arg);
}

Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "^person\\.." } option:

/* new-cap: ["error", { "capIsNewExceptionPattern": "^person\\.." }] */

var friend = person.Acquaintance();
var bestFriend = person.Friend();

Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "\\.Bar$" } option:

/* new-cap: ["error", { "capIsNewExceptionPattern": "\\.Bar$" }] */

foo.Bar();

Examples of additional correct code for this rule with the { "capIsNewExceptionPattern": "^Foo" } option:

/* new-cap: ["error", { "capIsNewExceptionPattern": "^Foo" }] */

var x = Foo(42);

var y = Foobar(42);

var z = Foo.Bar(42);

Examples of incorrect code for this rule with the default { "properties": true } option:

/* new-cap: ["error", { "properties": true }] */

var friend = new person.acquaintance();

Examples of correct code for this rule with the default { "properties": true } option:

/* new-cap: ["error", { "properties": true }] */

var friend = new person.Acquaintance();

Examples of correct code for this rule with the { "properties": false } option:

/* new-cap: ["error", { "properties": false }] */

var friend = new person.acquaintance();

Configuration

This rule accepts a configuration object with the following properties:

capIsNew

type: boolean

default: true

true to require that all functions with names starting with an uppercase letter to be called with new.

capIsNewExceptionPattern

type: string

A regex pattern to match exceptions for functions with names starting with an uppercase letter.

capIsNewExceptions

type: string[]

default: []

Exceptions to ignore for functions with names starting with an uppercase letter.

newIsCap

type: boolean

default: true

true to require that all constructor names start with an uppercase letter, e.g. new Person().

newIsCapExceptionPattern

type: string

A regex pattern to match exceptions for constructor names starting with an uppercase letter.

newIsCapExceptions

type: string[]

default: ["Array", "Boolean", "Date", "Error", "Function", "Number", "Object", "RegExp", "String", "Symbol", "BigInt"]

Exceptions to ignore for constructor names starting with an uppercase letter.

properties

type: boolean

default: true

true to require capitalization for object properties (e.g., new obj.Method()).

How to use

To enable this rule using the config file or in the CLI, you can use:

Config (.oxlintrc.json)

{
  "rules": {
    "new-cap": "error"
  }
}

Config (oxlint.config.ts)

import { defineConfig } from "oxlint";

export default defineConfig({
  rules: {
    "new-cap": "error",
  },
});

CLI

oxlint --deny new-cap

Version

This rule was added in v0.15.5.

References

• Rule Source
• Upstream rule docs
• Open rule in Oxc Playground