new-cap

A capital first letter is how you tell a constructor from an ordinary function. If names don't follow that convention, it's easy to forget new. A class then throws, and an older constructor function runs with the wrong this.

The rule reports two things: new with a name that starts with a lowercase letter, and a call without new to a name that starts with a capital letter.

The built-ins that work without new are allowed: Array, BigInt, Boolean, Date, Error, Function, Number, Object, RegExp, String, Symbol, and Date.UTC().

Configure

Off by default. To turn it on:

[lint.rules]
"new-cap" = "error" # or "warn"

Every option, set to its default. The ones that are commented out are unset by default.

[lint.rules]
"new-cap" = ["error", {
  newIsCap = true,
  capIsNew = true,
  properties = true,
  newIsCapExceptions = [], # list of strings
  # newIsCapExceptionPattern = <string>,
  capIsNewExceptions = [], # list of strings
  # capIsNewExceptionPattern = <string>,
}]

Examples

Reported:

const user = new person("Ada");
const team = Team("core");

Not reported:

const user = new Person("Ada");
const team = createTeam("core");
const id = String(42);
const start = Date.UTC(2026, 0, 1);

Options

[lint.rules]
new-cap = ["error", { capIsNewExceptions = ["Router"] }]

Option	Default	What it does
newIsCap	true	Report new with a lowercase name.
capIsNew	true	Report a call to a capitalized name without new.
properties	true	Also check names after a dot, as in new net.socket() and express.Router().
newIsCapExceptions	[]	Lowercase names that are allowed after new.
newIsCapExceptionPattern	none	A regular expression for those names.
capIsNewExceptions	[]	Capitalized names that can be called without new.
capIsNewExceptionPattern	none	A regular expression for those names.

Reported by default:

const router = express.Router();

Not reported with capIsNewExceptions: ["Router"]. "express.Router" works too:

const router = express.Router();

A pattern is tested against the whole express.Router. Not reported with capIsNewExceptionPattern: "^express\\.":

const router = express.Router();

Not reported with newIsCapExceptions: ["moment"]:

const now = new moment();

Not reported with properties: false:

const router = express.Router();
const socket = new net.socket();

When not to use it

Some libraries export capitalized functions that are meant to be called without new. If you use many of them, set capIsNew to false.