---
url: /docs/guide/usage/linter/rules/eslint/new-cap.md
---

### 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](https://docs.rs/regex/latest/regex/). Many JavaScript features
are not supported (Lookahead, Lookbehinds, ...).

### Examples

Examples of **incorrect** code for this rule:

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

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

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

var friend = new person();
```

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

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

var friend = new Person();
```

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

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

var friend = new person();
```

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

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

var colleague = Person();
```

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

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

var colleague = new Person();
```

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

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

var colleague = Person();
```

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

```js
/* 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:

```js
/* 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:

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

var friend = new person.bar();
```

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

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

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

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

```js
/* 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:

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

foo.Bar();
```

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

```js
/* 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:

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

var friend = new person.acquaintance();
```

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

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

var friend = new person.Acquaintance();
```

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

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

var friend = new person.acquaintance();
```

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

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

var friend = new person();
```

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

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

var friend = new Person();
```

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

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

var friend = new person();
```

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

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

var colleague = Person();
```

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

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

var colleague = new Person();
```

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

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

var colleague = Person();
```

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

```js
/* 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:

```js
/* 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:

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

var friend = new person.bar();
```

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

::: correct

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

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

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

```js
/* 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:

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

foo.Bar();
```

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

```js
/* 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:

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

var friend = new person.acquaintance();
```

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

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

var friend = new person.Acquaintance();
```

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

```js
/* 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

## Version

This rule was added in v0.15.5.

## References