TypeError: "x" is not a constructor

The JavaScript exception "is not a constructor" occurs when there was an attempt to use an object or a variable as a constructor, but that object or variable is not a constructor.

Message#

TypeError: x is not a constructor (V8-based & Firefox & Safari)

Error type#

{{jsxref("TypeError")}}

What went wrong?#

There was an attempt to use an object or a variable as a constructor, but that object or variable is not a constructor. See constructor or the new operator for more information on what a constructor is.

There are many global objects, like {{jsxref("String")}} or {{jsxref("Array")}}, which are constructable using new. However, some global objects are not and their properties and methods are static. The following JavaScript standard built-in objects are not a constructor: {{jsxref("Math")}}, {{jsxref("JSON")}}, {{jsxref("Symbol")}}, {{jsxref("Reflect")}}, {{jsxref("Intl")}}, {{jsxref("Atomics")}}.

Generator functions cannot be used as constructors either.

Examples#

Invalid cases#

```js example-bad const Car = 1; new Car(); // TypeError: Car is not a constructor

new Math(); // TypeError: Math is not a constructor

new Symbol(); // TypeError: Symbol is not a constructor

function* f() {} const obj = new f(); // TypeError: f is not a constructor

### A car constructor

Suppose you want to create an object type for cars. You want this type of object to be
called `Car`, and you want it to have properties for make, model, and year.
To do this, you would write the following function:

```js
function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}

Now you can create an object called myCar as follows:

const myCar = new Car("Eagle", "Talon TSi", 1993);

In Promises#

When returning an immediately-resolved or immediately-rejected Promise, you do not need to create a new Promise(...) and act on it. Instead, use the Promise.resolve() or Promise.reject() static methods.

This is not legal (the Promise constructor is not being called correctly) and will throw a TypeError: this is not a constructor exception:

```js example-bad function fn() { return new Promise.resolve(true); }

This is legal, but unnecessarily long:

```js
function fn() {
  return new Promise((resolve, reject) => {
    resolve(true);
  });
}

Instead, return the static method:

```js example-good function resolveAlways() { return Promise.resolve(true); }

function rejectAlways() { return Promise.reject(new Error()); } ```

See also#

• constructor
• new