An object.

When a function is called with new, the constructor's prototype property will become the resulting object's prototype.

function Ctor() {}
const inst = new Ctor();
console.log(Object.getPrototypeOf(inst) === Ctor.prototype); // true

You can read Inheritance and the prototype chain for more information about the interactions between a constructor function's prototype property and the resulting object's prototype.

A function having a prototype property is not sufficient for it to be eligible as a constructor. Generator functions have a prototype property, but cannot be called with new:

async function* asyncGeneratorFunction() {}
function* generatorFunction() {}

Instead, generator functions' prototype property is used when they are called without new. The prototype property will become the returned Generator object's prototype.

In addition, some functions may have a prototype but throw unconditionally when called with new. For example, the Symbol() and BigInt() functions throw when called with new, because Symbol.prototype and BigInt.prototype are only intended to provide methods for the primitive values, but the wrapper objects should not be directly constructed.

The following functions do not have prototype, and are therefore ineligible as constructors, even if a prototype property is later manually assigned:

const method = { foo() {} }.foo;
const arrowFunction = () => {};
async function asyncFunction() {}

The following are valid constructors that have prototype:

class Class {}
function fn() {}

A bound function does not have a prototype property, but may be constructable. When it's constructed, the target function is constructed instead, and if the target function is constructable, it would return a normal instance.

const boundFunction = function () {}.bind(null);

A function's prototype property, by default, is a plain object with one property: constructor, which is a reference to the function itself. The constructor property is writable, non-enumerable, and configurable.

If the prototype of a function is reassigned with something other than an Object, when the function is called with new, the returned object's prototype would be Object.prototype instead. (In other words, new ignores the prototype property and constructs a plain object.)

function Ctor() {}
Ctor.prototype = 3;
console.log(Object.getPrototypeOf(new Ctor()) === Object.prototype); // true

Class fields add properties to each instance. Class methods declare function properties on the prototype. However, there's no way to add a non-function property to the prototype. In case you want to share static data between all instances (for example, Error.prototype.name is the same between all error instances), you can manually assign it on the prototype of a class.

class Dog {
  constructor(name) {
    this.name = name;
  }
}

Dog.prototype.species = "dog";

console.log(new Dog("Jack").species); // "dog"

This can be made more ergonomic using static initialization blocks, which are called when the class is initialized.

class Dog {
  static {
    Dog.prototype.species = "dog";
  }
  constructor(name) {
    this.name = name;
  }
}

console.log(new Dog("Jack").species); // "dog"

• Function
• Inheritance and the prototype chain