# 07. Species Pattern

Sometimes a method creates new instances of its class. If you create a subclass – should the method return an instance of its class or an instance of the subclass? A few built-in ES6 methods let you configure how they create instances via the so-called *species pattern*.

The standard species pattern is used by `Promise.prototype.then()`, the `filter()` method of Typed Arrays and other operations. It works as follows:

* If `this.constructor[Symbol.species]` exists, use it as a constructor for the new instance.
* Otherwise, use a default constructor (e.g. `Array` for `Arrays`).

Implemented in JavaScript, the pattern would look like this:

```javascript
function SpeciesConstructor(O, defaultConstructor) {
  const C = O.constructor;
  if (C === undefined) {
    return defaultConstructor;
  }
  if (!isObject(C)) {
    throw new TypeError();
  }
  const S = C[Symbol.species];
  if (S === undefined || S === null) {
    return defaultConstructor;
  }
  if (!isConstructor(S)) {
    throw new TypeError();
  }
  return S;
}
```

## `Symbol.species`

This is the default getter for the property `[Symbol.species]`:

```javascript
static get [Symbol.species]() {
    return this;
}
```

You can override the default species via a static getter (line A):

```javascript
class MyArray1 extends Array {
  static get [Symbol.species]() {
    // (A)
    return Array;
  }
}
```

As a result, `map()` returns an instance of `Array`:

```javascript
const result1 = new MyArray1().map(x => x);
console.log(result1 instanceof Array); // true
```

If you don’t override the default species, `map()` returns an instance of the subclass:

```javascript
class MyArray2 extends Array {}

const result2 = new MyArray2().map(x => x);
console.log(result2 instanceof MyArray2); // true
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://strctr.gitbook.io/programming/01-languages/javascript/01-language/c-functions/c3-classes/07-species-pattern.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.