npm package: @stdlib/symbol-has-instance
Description: Has instance symbol.
Author: The Stdlib Authors
Latest version: 0.1.1

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<details>
  <summary>
    About stdlib...
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</details>

# HasInstanceSymbol

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

> Has instance [symbol][mdn-symbol] which is used to determine if a constructor object recognizes an object as its instance.

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<section class="intro">

</section>

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<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/symbol-has-instance
```

</section>

<section class="usage">

## Usage

```javascript
var HasInstanceSymbol = require( '@stdlib/symbol-has-instance' );
```

#### HasInstanceSymbol

Has instance [`symbol`][mdn-symbol] which is used to determine if a constructor object recognizes an object as its instance.

```javascript
var s = typeof HasInstanceSymbol;
// e.g., returns 'symbol'
```

</section>

<!-- /.usage -->

<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

## Notes

-   The [symbol][mdn-symbol] is only supported in environments which support [symbols][mdn-symbol]. In non-supporting environments, the value is `null`.

-   The `instanceof` operator uses the following algorithm to determine the return value of `object instanceof constructor`:

    -   If `constructor` has a `[HasInstanceSymbol]()` method, the `instanceof` operator calls the method with `object` as the first argument and returns the result (coerced to a boolean). If `constructor` is not an object or if `constructor[HasInstanceSymbol]` is neither `null`, `undefined`, nor a function, the `instanceof` operator raises an exception.
    -   Otherwise, if `constructor` does not have a `[HasInstanceSymbol]()` method (i.e., `constructor[HasInstanceSymbol]` is `null` or `undefined`), the `instanceof` operator determines the result using the same algorithm as `Function.prototype[HasInstanceSymbol]()`. If `constructor` is not a function, the `instanceof` operator raises an exception.

</section>

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<!-- Package usage examples. -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var isArray = require( '@stdlib/assert-is-array' );
var instanceOf = require( '@stdlib/assert-instance-of' );
var defineProperty = require( '@stdlib/utils-define-property' );
var HasInstanceSymbol = require( '@stdlib/symbol-has-instance' );

function ArrayLike() {
    return {
        'length': 3,
        '0': 4,
        '1': 5,
        '2': 6
    };
}

function hasInstance( instance ) {
    return isArray( instance );
}

var x = [ 1, 2, 3 ];

defineProperty( ArrayLike, HasInstanceSymbol, {
    'configurable': true,
    'value': null
});
console.log( instanceOf( x, ArrayLike ) );

defineProperty( ArrayLike, HasInstanceSymbol, {
    'configurable': true,
    'value': hasInstance
});
console.log( instanceOf( x, ArrayLike ) );
```

</section>

<!-- /.examples -->

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</section>

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<section class="related">

</section>

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<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

</section>

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<section class="links">

[npm-image]: http://img.shields.io/npm/v/@stdlib/symbol-has-instance.svg
[npm-url]: https://npmjs.org/package/@stdlib/symbol-has-instance

[test-image]: https://github.com/stdlib-js/symbol-has-instance/actions/workflows/test.yml/badge.svg?branch=v0.1.1
[test-url]: https://github.com/stdlib-js/symbol-has-instance/actions/workflows/test.yml?query=branch:v0.1.1

[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/symbol-has-instance/main.svg
[coverage-url]: https://codecov.io/github/stdlib-js/symbol-has-instance?branch=main

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[stdlib]: https://github.com/stdlib-js/stdlib

[stdlib-authors]: https://github.com/stdlib-js/stdlib/graphs/contributors

[umd]: https://github.com/umdjs/umd
[es-module]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules

[deno-url]: https://github.com/stdlib-js/symbol-has-instance/tree/deno
[deno-readme]: https://github.com/stdlib-js/symbol-has-instance/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/symbol-has-instance/tree/umd
[umd-readme]: https://github.com/stdlib-js/symbol-has-instance/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/symbol-has-instance/tree/esm
[esm-readme]: https://github.com/stdlib-js/symbol-has-instance/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/symbol-has-instance/blob/main/branches.md

[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/symbol-has-instance/main/LICENSE

[mdn-symbol]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol

</section>

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