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HasInstanceSymbol

Has instance symbol which is used to determine if a constructor object recognizes an object as its instance.

Installation

npm install @stdlib/symbol-has-instance

Alternatively,

• To load the package in a website via a script tag without installation and bundlers, use the ES Module available on the esm branch (see README).
• If you are using Deno, visit the deno branch (see README for usage intructions).
• For use in Observable, or in browser/node environments, use the Universal Module Definition (UMD) build available on the umd branch (see README).

The branches.md file summarizes the available branches and displays a diagram illustrating their relationships.

To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.

Usage

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

HasInstanceSymbol

Has instance symbol which is used to determine if a constructor object recognizes an object as its instance.

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

Notes

• The symbol is only supported in environments which support symbols. 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.

Examples

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 ) );

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Notice

This package is part of 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, see the main project repository.

Community

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License

See LICENSE.

Copyright

Copyright © 2016-2025. The Stdlib Authors.