@stdlib/assert/is-prototype-of/README.md

Standard assertion utilities.

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# isPrototypeOf

> Test if an object's prototype chain contains a provided prototype.

<section class="intro">

</section>

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<section class="usage">

## Usage

<!-- eslint-disable stdlib/no-redeclare -->

```javascript
var isPrototypeOf = require( '@stdlib/assert/is-prototype-of' );
```

#### isPrototypeOf( obj, prototype )

Tests if an `object`'s prototype chain contains a provided `prototype`.

<!-- eslint-disable stdlib/no-redeclare -->

```javascript
var inherit = require( '@stdlib/utils/inherit' );

function Foo() {
    return this;
}

function Bar() {
    return this;
}
inherit( Bar, Foo );

var bar = new Bar();

var bool = isPrototypeOf( bar, Foo.prototype );
// returns true
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   The function returns `false` if provided a primitive value.

    <!-- eslint-disable stdlib/no-redeclare -->

    ```javascript
    var Number = require( '@stdlib/number/ctor' );

    var bool = isPrototypeOf( 5, Number.prototype );
    // returns false

    bool = isPrototypeOf( 'beep', String.prototype );
    // returns false

    bool = isPrototypeOf( true, Boolean.prototype );
    // returns false
    ```

-   The function throws a `TypeError` if provided a `prototype` value which is neither an `object` (except `null`) or a `function`.

    <!-- eslint-disable stdlib/no-redeclare -->

    ```javascript
    var bool = isPrototypeOf( Object.create( null ), null );
    // throws <TypeError>
    ```

-   `isPrototypeOf()` is generally more robust than the `instanceof` operator. Consider the following example which does not use constructors:

    <!-- eslint-disable stdlib/no-redeclare -->

    ```javascript
    // Functionally similar to `Object.create()`...
    function createObject( proto ) {
        function Ctor() {
            // Empty constructor...
        }
        Ctor.prototype = proto;
        return new Ctor();
    }
    var superProto = {
        'beep': 'beep'
    };

    var subProto = createObject( superProto );
    subProto.boop = 'boop';

    var v = createObject( subProto );

    var bool;
    try {
        bool = ( v instanceof superProto );
    } catch ( error ) {
        // Encountered a type error...
        console.error( error.message );
    }

    bool = isPrototypeOf( v, superProto );
    // returns true
    ```

</section>

<!-- /.notes -->

<section class="examples">

## Examples

<!-- eslint-disable stdlib/no-redeclare -->

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

```javascript
var inherit = require( '@stdlib/utils/inherit' );
var isPrototypeOf = require( '@stdlib/assert/is-prototype-of' );

function A() {
    return this;
}

function B() {
    return this;
}
inherit( B, A );

function C() {
    return this;
}
inherit( C, B );

function D() {
    return this;
}
inherit( D, C );

var a = new A();
var b = new B();
var c = new C();
var d = new D();

var bool = isPrototypeOf( d, C.prototype );
// returns true

bool = isPrototypeOf( d, B.prototype );
// returns true

bool = isPrototypeOf( d, A.prototype );
// returns true

bool = isPrototypeOf( c, B.prototype );
// returns true

bool = isPrototypeOf( c, A.prototype );
// returns true

bool = isPrototypeOf( c, D.prototype );
// returns false

bool = isPrototypeOf( b, A.prototype );
// returns true

bool = isPrototypeOf( b, C.prototype );
// returns false

bool = isPrototypeOf( b, D.prototype );
// returns false

bool = isPrototypeOf( a, B.prototype );
// returns false

bool = isPrototypeOf( a, C.prototype );
// returns false

bool = isPrototypeOf( a, D.prototype );
// returns false
```

</section>

<!-- /.examples -->

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<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/utils/get-prototype-of`][@stdlib/utils/get-prototype-of]</span><span class="delimiter">: </span><span class="description">return the prototype of a provided object.</span>

</section>

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<section class="links">

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[@stdlib/utils/get-prototype-of]: https://www.npmjs.com/package/@stdlib/utils-get-prototype-of

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</section>

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