npm package: @stdlib/ndarray-promotion-rules
Description: Return the ndarray data type with the smallest size and closest kind to which ndarray data types can be safely cast.
Author: The Stdlib Authors
Latest version: 0.4.1

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# Promotion Rules

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

> Return the ndarray [data type][@stdlib/ndarray/dtypes] with the smallest size and closest "kind" to which ndarray [data types][@stdlib/ndarray/dtypes] can be **safely** cast.

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<section class="intro">

</section>

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<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/ndarray-promotion-rules
```

</section>

<section class="usage">

## Usage

```javascript
var promotionRules = require( '@stdlib/ndarray-promotion-rules' );
```

#### promotionRules( \[dtype1, dtype2] )

If provided [data types][@stdlib/ndarray/dtypes], returns the ndarray [data type][@stdlib/ndarray/dtypes] with the smallest size and closest "kind" to which ndarray [data types][@stdlib/ndarray/dtypes] can be **safely** cast.

```javascript
var out = promotionRules( 'float32', 'uint32' );
// returns 'float64'
```

If a [data type][@stdlib/ndarray/dtypes] to which [data types][@stdlib/ndarray/dtypes] can be safely cast does **not** exist (or is not supported), the function returns `-1`.

```javascript
var out = promotionRules( 'binary', 'generic' );
// returns -1
```

If not provided [data types][@stdlib/ndarray/dtypes], the function returns a promotion table.

```javascript
var out = promotionRules();
// returns {...}

var f32 = out[ 'float32' ];
// returns {...}

var rule = f32[ 'uint32' ];
// returns 'float64'
```

If provided an unrecognized or unsupported `dtype`, the function returns `null`.

```javascript
var out = promotionRules( 'foo', 'generic' );
// returns null
```

</section>

<!-- /.usage -->

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<section class="notes">

</section>

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<!-- Package usage examples. -->

<section class="examples">

## Examples

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

```javascript
var cartesianProduct = require( '@stdlib/array-cartesian-product' );
var dtypes = require( '@stdlib/ndarray-dtypes' );
var unzip = require( '@stdlib/utils-unzip' );
var logEachMap = require( '@stdlib/console-log-each-map' );
var promotionRules = require( '@stdlib/ndarray-promotion-rules' );

// Get the list of supported ndarray data types:
var dt = dtypes();

// Generate a list of data type pairs:
var pairs = cartesianProduct( dt, dt );

// Split the pairs into separate arrays:
var args = unzip( pairs );

// Print the promotion rule for each pair of ndarray data types:
logEachMap( '(%s, %s) => %s', args[ 0 ], args[ 1 ], promotionRules );
```

</section>

<!-- /.examples -->

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</section>

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<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/ndarray-casting-modes`][@stdlib/ndarray/casting-modes]</span><span class="delimiter">: </span><span class="description">list of ndarray casting modes.</span>
-   <span class="package-name">[`@stdlib/ndarray-dtypes`][@stdlib/ndarray/dtypes]</span><span class="delimiter">: </span><span class="description">list of ndarray data types.</span>
-   <span class="package-name">[`@stdlib/ndarray-safe-casts`][@stdlib/ndarray/safe-casts]</span><span class="delimiter">: </span><span class="description">return a list of ndarray data types to which a provided ndarray data type can be safely cast.</span>

</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].

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