npm package: @stdlib/utils-prepend
Description: Add elements from one collection to the beginning of another collection.
Author: The Stdlib Authors
Latest version: 0.2.3

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  <summary>
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# prepend

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

> Add elements from one collection to the beginning of another collection.

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/utils-prepend
```

</section>

<section class="usage">

## Usage

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

#### prepend( collection1, collection2 )

Adds elements from one `collection` to the beginning of another `collection`. A `collection` may be either an [`Array`][mdn-array], [`Typed Array`][mdn-typed-array], or an array-like [`Object`][mdn-object] (i.e., an [`Object`][mdn-object] having a valid writable `length` property).

```javascript
var arr = [ 1.0, 2.0, 3.0, 4.0, 5.0 ];

var out = prepend( arr, [ 6.0, 7.0 ] );
// returns [ 6.0, 7.0, 1.0, 2.0, 3.0, 4.0, 5.0 ]

var bool = ( out === arr );
// returns true
```

Note that the function returns the extended collection. For [typed arrays][mdn-typed-array], the returned value is a new [typed array][mdn-typed-array] view whose underlying [`ArrayBuffer`][mdn-arraybuffer] may **not** equal the underlying [`ArrayBuffer`][mdn-arraybuffer] for the input `collection`.

```javascript
var ArrayBuffer = require( '@stdlib/array-buffer' );
var Float64Array = require( '@stdlib/array-float64' );

var buf = new ArrayBuffer( 3*8 ); // 8 bytes per double

var arr = new Float64Array( buf, 8, 2 );
arr[ 0 ] = 1.0;
arr[ 1 ] = 2.0;

var out = prepend( arr, [ 3.0 ] );
// returns <Float64Array>[ 3.0, 1.0, 2.0 ]

var bool = ( out === arr );
// returns false

bool = ( out.buffer === arr.buffer );
// returns true

out = prepend( out, [ 4.0 ] );
// returns <Float64Array>[ 4.0, 3.0, 1.0, 2.0 ]

bool = ( out.buffer === arr.buffer );
// returns false
```

</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 function adds elements to a [typed array][mdn-typed-array] by setting values in the underlying [`ArrayBuffer`][mdn-arraybuffer]. If an [`ArrayBuffer`][mdn-arraybuffer] does not have enough bytes in which to store all elements, the function allocates a new [`ArrayBuffer`][mdn-arraybuffer] capable of holding `2^n` elements, where `n` is the next power of `2`. This procedure is similar to how environments internally handle dynamic memory allocation for [`Arrays`][mdn-array].
-   Beware when providing [typed arrays][mdn-typed-array] which are views pointing to a shared (or pooled) [`ArrayBuffer`][mdn-arraybuffer]. Because the function sets [`ArrayBuffer`][mdn-arraybuffer] bytes outside of a provided [view][mdn-typed-array], the function may overwrite bytes belonging to one or more external views. This could be a potential **security vulnerability**. Prefer providing [typed arrays][mdn-typed-array] which have an exclusive [`ArrayBuffer`][mdn-arraybuffer]; otherwise, be sure to plan for and guard against mutated state.

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

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

```javascript
var Float64Array = require( '@stdlib/array-float64' );
var prepend = require( '@stdlib/utils-prepend' );

var arr;
var i;
var j;

arr = new Float64Array();
for ( i = 0; i < 100; i++ ) {
    j = i * 3;
    arr = prepend( arr, [ j+2, j+1, j ] );
}
console.log( arr );
```

</section>

<!-- /.examples -->

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<section class="references">

</section>

<!-- /.references -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/utils-append`][@stdlib/utils/append]</span><span class="delimiter">: </span><span class="description">add elements from one collection to the end of another collection.</span>
-   <span class="package-name">[`@stdlib/utils-unshift`][@stdlib/utils/unshift]</span><span class="delimiter">: </span><span class="description">add one or more elements to the beginning of a collection.</span>

</section>

<!-- /.related -->

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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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[test-url]: https://github.com/stdlib-js/utils-prepend/actions/workflows/test.yml?query=branch:v0.2.3

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[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/utils-prepend/tree/deno
[deno-readme]: https://github.com/stdlib-js/utils-prepend/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/utils-prepend/tree/umd
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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-prepend/main/LICENSE

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

[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Typed_arrays

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

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

<!-- <related-links> -->

[@stdlib/utils/append]: https://www.npmjs.com/package/@stdlib/utils-append

[@stdlib/utils/unshift]: https://www.npmjs.com/package/@stdlib/utils-unshift

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</section>

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