unshift

Add one or more elements to the beginning of a collection.

Installation

npm install @stdlib/utils-unshift

Usage

var unshift = require( '@stdlib/utils-unshift' );

unshift( collection, ...items )

Adds one or more elements to the beginning of a collection. A collection may be either an Array, Typed Array, or an array-like Object (i.e., an Object having a valid writable length property).

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

var out = unshift( 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

In contrast to Array.prototype.unshift, the function returns the extended collection, rather than the collection length. For typed arrays, the returned value is a new typed array view whose underlying ArrayBuffer may not equal the underlying ArrayBuffer for the input collection.

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

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

Notes

• The function adds one or more elements to a typed array by setting values in the underlying ArrayBuffer. If an ArrayBuffer does not have enough bytes in which to store all elements, the function allocates a new 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.
• Beware when providing typed arrays which are views pointing to a shared (or pooled) ArrayBuffer. Because the function sets ArrayBuffer bytes outside of a provided view, the function may overwrite bytes belonging to one or more external views. This could be a potential security vulnerability. Prefer providing typed arrays which have an exclusive ArrayBuffer; otherwise, be sure to plan for and guard against mutated state.

Examples

var Float64Array = require( '@stdlib/array-float64' );
var unshift = require( '@stdlib/utils-unshift' );

var arr;
var i;

arr = new Float64Array();
for ( i = 0; i < 100; i++ ) {
    arr = unshift( arr, i );
}
console.log( arr );

See Also

• @stdlib/utils-pop: remove and return the last element of a collection.
• @stdlib/utils-push: add one or more elements to the end of a collection.
• @stdlib/utils-shift: remove and return the first element of a collection.

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

License

See LICENSE.

Copyright

Copyright © 2016-2024. The Stdlib Authors.