unshift

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

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 );