npm package: @stdlib/dstructs-doubly-linked-list
Description: Doubly linked list.
Author: The Stdlib Authors
Latest version: 0.1.1

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# Doubly Linked List

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

> Doubly linked list constructor.

<!-- 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/dstructs-doubly-linked-list
```

</section>

<section class="usage">

## Usage

```javascript
var doublyLinkedList = require( '@stdlib/dstructs-doubly-linked-list' );
```

#### doublyLinkedList()

Returns a new doubly linked list instance.

```javascript
var list = doublyLinkedList();
// returns <DoublyLinkedList>
```

##### list.clear()

Clears a list.

```javascript
var list = doublyLinkedList();
// returns <DoublyLinkedList>

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Peek at the first value:
var v = list.first().value;
// returns 'foo'

// Examine the list length:
var len = list.length;
// returns 2

// Clear all list items:
list.clear();

// Peek at the first value:
v = list.first();
// returns undefined

// Examine the list length:
len = list.length;
// returns 0
```

##### list.first()

Returns the first `node`. If the list is currently empty, the returned value is `undefined`.

```javascript
var list = doublyLinkedList();
// returns <DoublyLinkedList>

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Peek at the first value:
var v = list.first().value;
// returns 'foo'
```

##### list.insert( node, value\[, location] )

Inserts a `value` into the list either before or after a provided list `node`.

```javascript
var list = doublyLinkedList();
// returns <DoublyLinkedList>

// Add values to the list:
list.push( 'foo' ).push( 'bar' ).push( 'beep' );

// Determine the list length:
var len = list.length;
// returns 3

// Get the second node:
var node = list.first().next;

// Insert a value after the second node:
list.insert( node, 'boop' );

// Determine the list length:
len = list.length;
// returns 4

// Return a list of values:
var values = list.toArray();
// returns [ 'foo', 'bar', 'boop', 'beep' ]
```

The method supports the following insertion locations:

-   `'before'`: insert a `value` into the list **before** a provided list `node`.
-   `'after'`: insert a `value` into the list **after** a provided list `node`.

By default, the method inserts a `value` into the list **after** a provided list `node`. To insert a value **before** a provided list `node`, invoke the method with the `location` argument equal to `'before'`.

```javascript
var list = doublyLinkedList();
// returns <DoublyLinkedList>

// Add values to the list:
list.push( 'foo' ).push( 'bar' ).push( 'beep' );

// Determine the list length:
var len = list.length;
// returns 3

// Get the second node:
var node = list.first().next;

// Insert a value before the second node:
list.insert( node, 'boop', 'before' );

// Determine the list length:
len = list.length;
// returns 4

// Return a list of values:
var values = list.toArray();
// returns [ 'foo', 'boop', 'bar', 'beep' ]
```

##### list.iterator( \[direction] )

Returns an iterator for iterating over a list. If an environment supports `Symbol.iterator`, the returned iterator is iterable.

```javascript
var list = doublyLinkedList();

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Create an iterator:
var it = list.iterator();

// Iterate over the list...
var v = it.next().value;
// returns 'foo'

v = it.next().value;
// returns 'bar'

var bool = it.next().done;
// returns true
```

The method supports the following iteration directions:

-   `'forward'`: iterate over a list from the first value until the last value.
-   `'reverse'`: iterate over a list from the last value until the first value.

By default, the method returns an iterator which iterates over a list from the first value until the last value. To return an iterator which iterates in reverse order, invoke the method with the `direction` argument equal to `'reverse'`.

```javascript
var list = doublyLinkedList();

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Create an iterator:
var it = list.iterator( 'reverse' );

// Iterate over the list...
var v = it.next().value;
// returns 'bar'

v = it.next().value;
// returns 'foo'

var bool = it.next().done;
// returns true
```

**Note**: in order to prevent confusion arising from list mutation during iteration, a returned iterator **always** iterates over a list "snapshot", which is defined as the list of list elements at the time of `list.iterator()` invocation.

##### list.last()

Returns the last `node`. If the list is currently empty, the returned value is `undefined`.

```javascript
var list = doublyLinkedList();
// returns <DoublyLinkedList>

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Peek at the last value:
var v = list.last().value;
// returns 'bar'
```

##### list.length

List length.

```javascript
var list = doublyLinkedList();

// Examine the initial list length:
var len = list.length;
// returns 0

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Retrieve the current list length:
len = list.length;
// returns 2
```

##### list.pop()

Removes a value from the end of the list. If the list is currently empty, the returned value is `undefined`.

```javascript
var list = doublyLinkedList();

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Remove the last value:
var v = list.pop();
// returns 'bar'

// Add a new value to the list:
list.push( 'beep' );

// Remove the last value:
v = list.pop();
// returns 'beep'
```

##### list.push( value )

Adds a value to the end of the list.

```javascript
var list = doublyLinkedList();

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Remove the last value:
var v = list.pop();
// returns 'bar'

// Add a new value to the list:
list.push( 'beep' );

// Remove the last value:
v = list.pop();
// returns 'beep'
```

##### list.remove( node )

Removes a `node` from the list.

```javascript
var list = doublyLinkedList();

// Add values to the list:
list.push( 'foo' ).push( 'bar' ).push( 'beep' );

// Determine the list length:
var len = list.length;
// returns 3

// Get the second node:
var node = list.first().next;

// Remove the second node:
var v = list.remove( node );
// returns 'bar'

// Determine the list length:
len = list.length;
// returns 2
```

##### list.shift()

Removes a value from the beginning of the list. If the list is currently empty, the returned value is `undefined`.

```javascript
var list = doublyLinkedList();

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Remove the first value:
var v = list.shift();
// returns 'foo'

// Add a new value to the list:
list.push( 'beep' );

// Remove the first value:
v = list.shift();
// returns 'bar'
```

##### list.toArray()

Returns an array of list values.

```javascript
var list = doublyLinkedList();

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Get an array of list values:
var vals = list.toArray();
// returns [ 'foo', 'bar' ]
```

##### list.toJSON()

Serializes a list as JSON.

```javascript
var list = doublyLinkedList();

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Serialize to JSON:
var o = list.toJSON();
// returns { 'type': 'doubly-linked-list', 'data': [ 'foo', 'bar' ] }
```

**Note**: `JSON.stringify()` implicitly calls this method when stringifying a list instance.

##### list.unshift( value )

Adds a value to the beginning of the list.

```javascript
var list = doublyLinkedList();

// Add values to the list:
list.unshift( 'foo' ).unshift( 'bar' );

// Remove the last value:
var v = list.pop();
// returns 'foo'

// Add a new value to the list:
list.unshift( 'beep' );

// Remove the last value:
v = list.pop();
// returns 'bar'
```

</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

-   To manually traverse a list, use list node `next` and `prev` properties.

    ```javascript
    var list = doublyLinkedList();

    // Add values to the list:
    list.push( 'foo' ).push( 'bar' ).push( 'beep' ).push( 'boop' );

    // Get the first list node:
    var n = list.first();

    // Walk the list forward...
    while ( n ) {
        console.log( n.value );
        n = n.next;
    }

    // Get the last list node:
    n = list.last();

    // Walk the list backward...
    while ( n ) {
        console.log( n.value );
        n = n.prev;
    }
    ```

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

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

```javascript
var doublyLinkedList = require( '@stdlib/dstructs-doubly-linked-list' );

var list;
var iter;
var len;
var v;
var i;

// Create a new doubly linked list:
list = doublyLinkedList();

// Add some values to the list:
list.push( 'foo' );
list.push( 'bar' );
list.push( 'beep' );
list.push( 'boop' );

// Peek at the first and last list values:
v = list.first().value;
// returns 'foo'

v = list.last().value;
// returns 'boop'

// Inspect the list length:
len = list.length;
// returns 4

// Remove the last list value:
v = list.pop();
// returns 'boop'

// Inspect the list length:
len = list.length;
// returns 3

// Iterate over the list:
iter = list.iterator();
for ( i = 0; i < len; i++ ) {
    console.log( 'List value #%d: %s', i+1, iter.next().value );
}

// Clear the list:
list.clear();

// Inspect the list length:
len = list.length;
// returns 0
```

</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/dstructs-linked-list`][@stdlib/dstructs/linked-list]</span><span class="delimiter">: </span><span class="description">linked list.</span>
-   <span class="package-name">[`@stdlib/dstructs-stack`][@stdlib/dstructs/stack]</span><span class="delimiter">: </span><span class="description">stack.</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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