♻️ refactor: Improve and simplify from, push, and unshift.

src/Node.js

@@ -3,12 +3,15 @@
  *
  * @class
  * @param {any} value The value to hold.
+ * @param {Node|null} prev The value to hold.
+ * @param {Node|null} next The value to hold.
  */
-export default function Node(value) {
+// eslint-disable-next-line unicorn/prevent-abbreviations
+export default function Node(value, prev, next) {
 	/** @member {any} The value/key held by this node. */
 	this.value = value;
-	/** @member {Node} Pointer to previous (left) sibling */
-	this.prev = null;
-	/** @member {Node} Pointer to next (right) sibling */
-	this.next = null;
+	/** @member {Node|null} Pointer to previous (left) sibling */
+	this.prev = prev;
+	/** @member {Node|null} Pointer to next (right) sibling */
+	this.next = next;
 }

src/_push.js

@@ -0,0 +1,18 @@
+import assert from 'assert';
+import Node from './Node.js';
+
+/**
+ * Push value to list.
+ *
+ * @param {Node} z Last node of first input list (can be null).
+ * @param {any} value Value to push.
+ * @return {Node} The node at the front of the list (new node if empty, input
+ * node otherwise).
+ */
+export default function _push(z, value) {
+	assert(z instanceof Node);
+	assert(z.next === null);
+	const y = new Node(value, z, null);
+	z.next = y;
+	return y;
+}

src/_unshift.js

@@ -0,0 +1,17 @@
+import assert from 'assert';
+import Node from './Node.js';
+
+/**
+ * Unshift value to list.
+ *
+ * @param {Node} x First node of first input list (can be null).
+ * @param {any} value Value to unshift.
+ * @return {Node} The node at the front of the list (hence, the new node).
+ */
+export default function _unshift(x, value) {
+	assert(x instanceof Node);
+	assert(x.prev === null);
+	const y = new Node(value, null, x);
+	x.prev = y;
+	return y;
+}

src/from.js

@@ -1,5 +1,5 @@
 import Node from './Node.js';
-import _concat from './_concat.js';
+import _push from './_push.js';
 
 /**
  * Creates a list from an input iterable.
@@ -13,13 +13,11 @@ export default function from(iterable) {
 
 	if (event.done) return null;
 
-	const first = new Node(event.value);
+	const first = new Node(event.value, null, null);
 	let last = first;
 
 	for (const value of it) {
-		const next = new Node(value);
-		_concat(last, next);
-		last = next;
+		last = _push(last, value);
 	}
 
 	return first;

src/index.js

@@ -5,6 +5,7 @@ export {default as _iter_fast} from './_iter_fast.js';
 export {default as _last} from './_last.js';
 export {default as _len} from './_len.js';
 export {default as _pop} from './_pop.js';
+export {default as _push} from './_push.js';
 export {default as _remove} from './_remove.js';
 export {default as _rotate_left} from './_rotate_left.js';
 export {default as _rotate_left_modulo} from './_rotate_left_modulo.js';
@@ -15,6 +16,7 @@ export {default as _rotate_right_unknown_length} from './_rotate_right_unknown_l
 export {default as _rotate_to} from './_rotate_to.js';
 export {default as _shift} from './_shift.js';
 export {default as _split} from './_split.js';
+export {default as _unshift} from './_unshift.js';
 export {default as concat} from './concat.js';
 export {default as empty} from './empty.js';
 export {default as from} from './from.js';

src/push.js

@@ -1,6 +1,6 @@
 import assert from 'assert';
 import Node from './Node.js';
-import _concat from './_concat.js';
+import _push from './_push.js';
 
 /**
  * Push value to list.
@@ -12,11 +12,8 @@ import _concat from './_concat.js';
  * node otherwise).
  */
 export default function push(x, z, value) {
-	if (x === null) return new Node(value);
+	if (x === null) return new Node(value, null, null);
 	assert(x instanceof Node);
-	assert(z instanceof Node);
-	assert(z.next === null);
-	const y = new Node(value);
-	_concat(z, y);
+	_push(z, value);
 	return x;
 }

src/unshift.js

@@ -1,18 +1,14 @@
-import assert from 'assert';
 import Node from './Node.js';
-import _concat from './_concat.js';
+import _unshift from './_unshift.js';
 
 /**
  * Unshift value to list.
  *
  * @param {Node} x First node of first input list (can be null).
- * @param {Object} value Value to unshift.
+ * @param {any} value Value to unshift.
  * @return {Node} The node at the front of the list (hence, the new node).
  */
 export default function unshift(x, value) {
-	if (x === null) return new Node(value);
-	assert(x instanceof Node);
-	const y = new Node(value);
-	_concat(y, x);
-	return y;
+	if (x === null) return new Node(value, null, null);
+	return _unshift(x, value);
 }