Fix value property to return undefined for nodes without value concept (#260)

## Summary
Fixed a regression where certain CSS node types were incorrectly
returning `null` instead of `undefined` for their `value` property. This
distinction is semantically important: `null` means "has a value, but it
is absent", while `undefined` means "value is not a property of this
node type".

## Key Changes
- **Updated `CSSNode.value` getter** in `src/css-node.ts` to explicitly
return `undefined` for node types that don't have a value concept,
rather than attempting to read from the arena value fields
- **Added comprehensive regression tests** in `src/api.test.ts` covering
20+ node types that should return `undefined` for their value property,
including:
  - Structural nodes: StyleSheet, Rule, Block, SelectorList, Selector
- Selector types: TypeSelector, ClassSelector, IdSelector,
PseudoClassSelector, PseudoElementSelector, UniversalSelector,
Combinator, Nth
  - Value nodes: Value, Identifier, Hash, String
  - At-rule nodes: Atrule, MediaQuery
  - Other nodes: Raw

## Implementation Details
The fix adds an explicit type check that returns `undefined` early for
all node types except the five that actually store values in the arena:
- `DECLARATION`
- `FUNCTION`
- `ATTRIBUTE_SELECTOR`
- `SUPPORTS_QUERY`
- `PRELUDE_SELECTORLIST`

This ensures the correct semantic distinction between nodes that have no
value concept (undefined) and nodes that have a value but it's empty
(null).

https://claude.ai/code/session_01KETKttcckhFQtg8JzjxvuU

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: bartveneman

src/api.test.ts

@@ -1045,6 +1045,174 @@ describe('CSSNode', () => {
 	})
 })
 
+describe('value property - undefined for nodes without a value concept', () => {
+	// Regression: these node types were incorrectly returning null instead of undefined.
+	// null means "has a value, but it is absent"; undefined means "value is not a property of this node type".
+	//
+	// These types intentionally omit `value` from their TypeScript interfaces, so we
+	// access the runtime getter via the base CSSNode class to test the JS behavior.
+	function get_value(node: unknown) {
+		return (node as CSSNode).value
+	}
+
+	test('StyleSheet.value is undefined', () => {
+		const root = parse('div { color: red; }')
+		expect(get_value(root)).toBeUndefined()
+	})
+
+	test('Rule.value is undefined', () => {
+		const root = parse('div { color: red; }')
+		const rule = root.first_child!
+		expect(get_value(rule)).toBeUndefined()
+	})
+
+	test('SelectorList.value is undefined', () => {
+		const root = parse('div { color: red; }')
+		const rule = root.first_child!
+		const selectorList = rule.first_child!
+		expect(selectorList.type_name).toBe('SelectorList')
+		expect(get_value(selectorList)).toBeUndefined()
+	})
+
+	test('Selector.value is undefined', () => {
+		const root = parse('div { color: red; }')
+		const selector = root.first_child!.first_child!.first_child!
+		expect(selector.type_name).toBe('Selector')
+		expect(get_value(selector)).toBeUndefined()
+	})
+
+	test('TypeSelector.value is undefined', () => {
+		const root = parse('div { color: red; }')
+		const typeSelector = root.first_child!.first_child!.first_child!.first_child!
+		expect(typeSelector.type_name).toBe('TypeSelector')
+		expect(get_value(typeSelector)).toBeUndefined()
+	})
+
+	test('ClassSelector.value is undefined', () => {
+		const root = parse('.foo { color: red; }')
+		const classSelector = root.first_child!.first_child!.first_child!.first_child!
+		expect(classSelector.type_name).toBe('ClassSelector')
+		expect(get_value(classSelector)).toBeUndefined()
+	})
+
+	test('IdSelector.value is undefined', () => {
+		const root = parse('#bar { color: red; }')
+		const idSelector = root.first_child!.first_child!.first_child!.first_child!
+		expect(idSelector.type_name).toBe('IdSelector')
+		expect(get_value(idSelector)).toBeUndefined()
+	})
+
+	test('PseudoClassSelector.value is undefined', () => {
+		const root = parse('a:hover { color: red; }')
+		const selector = root.first_child!.first_child!.first_child!
+		const pseudo = selector.first_child!.next_sibling!
+		expect(pseudo.type_name).toBe('PseudoClassSelector')
+		expect(get_value(pseudo)).toBeUndefined()
+	})
+
+	test('PseudoElementSelector.value is undefined', () => {
+		const root = parse('p::before { content: ""; }')
+		const selector = root.first_child!.first_child!.first_child!
+		const pseudo = selector.first_child!.next_sibling!
+		expect(pseudo.type_name).toBe('PseudoElementSelector')
+		expect(get_value(pseudo)).toBeUndefined()
+	})
+
+	test('Combinator.value is undefined', () => {
+		const root = parse('div > span { color: red; }')
+		const selector = root.first_child!.first_child!.first_child!
+		const combinator = selector.first_child!.next_sibling!
+		expect(combinator.type_name).toBe('Combinator')
+		expect(get_value(combinator)).toBeUndefined()
+	})
+
+	test('UniversalSelector.value is undefined', () => {
+		const root = parse('* { color: red; }')
+		const universal = root.first_child!.first_child!.first_child!.first_child!
+		expect(universal.type_name).toBe('UniversalSelector')
+		expect(get_value(universal)).toBeUndefined()
+	})
+
+	test('NthSelector.value is undefined', () => {
+		const root = parse('li:nth-child(2n+1) { color: red; }')
+		const selector = root.first_child!.first_child!.first_child!
+		const pseudo = selector.first_child!.next_sibling!
+		const nth = pseudo.first_child!
+		expect(nth.type_name).toBe('Nth')
+		expect(get_value(nth)).toBeUndefined()
+	})
+
+	test('Block.value is undefined', () => {
+		const root = parse('div { color: red; }')
+		const block = (root.first_child! as Rule).block!
+		expect(get_value(block)).toBeUndefined()
+	})
+
+	test('Value node.value is undefined', () => {
+		const root = parse('div { color: red; }')
+		const block = (root.first_child! as Rule).block!
+		const decl = block.first_child! as Declaration
+		const valueNode = decl.value!
+		expect(valueNode.type_name).toBe('Value')
+		expect(get_value(valueNode)).toBeUndefined()
+	})
+
+	test('Identifier.value is undefined', () => {
+		const root = parse('div { color: red; }')
+		const block = (root.first_child! as Rule).block!
+		const decl = block.first_child! as Declaration
+		const identifier = decl.value!.first_child!
+		expect(identifier.type_name).toBe('Identifier')
+		expect(get_value(identifier)).toBeUndefined()
+	})
+
+	test('Hash.value is undefined', () => {
+		const root = parse('div { color: #fff; }')
+		const block = (root.first_child! as Rule).block!
+		const decl = block.first_child! as Declaration
+		const hash = decl.value!.first_child!
+		expect(hash.type_name).toBe('Hash')
+		expect(get_value(hash)).toBeUndefined()
+	})
+
+	test('String.value is undefined', () => {
+		const root = parse('p::before { content: "hello"; }')
+		const block = (root.first_child! as Rule).block!
+		const decl = block.first_child! as Declaration
+		const str = decl.value!.first_child!
+		expect(str.type_name).toBe('String')
+		expect(get_value(str)).toBeUndefined()
+	})
+
+	test('Atrule.value is undefined', () => {
+		const root = parse('@media screen { div { color: red; } }', {
+			parse_atrule_preludes: false,
+		})
+		const atrule = root.first_child!
+		expect(atrule.type_name).toBe('Atrule')
+		expect(get_value(atrule)).toBeUndefined()
+	})
+
+	test('MediaQuery.value is undefined', () => {
+		const root = parse('@media screen and (min-width: 600px) { div { color: red; } }')
+		const prelude = root.first_child!.first_child!
+		const mediaQuery = prelude.first_child!
+		expect(mediaQuery.type_name).toBe('MediaQuery')
+		expect(get_value(mediaQuery)).toBeUndefined()
+	})
+
+	test('Raw.value is undefined', () => {
+		const root = parse('div { color: red; }', {
+			parse_selectors: false,
+			parse_values: false,
+		})
+		const rule = root.first_child!
+		const raw = rule.first_child!
+		expect(raw.type_name).toBe('Raw')
+		expect(get_value(raw)).toBeUndefined()
+	})
+})
+
 describe('NODE_TYPES namespace', () => {
 	test('should work as alternative to individual imports', async () => {
 		// Import namespace object

src/css-node.ts

@@ -415,7 +415,18 @@ export class CSSNode {
 			return this.get_content()
 		}
 
-		// For other nodes, return as string
+		// Only these types store a value in the arena value_start/value_length field.
+		// Every other node type has no value concept and must return undefined.
+		if (
+			type !== DECLARATION &&
+			type !== FUNCTION &&
+			type !== ATTRIBUTE_SELECTOR &&
+			type !== SUPPORTS_QUERY &&
+			type !== PRELUDE_SELECTORLIST
+		) {
+			return undefined
+		}
+
 		let start = this.arena.get_value_start(this.index)
 		let length = this.arena.get_value_length(this.index)
 		if (length === 0) return null