Empty component props block throws `expected a document, but the input is empty`

[adamdehaven]

Environment

• comark: 0.5.1
• js-yaml: 5.2.2 (transitive)
• Node: 24.x
• OS: macOS (platform-independent)

Comark Version

0.5.1

Reproduction

import { parse } from 'comark'

// Empty props block (opening `---` immediately followed by closing `---`)
await parse('::page-section\n---\n---\n#title\nHello\n::')
// → throws: YAMLException: expected a document, but the input is empty

Blocks that are not empty parse correctly, at any nesting/indentation level:

await parse('::hero\n---\ntitle: x\n---\n#title\nHi\n::')            // OK
await parse('::a\n  ::b\n  ---\n  title: x\n  ---\n  ::\n::')        // OK (nested + indented)
await parse('::hero\n---\n   \n---\ncontent\n::')                    // throws (whitespace-only block)

• Expected: an empty (or whitespace-only) props block yields a component with no prop (equivalent to omitting the block). No throw.
• Actual: parse() rejects with expected a document, but the input is empty.

Description

A block component whose --- fenced YAML props block is empty (or contains only whitespace) crashes the parser. Comark passes the empty block to js-yaml, which throws YAMLException: expected a document, but the input is empty, and the error propagates out of parse(), failing the entire document rather than the one empty block.

@nuxtjs/mdc (Comark's predecessor) tolerated empty props blocks and simply produced a component with no props. Real-world authored content contains empty props blocks, so this is a migration blocker.

Additional context

Root cause

The block-props rule slices the YAML between the fences and calls parseYaml unconditionally:

packages/comark/src/plugins/syntax.ts — comark_block_yaml (dist: dist/plugins/syntax.js:~245):

const yaml = state.src.slice(state.bMarks[startLine + 1], state.eMarks[lineEnd - 1])
const data = parseYaml(yaml)   // yaml === '' when the block is empty

packages/comark/src/internal/yaml.ts — parseYaml (dist: dist/internal/yaml.js):

import { load, JSON_SCHEMA } from 'js-yaml'

export function parseYaml(content) {
  return load(content, { schema: JSON_SCHEMA })   // js-yaml throws on empty input
}

js-yaml's load('') (and whitespace-only input) throws expected a document, but the input is empty instead of returning undefined.

Proposed fix

Guard empty/whitespace input in parseYaml so it returns undefined (Comark already handles a falsy result via data || {} at the call site):

// packages/comark/src/internal/yaml.ts
export function parseYaml(content: string) {
  if (!content || content.trim() === '') {
    return undefined
  }
  return load(content, { schema: JSON_SCHEMA })
}

Alternatively (or additionally), short-circuit in comark_block_yaml when the sliced block is blank:

const yaml = state.src.slice(state.bMarks[startLine + 1], state.eMarks[lineEnd - 1])
const data = yaml.trim() === '' ? undefined : parseYaml(yaml)

Either change makes an empty props block behave like "no props", matching @nuxtjs/mdc.

Note: the fenced ```yaml [props] props variant likely shares the same code path and should be
covered by the same guard — worth adding a test for both forms.

Suggested tests

expect((await parse('::x\n---\n---\ncontent\n::')).nodes).toBeDefined()   // no throw
expect((await parse('::x\n---\n   \n---\ncontent\n::')).nodes).toBeDefined() // whitespace-only
expect((await parse('::x\n```yaml [props]\n```\ncontent\n::')).nodes).toBeDefined() // empty fenced props

Logs

[adamdehaven]

@farnabaz I ran across this as I'm investigating converting our production app from @nuxtjs/mdc to Comark