yaml-frontmatter-parsing

name: "YAML Frontmatter Parsing" description: "Lightweight YAML frontmatter extraction from markdown files without external dependencies" domain: "parsing" confidence: "low" source: "earned"

Context

Markdown files often use YAML frontmatter (delimited by --- lines) for metadata. Full YAML parsers like js-yaml are overkill when you only need simple key: value or key: "value" pairs. This pattern extracts frontmatter fields with zero dependencies.

Patterns

Detect and Extract Frontmatter Block

Check if line 0 is ---, then find the closing ---. Lines between are frontmatter; everything after is body content.

const lines = content.split('\n');
let bodyStartIndex = 0;
const metadata: Record<string, string> = {};

if (lines[0]?.trim() === '---') {
    const closingIndex = lines.indexOf('---', 1);
    if (closingIndex > 0) {
        bodyStartIndex = closingIndex + 1;
        for (let i = 1; i < closingIndex; i++) {
            const match = /^(\w+):\s*"?([^"]*)"?\s*$/.exec(lines[i]);
            if (match) {
                metadata[match[1].toLowerCase()] = match[2].trim();
            }
        }
    }
}

Fall Back Gracefully

Always provide a fallback for files without frontmatter. Check headings or use filename as the default:

let name = metadata.name || dirName;
if (name === dirName) {
    const headingMatch = /^#\s+(.+)/.exec(lines[bodyStartIndex] ?? '');
    if (headingMatch) { name = headingMatch[1].trim(); }
}

Body Content Starts After Frontmatter

When extracting description or body text, start scanning from bodyStartIndex (after the closing ---), not from line 0 or line 1.

Anti-Patterns

• Parsing from line 1 when frontmatter exists — will read YAML keys as body text.
• Using full YAML parser for simple key-value frontmatter — unnecessary dependency; js-yaml is 50KB+ and handles features you don't need.
• Assuming frontmatter always exists — always code the no-frontmatter path first, then add frontmatter as an enhancement.
• Modifying the raw content to strip frontmatter — keep the original content intact; use bodyStartIndex for display-layer parsing only.

Limitations

• Only handles flat key: value and key: "value" pairs — no nested YAML, arrays, or multiline values.
• Does not validate YAML syntax — malformed lines are silently skipped.
• The closing --- must be an exact match on its own line.