@mathssyfy/plugin-markdown/src/lineNumbers.js

// markdown-it plugin for generating line numbers.
// It depends on preWrapper plugin.

module.exports = md => {
    const fence = md.renderer.rules.fence
    md.renderer.rules.fence = (...args) => {
      const rawCode = fence(...args)
      const code = rawCode.slice(
        rawCode.indexOf('<code>'),
        rawCode.indexOf('</code>')
      )
  
      const lines = code.split('\n')
      const lineNumbersCode = [...Array(lines.length - 1)]
        .map((line, index) => `<span class="line-number">${index + 1}</span><br>`).join('')
  
      const lineNumbersWrapperCode =
        `<div class="line-numbers-wrapper">${lineNumbersCode}</div>`
  
      const finalCode = rawCode
        .replace('<!--beforeend-->', `${lineNumbersWrapperCode}<!--beforeend-->`)
        .replace('extra-class', 'line-numbers-mode')
  
      return finalCode
    }
  }