Support katex for marked.

marked-katex.js

const renderer = new marked.Renderer()
let originParagraph = renderer.paragraph.bind(renderer)
renderer.paragraph = (text) => {
  const blockRegex = /\$\$[^\$]*\$\$/g
  const inlineRegex = /\$[^\$]*\$/g
  let blockExprArray = text.match(blockRegex)
  let inlineExprArray = text.match(inlineRegex)
  for (let i in blockExprArray) {
    const expr = blockExprArray[i]
    const result = renderMathsExpression(expr)
    text = text.replace(expr, result)
  }
  for (let i in inlineExprArray) {
    const expr = inlineExprArray[i]
    const result = renderMathsExpression(expr)
    text = text.replace(expr, result)
  }
  return originParagraph(text)
}
function renderMathsExpression (expr) {
  if (expr[0] === '$' && expr[expr.length - 1] === '$') {
    let displayStyle = false
    expr = expr.substr(1, expr.length - 2)
    if (expr[0] === '$' && expr[expr.length - 1] === '$') {
      displayStyle = true
      expr = expr.substr(1, expr.length - 2)
    }
    let html = null
    try {
      html = katex.renderToString(expr)
    } catch (e) {
      console.err(e)
    }
    if (displayStyle && html) {
      html = html.replace(/class="katex"/g, 'class="katex katex-block" style="display: block;"')
    }
    return html
  } else {
    return null
  }
}
marked.setOptions({renderer: renderer})