I want to prerender it on server by passing this string into katex.renderToString on js interpretator Jint. So processTexFormula will return katex html output.

If you'd like to automatically convert \(...\) passages into math, then use auto-render's renderMathInText, which supports a preProcess option. If you'd like to look for <span class="math">, I'd recommend using a regular expression replace and calling katex.renderToString after modifying the string how you'd like.

renderMathInText looks like nice fast solution, I will try it.