npm install markdown-it

npm install @iktakahiro/markdown-it-katex

var md = require('markdown-it')(),
    mk = require('@iktakahiro/markdown-it-katex');

md.use(mk);

// double backslash is required for javascript strings, but not html input
var result = md.render('# Math Rulez! \n  $\\sqrt{3x-1}+(1+x)^2$');

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.9.0/katex.min.css">

<link rel="stylesheet" href="https://cdn.jsdelivr.net/github-markdown-css/2.2.1/github-markdown.css"/>

md.use(mk, {"throwOnError" : false, "errorColor" : " #cc0000"});

$\sqrt{3x-1}+(1+x)^2$

$$\begin{array}{c}

\nabla \times \vec{\mathbf{B}} -\, \frac1c\, \frac{\partial\vec{\mathbf{E}}}{\partial t} &
= \frac{4\pi}{c}\vec{\mathbf{j}}    \nabla \cdot \vec{\mathbf{E}} & = 4 \pi \rho \\

\nabla \times \vec{\mathbf{E}}\, +\, \frac1c\, \frac{\partial\vec{\mathbf{B}}}{\partial t} & = \vec{\mathbf{0}} \\

\nabla \cdot \vec{\mathbf{B}} & = 0

\end{array}$$

Anything between two $ characters will be treated as TeX math. The opening $ must
have a non-space character immediately to its right, while the closing $ must
have a non-space character immediately to its left, and must not be followed
immediately by a digit. Thus, $20,000 and $30,000 won’t parse as math. If for some
reason you need to enclose text in literal $ characters, backslash-escape them and
they won’t be treated as math delimiters.