render_katex.js

var fs = require("fs");
var katex = require("katex");

function walk(dir) {
    fs.readdir(dir, function(err, list) {
        if (err) { console.log(err); return; }

        list.forEach(function(file) {
            var path = dir + "/" + file;
            fs.stat(path, function(err, stat) {
                if (stat && stat.isDirectory()) {
                    walk(path);
                } else if (path.slice(-5) == ".html") {
                    console.log(path);
                    fs.readFile(path, "utf8", function(err, text) {
                        if (err) { console.log(err); return; }

                        var segments = [];

                        var start = 0;
                        var i = 0;
                        while (i < text.length) {
                            if (text.slice(i, i + 2) == "\\[") {
                                segments.push(text.slice(start, i));
                                start = i + 2;
                                i = start;
                                while (text.slice(i, i + 2) != "\\]" && i < text.length) {
                                    i++;
                                }
                                segments.push(katex.renderToString(text.slice(start, i), { displayMode: true }));
                                start = i + 2;
                                i = start;
                            } else if (text.slice(i, i + 2) == "\\(") {
                                segments.push(text.slice(start, i));
                                start = i + 2;
                                i = start;
                                while (text.slice(i, i + 2) != "\\)" && i < text.length) {
                                    i++;
                                }
                                segments.push(katex.renderToString(text.slice(start, i)));
                                start = i + 2;
                                i = start;
                            } else {
                                i++;
                            }
                        }
                        if (i > start) {
                            segments.push(text.slice(start, i));
                        }

                        var output = segments.join("");
                        
                        fs.writeFile(path, output, "utf8", function(err) {
                            if (err) { console.log(err); return; }
                        });
                    });
                }
            });
        });
    });
}

walk("public");