Mastering-Node.js-Web-Development/Chapter 10/End of Chapter/part2app/src/server/custom_engine.ts

import { readFile } from "fs";
import { Express } from "express";
import * as features from "./custom_features";

const renderTemplate = (path: string, context: any, 
    callback: (err: any, response: string | undefined) => void) => {

    readFile(path, (err, data) => {
        if (err != undefined) {
            callback("Cannot generate content", undefined);
        } else {
            callback(undefined, parseTemplate(data.toString(), 
                { ...context, features }));            
        }
    });
};

const parseTemplate = (template: string, context: any) => {
    const ctx = Object.keys(context)
        .map((k) => `const ${k} = context.${k}`)
        .join(";");
    const expr = /{{(.*)}}/gm;
    return template.toString().replaceAll(expr, (match, group) => {
        const evalFunc= (expr: string) => {
            return eval(`${ctx};${expr}`)
        }
        try {
            if (group.trim()[0] === "@") {
                group = `features.${group.trim().substring(1)}`;
                group = group.replace(/\)$/m, ", context, evalFunc)");
            }
            let result = evalFunc(group);
            if (expr.test(result)) {
                result = parseTemplate(result, context);
            }
            return result;
        } catch (err: any) {
            return err;
        }
    });
}

export const registerCustomTemplateEngine = (expressApp: Express) => 
    expressApp.engine("custom", renderTemplate);