Example of decorator use in JavaScript

decorators.js

function log(target, name, descriptor) {
  const original = descriptor.value;
  if (typeof original === 'function') {
    descriptor.value = function(...args) {
      console.log(`[${original.name}] Arguments: ${args}`);
      try {
        const result = original.apply(this, args);
        console.log(`[${original.name}] Result: ${result}`);
        return result;
      } catch (e) {
        console.log(`[${original.name}] Error: ${e}`);
        throw e;
      }
    }
  }
  return descriptor;
}

function logClass(Class) {
  console.log(`[${Class.name}] created. Methods:`, Object.getOwnPropertyNames(Class.prototype));
  return Class;
}

@logClass
class Example {

  @log
  sum(a, b) {
    return a + b;
  }

  test(a, b) {
    return a + b;
  }
}

const e = new Example();

console.log("logged", e.sum(1, 2));
console.log("not logged", e.test(3, 4));

/*
[LOG]: "[Example] created. Methods:",  ["constructor", "sum", "test"] 
[LOG]: "[sum] Arguments: 1,2" 
[LOG]: "[sum] Result: 3" 
[LOG]: "logged",  3 
[LOG]: "not logged",  7 
*/

function readonly(target, name, descriptor) {
  descriptor.writable = false;
  return descriptor;
}

class Example2 {
  a() { }
  @readonly
  b() { }
}

const e2 = new Example2();
e2.a = () => { };
e2.b = () => { };
/*
[ERR]: Cannot assign to read only property 'b' of object '#<Example2>'
*/