log2Deco.js

var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
var ARGUMENT_NAMES = /([^\s,]+)/g;
function getParamNames(func) {
  var fnStr = func.toString().replace(STRIP_COMMENTS, '');
  var result = fnStr.slice(fnStr.indexOf('(') + 1, fnStr.indexOf(')')).match(ARGUMENT_NAMES);
  if (result === null)
    result = [];
  return result;
}

const log2 = (target, name, descriptor) => {
  const original = descriptor.value;  

  if (typeof original === 'function') { 
    const paramNames = getParamNames(original)
    
    descriptor.value = function (...args) {
      const params = paramNames.reduce((obj, pn, i) => {
        obj[pn] = args[i];
        return obj;}, {} )

      const result = original.apply(this, args);
      console.log(`${name}(${JSON.stringify(params)}) = ${result}`)
    }
  }
}

class MyClass {
  @log2
  sum(a, b) {
    return a + b;
  }
}

const instance = new MyClass();
instance.sum(4, 5);    // decorator outputs:  sum2({"a":4,"b":5}) = 9