decorators.js

// This is the decorator function that times our functions
function timer(target, name, descriptor) {
    console.log(target, name, descriptor);
    let original = descriptor.value;
    descriptor.value = function(...args){
      console.time('timer');
      var result = original.apply(this, args);
      console.timeEnd('timer');
      return result;
    }
    
    return descriptor;
}


class Test {
    constructor() {
      let a = 50;
      console.log(this.recursiveFactorial(a));
      console.log(this.loopingFactorial(a));
    }

    
    @timer
    recursiveFactorial(x){
      function factorial(num){
       if(num===0 || num===1 ) {
          return 1;
       }
       return num * factorial(num-1);        
      }

       return factorial(x);
    }

    @timer
    loopingFactorial(x){
      if(x == 0) {
           return 1;
       }
       if(x < 0 ) {
           return undefined;
       }
       for(var i = x; --i; ) {
           x *= i;
       }
       return x;      
    }
}

let test = new Test();