Adding console.log to every function automatically

If you want more targeted logging, the following code will log function calls for a particular object. You can even modify Object prototypes so that all new instances get logging too. I used Object.getOwnPropertyNames instead of for...in, so it works with ECMAScript 6 classes, which don't have enumerable methods.

function inject(obj, beforeFn) {
    for (let propName of Object.getOwnPropertyNames(obj)) {
        let prop = obj[propName];
        if (Object.prototype.toString.call(prop) === '[object Function]') {
            obj[propName] = (function(fnName) {
                return function() {
                    beforeFn.call(this, fnName, arguments);
                    return prop.apply(this, arguments);
                }
            })(propName);
        }
    }
}

function logFnCall(name, args) {
    let s = name + '(';
    for (let i = 0; i < args.length; i++) {
        if (i > 0)
            s += ', ';
        s += String(args[i]);
    }
    s += ')';
    console.log(s);
}

inject(Foo.prototype, logFnCall);

As to me, this looks like the most elegant solution:

(function() {
    var call = Function.prototype.call;
    Function.prototype.call = function() {
        console.log(this, arguments); // Here you can do whatever actions you want
        return call.apply(this, arguments);
    };
}());

Proxy Method to log Function calls

There is a new way using Proxy to achieve this functionality in JS. assume that we want to have a console.log whenever a function of a specific class is called:

class TestClass {
  a() {
    this.aa = 1;
  }
  b() {
    this.bb = 1;
  }
}

const foo = new TestClass()
foo.a() // nothing get logged

we can replace our class instantiation with a Proxy that overrides each property of this class. so:

class TestClass {
  a() {
    this.aa = 1;
  }
  b() {
    this.bb = 1;
  }
}


const logger = className => {
  return new Proxy(new className(), {
    get: function(target, name, receiver) {
      if (!target.hasOwnProperty(name)) {
        if (typeof target[name] === "function") {
          console.log(
            "Calling Method : ",
            name,
            "|| on : ",
            target.constructor.name
          );
        }
        return new Proxy(target[name], this);
      }
      return Reflect.get(target, name, receiver);
    }
  });
};



const instance = logger(TestClass)

instance.a() // output: "Calling Method : a || on : TestClass"

check that this actually works in Codepen

---

Remember that using Proxy gives you a lot more functionality than to just logging console names.

Also this method works in Node.js too.

Here's a way to augment all functions in the global namespace with the function of your choice:

function augment(withFn) {
    var name, fn;
    for (name in window) {
        fn = window[name];
        if (typeof fn === 'function') {
            window[name] = (function(name, fn) {
                var args = arguments;
                return function() {
                    withFn.apply(this, args);
                    return fn.apply(this, arguments);

                }
            })(name, fn);
        }
    }
}

augment(function(name, fn) {
    console.log("calling " + name);
});

One down side is that no functions created after calling augment will have the additional behavior.