before and after hooks for api methods

beforeAndAfter.js

'use strict';


// call something before the method(s)

function toArr(arrLike) {
  return Array.prototype.slice.apply(arrLike);
}

function beforeMethod(parent, methodName, beforeFn) {
  const oldFn = parent[methodName];
  parent[methodName] = function() {
    const args = toArr(arguments)
    beforeFn(methodName, args);
    return oldFn.apply(parent, args);
  };
}

function afterMethod(parent, methodName, afterFn) {
  const oldFn = parent[methodName];
  parent[methodName] = function() {
    const args = toArr(arguments)
    const res = oldFn.apply(parent, args);
    afterFn(methodName, args, res);
    return res;
  };
}



// object with method we want to listen to

const api = {
  f1: function(a, b) {
    return a + b;
  }
}



// annotate

function before(methodName, args) {
  console.warn(`method:%s, args:%s`, methodName, JSON.stringify(args));
}
beforeMethod(api, 'f1', before);

function after(methodName, args, res) {
  console.warn(`method:%s, args:%s, result:%s`, methodName, JSON.stringify(args), JSON.stringify(res));
}
afterMethod(api, 'f1', after);



// stuff

const a = 2;
const b = 3;
const c = api.f1(a, b);
console.log(`f1(${a}, ${b}) = ${c}`);

DYI spy. Of course this isn't performant. Skip toArr() call or args passing altogether if you're not using it.

You can try it out here:
http://jsbin.com/cabekuz/edit?js,console

Example use case: when you can't use a debugger statement but can print a stacktrace..., use console.warn() in a beforeMethod() to understand who's calling it.