functional-javascript-workshop/exercises/function_spies/solution/solution.js

The basics of functional programming in JavaScript. No libraries required.

function Spy(target, method) {
  var originalFunction = target[method]

  // use an object so we can pass by reference, not value
  // i.e. we can return result, but update count from this scope
  var result = {
    count: 0
  }

  // replace method with spy method
  target[method] = function() {
    result.count++ // track function was called
    return originalFunction.apply(this, arguments) // invoke original function
  }

  return result
}

module.exports = Spy