default_parameters_bug.js

'use strict'

/**
 * I'm using node 5.10.1 --harmony_default_parameters reporting v8 '4.6.85.31'
 *
 * [Bug] Default Parameters referring to class names do not work with Fat Arrow functions
 * So I figured, maybe it's just a difference between Fat Arrow Functions and Full Functions,
 * but then I realised that by declaring a Full Function referencing the class,
 * it caused the Fat Arrow Function to execute fine. This tells me it's a bonafide bug.
 */

/**
 * This works as expected
 */
const thisWorks = () => {
  class myClass {}
  const myFunc = function (a = new myClass) {}

  myFunc()
}

/**
 * This causes 'ReferenceError: myClass is not defined'
 */
const thisProducesRuntimeError = () => {
  class myClass {}
  const myFunc = (a = new myClass) => {} /* Fat Arrow causes issue */

  myFunc() /* ReferenceError: myClass is not defined */
}

/**
 * But: This works
 * It's functionally identical to thisProducesRuntimeError
 * except it also declares a Full Function which is never invoked
 * yet it avoids the ReferenceError issue on the Fat Arrow Function
 */
const thisOddlyWorksToo = () => {
  class myClass {}
  const myFunc = (a = new myClass) => {} /* Fat Arrow is *expected* to cause issue */

  const neverCalled = function (a = new myClass) {} /* but this line somehow avoids issue with Fat Arrow */

  myFunc()
}

thisWorks() /* undefined */
thisOddlyWorksToo() /* undefined */
thisProducesRuntimeError() /* ReferenceError: myClass is not defined */