babelでlet a = () => {} が let a = function a(){}と匿名関数に名前が付けられるのはなんでなんでなの？というのを調べた ref: http://qiita.com/inuscript/items/e820652ee0d68dd2c190

file0.js

let a = () => {}       // arrow function
let b = function(){}   // 普通のfunction(無名）
let c = function d(){} // 名前付きfunction

file1.txt

% $(npm bin)/babel foo.js --presets es2015               

file10.js

function hogeFunctionGenerator(someFlag, defaultCallback){
  if(someFlag){
    return function someFlagFn(){}
  }
  return defaultCallback
}

// どの関数が結局コールされるのか知りたい場合
console.log(hogeFunctionGenerator(true, function (){}).name) // "someFlagFn"
console.log(hogeFunctionGenerator(false, function (){}).name) // ""
console.log(hogeFunctionGenerator(false, function cb(){}).name) // "cb"

file11.js

class Hoge {}

function createClassZ() {
  // Z classを返す
  return class Z{ 
    constructor(){
    }
  }
}
function createAnonymousClass() {
  // 匿名classを返す
  return class { 
    constructor(){
    }
  }
}
let ZX = createClassZ()
let zx = new ZX()

let X = createAnonymousClass()
let x = new X()

console.log("zx", zx.constructor.name)
console.log("x", x.constructor.name)

file12.txt

zx Z
x undefined

file13.txt

zx Z
x _class

file2.js

"use strict";

var a = function a() {};
var b = function b() {};
var c = function d() {};

file3.js

% $(npm bin)/babel foo.js --plugins transform-es2015-arrow-functions                        

file4.js

let a = function () {}; // まだ無名！
let b = function () {};
let c = function d() {};

file5.txt

% $(npm bin)/babel foo.js --plugins transform-es2015-function-name                          

file6.js

let a = () => {};
let b = function b() {}; // 名前がついた！
let c = function d() {};

file7.txt

% $(npm bin)/babel foo.js --plugins transform-es2015-function-name,transform-es2015-arrow-functions

file8.js

let a = function a() {};
let b = function b() {};
let c = function d() {};

file9.js

// Function Statements
function fn () {}
console.log(fn.name) // "fn"

// variables
const a = function(){}
console.log(a.name) // "a"
const b = () => {}
console.log(b.name) // "b"