Hoisting

var 存在变量提升的情况，即 var 声明会被提升至该作用域的顶部，但是他们的赋值并不会。而 const 和 let 并不存在这种情况，他们被赋予了 Temporal Dead Zones, TDZ， 了解 typeof 不再安全很重要

function example () {
  console.log(notDefined)   // => throws a ReferenceError
}

function example () {
  console.log(declareButNotAssigned)  // => undefined
  var declaredButNotAssigned = true
}

function example () {
  let declaredButNotAssigned
  console.log(declaredButNotAssigned)   // => undefined
  declaredButNotAssigned = true
}

function example () {
  console.log(declaredButNotAssigned)   // => throws a ReferenceError
  console.log(typeof declaredButNotAssigned)  // => throws a ReferenceError
  const declaredButNotAssigned = true
}

匿名函数的变量名会提升，但函数内容不会

function example () {
  console.log(anonymous)  // => undefined

  anonymous() // TypeError: anonymous is not a function

  var anonymous = function () {
    console.log('test')
  }
}

命名的函数表达式的变量名会被提升，但函数名和函数函数内容并不会

function example() {
  console.log(named)  // => undefined

  named()   // => TypeError named is not a function

  superPower()  // => ReferenceError superPower is not defined

  var named = function superPower () {
    console.log('Flying')
  }
}

function example() {
  console.log(named)  // => undefined

  named()   // => TypeError named is not a function

  var named = function named () {
    console.log('named')
  }
}