求大神解释一下下面示例的原因：

• 变量定义

<script>
  var a = 1
  let b = 2
  const c = 3
  window.d = 4
  Object.defineProperty(window, 'e', {
    value: 5
  })
  console.log(a, b, c, d, e)
  console.log(window.a, window.b, window.c, window.d, window.e)
</script>
输出：
1,2,3,4,5
1 undefined undefined 4 5

• 重复声明

<script>
  Object.defineProperty(window, 'a', {
    value: 1,
    writable: false
  })
  let a = 2
  console.log(a)
  console.log(window.a)
</script>
输出：
2
1

<script>
  Object.defineProperty(window, 'a', {
    value: 1
  })
</script>
<script>
  let a = 2
  console.log(a)
</script>
<script>
  console.log(a)
</script>
输出：
Uncaught SyntaxError: Identifier 'a' has already been declared
1