javascript 函数在块语句内提升

{function foo(){};foo=1;function foo(){};foo=2;}
console.log(foo); // 1

谁能解释一下为什么这里输出“1”？

Edit： 似乎存在实现差异，在“Chrome”、“Firefox”、“Nodejs”中输出为“1”，但在“Safari”中输出为“2”

让我们美化该块并将其分解：

{
  function foo() {};
  foo = 1;

  function foo() {};
  foo = 2;
}
console.log(foo); // 1

块作用域函数声明通常将其变量名称提升到其块之外（其值为undefined），但仅在块内部接收一次值。您可以查看有关该行为的更多详细信息here https://stackoverflow.com/questions/31419897/what-are-the-precise-semantics-of-block-level-functions-in-es6.

但这种情况有点不同——你声明了一个函数twice在一个块中。看起来像第一个foo函数被分配给块外部可见的名称。如果您将属性描述符记录在window在代码中的不同点。

看起来，一旦达到重复的函数声明，就会进一步引用foo块内的变量名称引用绑定仅在块内。所以foo = 2在底部，因为它位于重复声明之后，所以只会导致2值绑定到foo在块内引用时的名称。块外的值仍然是块内的值foo最后举行before重复的函数声明：

// Variable name is hoisted: it exists on the global object, but is undefined
console.log(Object.getOwnPropertyDescriptor(window, 'foo'));
{
  // Function declaration: global foo gets assigned to
  function foo() {};
  console.log(window.foo);
  // Assignment to foo name: global foo gets assigned to
  foo = 1;
  // Assignment to foo name: global foo gets assigned to
  foo = 3;
  console.log(window.foo);

  // Duplicate function declaration: past this point, foo now no longer refers to the global foo
  // but to a locally-scoped identifier
  function foo() {};
  // See how the window value remains at 3:
  console.log(window.foo);

  // So this assignemnt only changes the binding of the `foo` inside this block
  // while window.foo remains at 3:
  foo = 2;
}
console.log(foo); // 3

查看原始代码的另一种方式：

{
  function foo() {}; // this ALSO does: window.foo = function foo() {};
  foo = 1; // this ALSO does: window.foo = 1

  function foo() {};  // this actually does: fooLocal = function foo() {};
  // further references to "foo" in this block refer to fooLocal
  foo = 2; // this actually does: fooLocal = 2
}
console.log(foo); // references window.foo