javascript - 在 for 循环的第一个语句中声明的变量是否有作用域或特殊处理？

Question

这两个示例之间是否存在任何(我的意思是任何)差异，如键入的那样——甚至是细微的差异？

for (var foo = 0; …; …)
    statement;

和

var foo = 0;
for (; …; …)
    statement;

我似乎记得我读过的一些评论说它的行为略有不同，但据我所知，foo 在这两种情况下仍然是函数范围的。有什么区别？

(我试图通读 ECMA-262 13.7.4 ，但最终还是有点难以理解。)

Answer 1

是的，有区别。

for (var foo = something; …; …)
    statement;

相当于:

var foo;                               // hoist foo (declare it at top)
for (foo = something; …; …)            // but doesn't assign the value at top, it will assign it where it was before the hoisting
    statement;

但不等同于:

var foo = something;                   // wrong assumption: it should not move the assignemet to top too, it should move just the declaration
for (; …; …)
    statement;

证明:

1- 如果未声明变量，将抛出错误:

console.log(foo);

2- 如果一个变量从未被赋值，它的值是undefined:

var foo;

console.log(foo);

3- 将声明移动到顶部(提升)但不移动赋值:

console.log(foo); // undefined value but doesn't throw an error

var foo = "Hello, world!";

所以它等价于:

var foo;  // declared first so it doesn't throw an error in the next line

console.log(foo);  // undefined so the assignment is still after this line (still at the same place before hoisting)

var foo = "Hello, world!";  // assignment here to justify the logged undefined value