javascript - 是: any operator?

Question

如果我将这一行放入 JavaScript 控制台(你不需要声明“foo”)

foo : 4;

这行到底是什么意思？ “foo”存在于任何上下文中吗？是 : 任何运算符？

Answer 1

这是一个 label :

来自文档:

The labeled statement can be used with break or continue statements. It is prefixing a statement with an identifier which you can refer to.

在 C 等其他编程语言中，标签通常与 goto 一起使用陈述。 JavaScript 没有 goto。在 javaScript 中，它可以与 break 或 continue 语句一起使用。

文档中的示例使用带标签的 continue 和 for 循环:

var i, j;

loop1:
for (i = 0; i < 3; i++) {      //The first for statement is labeled "loop1"
   loop2:
   for (j = 0; j < 3; j++) {   //The second for statement is labeled "loop2"
      if (i === 1 && j === 1) {
         continue loop1;
      }
      console.log('i = ' + i + ', j = ' + j);
   }
}

// Output is:
//   "i = 0, j = 0"
//   "i = 0, j = 1"
//   "i = 0, j = 2"
//   "i = 1, j = 0"
//   "i = 2, j = 0"
//   "i = 2, j = 1"
//   "i = 2, j = 2"
// Notice how it skips both "i = 1, j = 1" and "i = 1, j = 2"