javascript - JavaScript 中 "break Identifier"的用例是什么？

Question

规范 goes

BreakStatement :   
    break ;
    break [no LineTerminator here] Identifier ;

然后就可以了

The program contains a break statement with the optional Identifier, where Identifier does not appear in the label set of an enclosing (but not crossing function boundaries) Statement.

...

A BreakStatement with an Identifier is evaluated as follows:

Return (break, empty, Identifier).

这到底是什么意思？

Answer 1

标签是这样的:

// ...
mylabel:
// ...

这可以作为语句放在任何地方。

当有多个嵌套的 for 循环时，break/continue 很有用。

使用示例:

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"

Source .