JavaScript中“break Identifier”的用例是什么？

Question

the spec goes

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

then it goes

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). 

What on bloody earth does this mean?

Answer 1

A label is something like this:

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

This can be placed anywhere as a statement.

It is useful to break / continue when having multiple nested for loops.

An example of its usage:

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 .

Answer 2

If you look on MDN , there's examples

outer_block: {
    inner_block: {
        console.log('1');
        break outer_block; // breaks out of both inner_block and outer_block
        console.log(':-('); // skipped
    }
    console.log('2'); // skipped
}

as you can see, you can break with an identifier that selects a label higher up in the chain than just the first immediate parent statement.

The default action without an identifier would be

outer_block: {
    inner_block: {
        console.log('1');
        break; // breaks out of the inner_block only
        console.log(':-('); // skipped
    }
    console.log('2'); // still executed, does not break
}

The break has to be inside the label, you can't break labels based on indentifiers that the break is outside of.