labeled statement
can be used with
break
or
continue
statements. It is prefixing a statement with an identifier which you can refer to.
The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
注意： Labeled loops or blocks are very uncommon. Usually function calls can be used instead of loop jumps.
label : statement
label
Any JavaScript identifier that is not a reserved word.
statement
break
can be used with any labeled statement, and
continue
can be used with looping labeled statements.
You can use a label to identify a loop, and then use the
break
or
continue
statements to indicate whether a program should interrupt the loop or continue its execution.
Note that JavaScript has
no
goto
statement, you can only use labels with
break
or
continue
.
在
严格模式
code, you can't use "
let
" as a label name. It will throw a
SyntaxError
(let is a reserved identifier).
continue
with
for
loops
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"
continue
statement
Given an array of items and an array of tests, this example counts the number of items that passes all the tests.
var itemsPassed = 0;
var i, j;
top:
for (i = 0; i < items.length; i++) {
for (j = 0; j < tests.length; j++) {
if (!tests[j].pass(items[i])) {
continue top;
}
}
itemsPassed++;
}
break
with
for
loops
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) {
break 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"
// Notice the difference with the previous continue example
break
statement
Given an array of items and an array of tests, this example determines whether all items pass all tests.
var allPass = true;
var i, j;
top:
for (i = 0; items.length; i++)
for (j = 0; j < tests.length; i++)
if (!tests[j].pass(items[i])) {
allPass = false;
break top;
}
break
You can use labels within simple blocks, but only
break
statements can make use of non-loop labels.
foo: {
console.log('face');
break foo;
console.log('this will not be executed');
}
console.log('swap');
// this will log:
// "face"
// "swap"
Starting with ECMAScript 2015, labeled function declarations are now standardized for non-strict code in the web compatibility annex of the specification .
L: function F() {}
在
严格模式
code, however, this will throw a
SyntaxError
:
'use strict';
L: function F() {}
// SyntaxError: functions cannot be labelled
生成器函数 can neither be labeled in strict code, nor in non-strict code:
L: function* F() {}
// SyntaxError: generator functions cannot be labelled
| 规范 |
|---|
|
ECMAScript (ECMA-262)
The definition of 'Labelled statement' in that specification. |
| Desktop | Mobile | Server | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
label
|
Chrome 1 | Edge 12 | Firefox 1 | IE 4 | Opera 4 | Safari 1 | WebView Android 1 | Chrome Android 18 | Firefox Android 4 | Opera Android 10.1 | Safari iOS 1 | Samsung Internet Android 1.0 | nodejs 0.1.100 |
完整支持