label
The labeled statement can be used with break
or continue
statements. It is prefixing
a statement with an identifier which you can refer to.
Syntax
label :
statement
label
-
Any JavaScript identifier that is not a reserved word.
statement
-
A JavaScript statement.
break
can be used with any labeled statement, andcontinue
can be used with looping labeled statements.
Description
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
.
In strict mode code, you
can't use "let
" as a label name. It will throw a SyntaxError
(let is a reserved identifier).
Examples
Using a labeled continue with for loops
let 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"
Using a labeled continue statement
Given an array of items and an array of tests, this example counts the number of items that passes all the tests.
let itemsPassed = 0;
let 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++;
}
Using a labeled break with for loops
let 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
Using a labeled break statement
Given an array of items and an array of tests, this example determines whether all items pass all tests.
let allPass = true;
let i, j;
top:
for (i = 0; i < items.length; i++) {
for (j = 0; j < tests.length; j++) {
if (!tests[j].pass(items[i])) {
allPass = false;
break top;
}
}
}
Using a labeled block with 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"
Labeled function declarations
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() {}
In strict mode code,
however, this will throw a SyntaxError
:
'use strict';
L: function F() {}
// SyntaxError: functions cannot be labelled
Generator functions can neither be labeled in strict code, nor in non-strict code:
L: function* F() {}
// SyntaxError: generator functions cannot be labelled
Specifications
Specification |
---|
ECMAScript Language Specification # sec-labelled-statements |
Browser compatibility
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