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History
This warning has existed in three forms across the three main linters. It was
introduced in the original version of JSLint and has remained (in a way) in all
three tools ever since.

In JSLint the warning given is "'{a}' is not a label"

In JSHint the warning given has always been "'{a}' is not a statement label"

In ESLint the Esprima parser fails to parse the code so the message given
matches the V8 engine error "Undefined label '{a}'"


The situations that produce the warning have not changed despite changes to the
text of the warning itself.
When do I get this error?
The "'{a}' is not a label" error, and the alternatives "'{a}' is not a statement
label" and "Undefined label '{a}'", is thrown when JSLint, JSHint or ESLint
encounters a break or continue statement referencing a label that does not
exist. In the following example we try to break out of a for loop to the
non-existent example label:
​x
 
1
function demo() {
2
    "use strict";
3
    var i;
4
    for (i = 0; i < 10; i += 1) {
5
        if (i === 5) {
6
            break example;
7
        }
8
    }
9
}
10
​
JSLint found 2 errors
Which linter should be used for this code snippet:
 JSLint JSHint ESLint
Which version of the linter should be used:
Update
Version 2015-09-23
Line 3:
Unexpected 'for'.
Line 5:
'example' is not a label.

Why do I get this error?
This error is raised to highlight a fatal JavaScript syntax error. It is not
valid to reference an identifier that does not appear in the label set of the
enclosing statement. This is stated in the ECMAScript 5 specification (section
§12.7 and section §12.8):

A program is considered syntactically incorrect if...

...
The program contains a break statement with the optional Identifier, where
Identifier does not appear in the label set of an enclosing [statement]


In the snippet above, the for statement has not been explicitly labelled, and
therefore there is no label in its label set with the identifier example. When
the interpreter reaches the break statement a syntax error will be thrown.
This can be avoided by removing the identifier from the break statement:
10
 
1
function demo() {
2
    "use strict";
3
    var i;
4
    for (i = 0; i < 10; i += 1) {
5
        if (i === 5) {
6
            break;
7
        }
8
    }
9
}
10
​
JSLint found 1 error
Which linter should be used for this code snippet:
 JSLint JSHint ESLint
Which version of the linter should be used:
Update
Version 2015-09-23
Line 3:
Unexpected 'for'.

Or alternatively by adding a label with the correct identifer to the label set
of the for statement (although since November 2013 this will cause JSLint to
raise a different warning, because it is now of the opinion that label
statements should not be used at all):
11
 
1
function demo() {
2
    "use strict";
3
    var i;
4
example:
5
    for (i = 0; i < 10; i += 1) {
6
        if (i === 5) {
7
            break example;
8
        }
9
    }
10
}
11
​
JSLint found 1 error
Which linter should be used for this code snippet:
 JSLint JSHint ESLint
Which version of the linter should be used:
Update
Version 2015-09-23
Line 4:
Unexpected 'for'.

In JSHint 1.0.0 and above you have the ability to ignore any warning with a
special option syntax. The identifier of this warning is W090.
This means you can tell JSHint to not issue this warning with the /*jshint
-W090 */ directive.