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rules.jshint.W082.html Maven / Gradle / Ivy

    
When do I get this error?
The "Function statements should not be placed in blocks" error (and the
alternative "Function declarations should not be placed in blocks" error) is
thrown when JSLint or JSHint encounters a function declaration inside a block
statement. In the following example we attempt to declare the example
function only if some condition is true:
​x
 
1
var x = true;
2
if (x) {
3
    function example() {
4
        "use strict";
5
        return true;
6
    }
7
}
8
​
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 2:
Unexpected 'function'.

Why do I get this error?
This error is raised to highlight code that may not work as you expect it
to. In most environments Your code will run without error, but maybe not in
the way you expect. In some environments it could cause a fatal syntax
error.
Function declarations (or "function statements" as they are misleadingly called
in the JSLint error message) are hoisted to the top of the scope in which they
appear, as described by Declaration Binding Instantiation (ES5
§10.5). Therefore, it is not possible to conditionally declare a
function with a function statement. The above example is actually interpreted as
follows:
7
 
1
function example() {
2
    "use strict";
3
    return true;
4
}
5
var x = true;
6
if (x) {}
7
​
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 5:
Empty block.

As you can see, regardless of the result of the condition, the example function
is always declared. If you were to, for example, declare it twice (once in an if
block and once in the corresponding else block) you would actually end up with
the second declaration overwriting the first regardless of the result of the
condition.
Since assignments are not hoisted (they happen where you expect them to), if you
want to declare a function conditionally, you can use a function expression,
instead of a function declaration. A function expression can easily by produced
by assigning a function to a variable:
10
 
1
var x = true,
2
    example;
3
​
4
if (x) {
5
    example = function () {
6
        "use strict";
7
        return true;
8
    };
9
}
10
​
JSLint found no 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

Syntax errors
It's important to note that there is no support in the ECMAScript 5
specification for function declarations within block statements. However, most
engines are happy to parse the code and not generate a syntax error. There is a
note about this in the spec (ES5 §12):

Several widely used implementations of ECMAScript are known to support the use
of FunctionDeclaration as a Statement. However there are significant and
irreconcilable variations among the implementations in the semantics applied
to such FunctionDeclarations. Because of these irreconcilable difference,
the use of a FunctionDeclaration as a Statement results in code that is
not reliably portable among implementations. It is recommended that ECMAScript
implementations either disallow this usage of FunctionDeclaration or issue a
warning when such a usage is encountered. Future editions of ECMAScript may
define alternative portable means for declaring functions in a Statement
context.

Unfortunately the misleading use of the term "statement" in the message produced
by JSLint doesn't help in this regard. A function declaration is not a statement
as defined by the spec. There is such thing as a function statement in
Mozilla's implementation but this is a non-standard addition that should not be
relied upon.
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 W082.
This means you can tell JSHint to not issue this warning with the /*jshint
-W082 */ directive.