What does this option do?

In JSLint the forin option is used to allow the usage of unfiltered for-in statements. Since the for-in statement will enumerate properties from the prototype chain and not just from the object in question, it can potentially cause problems if other code is modifying native prototypes without your knowledge. In the following example we use a for-in loop to iterate over the elements of an array. This is commonly called out as bad practice because enumerable methods added to Array.prototype will be produced:

/*jslint forin: true */
/*global doSomething */

var arr = [],
    i;

for (i in arr) {
    doSomething(arr[i]);
}

The JSHint forin option is used to require the filtering of such loops. Here's the same example again:

/*jshint forin: true */
/*global doSomething */

var arr = [],
    i;

for (i in arr) {
    doSomething(arr[i]);
}

A "filtered" for-in statement is one that ensures only properties that belong to the object in question are operated on. This is usually achieved by wrapping the body of the for-in in an if statement that makes sure each property is an "own" property of the object:

/*jshint forin: true */
/*global doSomething */

var arr = [],
    i;

for (i in arr) {
    if (arr.hasOwnProperty(i)) {
        doSomething(arr[i]);
    }
}

When should I use this option?

With JSLint, if the forin option is not set, you'll get a "The body of a for in should be wrapped in an if statement to filter unwanted properties from the prototype" error any time an unfiltered for-in statement is used. In JSHint the opposite is true and you'll receive the same error message for each unfiltered for-in when the option is set.

Note that in JSHint this is an enforcing option which means JSHint does not apply it by default. If you do not explicitly set this option to true JSHint will allow the use of bitwise operators anywhere in your code.

Recommendation

• JSLint - Do not set this option (you will not be able to use unfiltered for-in statements).

• JSHint - Set this option to true (you will not be able to use unfiltered for-in statements).