What does the JSLint error 'body of a for in should be wrapped in an if statement' mean?

Question

I used JSLint on a JavaScript file of mine. It threw the error:

for( ind in evtListeners ) {

Problem at line 41 character 9: The body of a for in should be wrapped in an if statement to filter unwanted properties from the prototype.

What does this mean?

Answer 1

First of all, never use a for in loop to enumerate over an array. Never. Use good old for(var i = 0; i<arr.length; i++).

The reason behind this is the following: each object in JavaScript has a special field called prototype. Everything you add to that field is going to be accessible on every object of that type. Suppose you want all arrays to have a cool new function called filter_0 that will filter zeroes out.

Array.prototype.filter_0 = function() {
    var res = [];
    for (var i = 0; i < this.length; i++) {
        if (this[i] != 0) {
            res.push(this[i]);
        }
    }
    return res;
};

console.log([0, 5, 0, 3, 0, 1, 0].filter_0());
//prints [5,3,1]

This is a standard way to extend objects and add new methods. Lots of libraries do this. However, let's look at how for in works now:

var listeners = ["a", "b", "c"];
for (o in listeners) {
    console.log(o);
}
//prints:
//  0
//  1
//  2
//  filter_0

Do you see? It suddenly thinks filter_0 is another array index. Of course, it is not really a numeric index, but for in enumerates through object fields, not just numeric indexes. So we're now enumerating through every numeric index and filter_0. But filter_0 is not a field of any particular array object, every array object has this property now.

Luckily, all objects have a hasOwnProperty method, which checks if this field really belongs to the object itself or if it is simply inherited from the prototype chain and thus belongs to all the objects of that type.

for (o in listeners) {
    if (listeners.hasOwnProperty(o)) {
       console.log(o);
    }
}
 //prints:
 //  0
 //  1
 //  2

Note, that although this code works as expected for arrays, you should never, never, use for in and for each in for arrays. Remember that for in enumerates the fields of an object, not array indexes or values.

var listeners = ["a", "b", "c"];
listeners.happy = "Happy debugging";

for (o in listeners) {
    if (listeners.hasOwnProperty(o)) {
       console.log(o);
    }
}

 //prints:
 //  0
 //  1
 //  2
 //  happy