Question

Some times when I call methods of this object I got “Oops” error because “i” refers to unexisted attribute. How its possible?

m.derp();
m.herp(); // Sometimes throws error

Methods:

this.movements = new Array();
this.herp = function() {
    for (var i in this.movements) {
        if (!(this.movements[i] instanceof Movement)) {
            throw new Error("Oops"); // this.movements[i] is undefined
        }
    }
}
this.derp = function() {
    var newArray = new Array();
    for (var i in this.movements) {
        if (!this.movements[i].isFinished()) {
            newArray.push(this.movements[i]);
        }
    }
    this.movements = newArray;
}

Answer 1

For arrays, you should not be using for-in. It’s a general enumerator that includes all enumerable properties, including those inherited via prototype.

You should do this…

for (var i = 0; i < this.movements.length; i++) {

Any properties added to Array.prototype or Object.prototype will be encountered when using for-in.

If you only want numeric indices, then the for statement is the correct statement to use. …except in certain narrow circumstances, as noted by T.J. Crowder below.