How do I unset an element in an array in javascript?

Don't use delete as it won't remove an element from an array it will only set it as undefined, which will then not be reflected correctly in the length of the array.

If you know the key you should use splice i.e.

myArray.splice(key, 1);

For someone in Steven's position you can try something like this:

for (var key in myArray) {
    if (key == 'bar') {
        myArray.splice(key, 1);
    }
}

or

for (var key in myArray) {
    if (myArray[key] == 'bar') {
        myArray.splice(key, 1);
    }
}

delete foo[key];

:D

If you know the key name simply do like this:

delete array['key_name']

An important note: JavaScript Arrays are not associative arrays like those you might be used to from PHP. If your "array key" is a string, you're no longer operating on the contents of an array. Your array is an object, and you're using bracket notation to access the member named <key name>. Thus:

var myArray = [];
myArray["bar"] = true;
myArray["foo"] = true;
alert(myArray.length); // returns 0.

because you have not added elements to the array, you have only modified myArray's bar and foo members.

This is how I would do it

 myArray.splice( myArray.indexOf('bar') , 1)