Detect empty value on prompt

How to detect an empty value when user presses ok on prompt (and previously empties the prompt field)? I need to overwrite the old value with the new (empty) value. I’m doing this:

var oldVal = 'something';
var newVal = prompt("Enter new value:", oldVal);

Currently it returns null if user empties value and clicks ok.

But at the same time I need to check for null value because if user clicks ‘cancel’, it will return null, which I don’t want as new value.

Answer

It does not return null if the user hits OK – it will return an empty string. You are probably not testing the return value properly. If you want to test between the three different return states, you can do that like this:

var oldVal = 'something';
var newVal = prompt("Enter new value:", oldVal);
if (newVal === "") {
    // user pressed OK, but the input field was empty
} else if (newVal) {
    // user typed something and hit OK
} else {
    // user hit cancel
}

Working demo: http://jsfiddle.net/jfriend00/Kx2EK/

---

Your comment suggests that you’re using this code to test the result:

if(!newVal || oldVal == newVal)return false;

When the user clears the field and presses OK, newVal will be "" (an empty string). !newVal will be true so you will return false. An empty string is a falsey value just like null. You need to more explicitly check for null like this:

if (newVal === null || newVal === oldVal) {
    // cancel button was hit
    // or the same value was entered
    return false;
}

Working demo of this logic: http://jsfiddle.net/jfriend00/ynwBx/

Note: I’m using === to prevent the javascript interpreter from doing any type casting as I want to only explicitly check for null.