Adding a Prompt to a Variable

I am a beginner in scripting, self taught – finding it easier to learn from examples when I stumble over one that interests me.

I found the following script:

// https://graphicdesign.stackexchange.com/questions/58812/specific-resize-layer-action

#target photoshop

(function (){

    var startRulerUnits = app.preferences.rulerUnits; 

    app.preferences.rulerUnits = Units.PIXELS; 

    var bounds = activeDocument.activeLayer.bounds; 

    var width = bounds[2].value - bounds[0].value;

    var newSize = (100 / width) * 300;  // 300px

    activeDocument.activeLayer.resize(newSize, newSize, AnchorPosition.MIDDLECENTER);

    app.preferences.rulerUnits = startRulerUnits; 

})();

To which I added a prompt (line 08) to enter a user defined variable rather than a hard coded value.

// https://graphicdesign.stackexchange.com/questions/58812/specific-resize-layer-action

#target photoshop

(function (){

    var startRulerUnits = app.preferences.rulerUnits; 

    app.preferences.rulerUnits = Units.PIXELS; 

    var bounds = activeDocument.activeLayer.bounds; 

    var width = bounds[2].value - bounds[0].value;

    var newWidth = prompt("Enter a new width in pixels.", 300);

    var newSize = (100 / width) * newWidth;

    activeDocument.activeLayer.resize(newSize, newSize, AnchorPosition.MIDDLECENTER);

    app.preferences.rulerUnits = startRulerUnits; 

})();

All is good if one hits OK, but if one presses Cancel – the actual active layer that should be resized has all content deleted. My guess is that cancel returns a value of null and that null is not a number so therefore the layer content is scaled to nothing.

So I tried adding the following line of code suggested in an Illustrator scripting discussion, however it does not stop the layer content being removed:

// User pressed the Cancel button: 

if( newWidth == null ){ break };

I read up on prompts, however I didn’t find anything that helped:

user-notification-dialogs

Hoping somebody can help set me straight, thanks!