Cancel in prompt browser Script

Hello,

I have written one browser Script,in which I have to take some input from user as follows:

var input = prompt("Please provide input data")

now it is working when i give some input data and click on Ok, but it is not cancelling the operation when i click on Cancel.

By default there is some value in the input as 'Undefined' . So how the logic will work.I tried like

if (input == "True")

{ do this}

if (input == "False")

{ return(CancelOperation)}

bt it always take the value of input as "Undefined" or what i enter in the prompt box, it is not treating the value of input as "True" or "False". Can someone suggest on this.