Mobile phone Pop Up alert

I am setting up a button with javascript pop-up alert if certain criteria is not met
This button works fine on browser but it stops working when click it through my APP
it will take me straight to a browser. Has anyone had any idea how i can get around this. Thanks 
below is my code. any help is very much appreciated


If(not IsNull([Actual Completion Date]),
"<a " & $style & " href=\"javascript:alert ('Task has been completed!!')\">" &[Task Status]&"</a>",

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Leon Wong
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