Java Script Need help finding my error

<script type="text/javascript">
 var score = prompt("Enter your score. Grade must be between 100-0.");

 if ( score>==90 ) 
    {
        document.write ( "Your grade is an A." )
    }
else if ( score <89>==80 )
    {
        document.write ( "Your grade is a B." )
    }
else if ( score <79>==70 )
    {
        document.write ( "Your grade is a C.")
    }
else if ( score <69>==60 ) 
    {
        document.write ( "Your grade is a D.")
    }
else if ( score <59>==0 )
    {
        document.write ( "Your grade is a F")
   }

}//end if
</script>