JS Grade Calculator

//get score from user
var score = prompt("Enter score: ");

//display correcponding letter grade
console.log("Your grade is: " + gradeCalc(score));

//converts from number score to letter grade
function gradeCalc(sco){
    var gra;

    //catch invalid scores
    if (sco<0 || sco>100){
        console.log("ERROR: ");
        return 'invalid';
    }else if (sco>90){gra='A';}
    else if (sco>80){gra='B';}
    else if (sco>70){gra='C';}
    else if (sco>60){gra='D';}
    else{gra='F';}

    return gra;
}