terminate a prompt dialogue box when user enters "-1" [javascript]

Question

I am having some troubles with js code. I need to prompt the user to enter a valid number (between 0 and 120). If the entry is invalid, it should just display the starting prompt. However, if the user enters "-1", it should terminate the dialogue box.

My dialogue box does not terminate when I enter "-1". What could be the issue? Here is my code

let numGrade; //variable that holds number grade entry
let letter; //this variable holds grade as a letter
//this function checks whether our grade within the range
    numGrade = window.prompt("Enter number grades from 0 to 120\nOr enter - 1 to end entries")
    if (numGrade == -1){
        return false;
    }
    else if(numGrade<=120 && numGrade>=0){
        //calling replaceGrades function, passing a number grade to convert it to a letter grade
        //  store a grade letter in "final"
        final = replaceGrades(numGrade)
        //display a number grade and letter grade
        alert("Number grade = " + numGrade + "\nLetter grade = " + final)

        //this function takes in number grade as a parameter--
        // and return a letter grade
        function replaceGrades(grade) {

            let letterGrade; //variable that returns a  letter grade

            //if else-if to convert number grades to letter grades{
            if (grade >= 100 && grade <= 120) {
                return letterGrade = "A";
            } else if (grade >= 80 && grade <= 99) {
                return letterGrade = "B";
            } else if (grade >= 70 && grade <= 79) {
                return letterGrade = "C";
            } else if (grade >= 60 && grade <= 69) {
                return letterGrade = "D"
            } else {
                return letterGrade = "F";
            }

        }
    }
    else {
        numGrade = window.prompt("Enter number grades from 0 to 120\nOr enter - 1 to end entries")
    }