when to ue switch statement and if statement in javascript code example

Example: js convert if/else statement to switch case

var gradeOut = 70;
var totalGrade = 80;

if (gradeOut >= 90)
    gradeOut+= "My letter grade is: A";
  else if (totalGrade < 90 && totalGrade >= 80)
    gradeOut+= "My letter grade is: B";
  else if (totalGrade < 80 && totalGrade >= 70)
    gradeOut+= "My letter grade is: C";
  else if (totalGrade < 70 && totalGrade >= 60)
    gradeOut+= "My letter grade is: D";
  else if (totalGrade < 60)
    gradeOut+= "My letter grade is: F";

//here's the same thing using switch(true). Requires var gradeOut (is currently at top of page
switch(true) {
case gradeOut<60:
    gradeOut= "My letter grade is: F";
    break;
case gradeOut<70:
    gradeOut= "My letter grade is: D";
    break;
case gradeOut<80:
    gradeOut="My letter grade is: C";
    break;
}