Ultimate JavaScript Exercise 9- Grade

1

Hi everyone. I did this “Grade” Exercise:
const marks = [80, 80, 50];
console.log(calculateGrade(marks));
function calculateGrade(marks) {
let sum = 0;
for (let mark of marks) sum += mark;
let average = sum / marks.length;
if (average < 60) return “F”;
if (average < 70) return “D”;
if (average < 80) return “C”;
if (average < 90) return “B”;
return “A”;
}
i understud everything exept this expression sum += mark.
I know that variable include three numbers collected from arrey. If i will console.log (mark) it will give me 80, 80, 50 - three sepparete numbers. If i will shift this expressoin sum += mark with anather aruthmetic expression like - mark + 1 it still gives me three sepparete number but in a case of sum += mark number are summing. Please explane me how does it happend or shere me a right article abaut this topic.
Thank you :slight_smile:

2 
function calculateGrade(marks) {
    let sum = 0;

    for (let mark of marks) {
        sum += mark;
    }
    
    let average = sum / marks.length;
    let grade = null;

    if (average < 60) {
        grade = "F";
    } else if (average < 70) {
        grade = "D";
    } else if (average < 80) {
        grade = "C";
    } else if (average < 90) {
        grade = "B";
    } else {
        grade = "A";
    }

    return grade;
}

const marks = [80, 80, 50];

console.log(calculateGrade(marks));

sum += mark is the shorthand of sum = sum + mark. You declared and initialized a variable let sum = 0. Then, you are looping through the array, and taking the value at each index and adding it to sum for your total.

sum = 0
sum = sum + 80 (80 + 0 = 80)
sum = sum + 80 (80 + 80 = 160)
sum = sum + 50 (160+ 50 = 210)
sum = 210;