JSFiddle - React, Tailwind, and code Playground

JavaScript

var grades = [100, 90, 100, 50, 80, 60];
function countFailing(gradesArr){
    var counter = 0;
    for (var i = 0; i < gradesArr.length; i++){
        if (gradesArr[i] <= 70 )
            counter++;
    } 
    return counter;
}

alert(countFailing(grades));