Insert new element in an array of object and sort the array

I have used non ES6 functins, instead traditional for loop and other. Please, find answers for your three questions:

• Creating a list/array of object in this format: [ { name: 'Alin', average: 68.8 }, ..]

• Sorting a list/array with average value

• Getting students with the highest average grade.

       const Students = [{
       name: 'Bob',
       marks: [78, 80, 89, 90, 68],
   },
       {
           name: 'Alin',
           marks: [87, 60, 59, 70, 68],
       },
       {
           name: 'bikash',
           marks: [82, 60, 79, 60, 80],
       },
       {
           name: 'Doston',
           marks: [78, 80, 89, 90, 68],
       },
   ];
  
   var average;
   var data = [];
   for (var i = 0; i < Students.length; i++){
       var marks = Students[i]["marks"];
       var total = 0;
       console.log(marks);
       for (var j = 0; j < marks.length; j++ ) {
           total += marks[j];
       }
       average = total / marks.length;
  
       var msg = Students[i]["name"] + " has average mark: " + average;
       console.log(msg);
  
       // Answer for the first question:
       var item = {"name": Students[i]["name"], "average": average};
       data.push(item);
  
   }
  
   function compareAndSort(a,b) {
       return parseInt(a.average, 10) - parseInt(b.average, 10);
   }
  
   data.sort(compareAndSort);
  
   // Answer for the second question: sort [ { name: 'Alin', average: 68.8 }, ..]
   console.log(data);
  
   // Answer for the third question - getting students who has highest mark is last since it is sorted
   var bestStudent = data[data.length - 1];
   console.log(bestStudent.name + " has got the highest mark: " + bestStudent.average);
  
   var bestStudents = [bestStudent];
  
   // check if there is only one person with this highest mark
   for (var i = 0; i < data.length; i ++) {
       if (data[i].average >= bestStudent.average && data[i].name !== bestStudent.name) {
           bestStudents.push(data[i]);
       }
   }
  
   // all best students
   console.log(bestStudents);
  

I have added some data to the JSON input, so that you get two best students

You don't need an additional array. In theory, calling sort is less efficient than to gather the best student(s) during your loop.

So:

var students = [{name: "Bob",marks: [78,80,89,90,68] },{name: "Alin",marks: [87,60,59,70,68]},{name: "bikash",marks: [82,60,79,60,80]}];

var topAverage = -1; 
var topStudents = [];
var newArray = [];
for (let i = 0; i < students.length; i++){
    var marks = students[i].marks;
    var total = 0;
    for (var j = 0; j < marks.length; j++ ) {
        total += marks[j];
    }
    var average = total / marks.length;
    // Keep topAverage & topStudents up to date:
    if (average >= topAverage) {
        if (average > topAverage) {
            topStudents.length = 0; // clear previous content
            topAverage = average;
        }
        topStudents.push(students[i].name);
    }
}

console.log("Top student(s): " + topStudents + " with an average of " + topAverage);

Instead of using loops, I used the built-in array methods, which tend to be much more readable (although they would be a lot more readable if you used ES6 syntax).

I use the Array.prototype.map method to convert the list of marks into the average. In order to find this average, I use Array.prototype.reduce to add up all of the values, and then I divide by the length of the marks array. I then sort the array using Array.prototype.sort.

In order to get the top students, I use Array.prototype.filter in order to select only the students will average marks equal to the top average mark, which since the averages array is sorted, will be averages[0].average.

var Students = [
  {
    name: "Bob",
    marks: [78, 80, 89, 90, 68]
  },
  {
    name: "Alin",
    marks: [87, 60, 59, 70, 68]
  },
  {
    name: "bikash",
    marks: [82, 60, 79, 60, 80]
  }
];

var averages = Students.map(function(student) {
  return {
    name: student.name,
    average: student.marks.reduce(function(a, b) {
      return a + b;
    }) / student.marks.length
  };
}).sort(function(a, b) {
  return b.average - a.average;
});

var top_students = averages.filter(function(student, _i, arr) {
  return student.average === arr[0].average;
});

console.log("Averages:");
console.log(averages);

console.log("Top Students:\n");
console.log(top_students);