Finding the average of an array using JS

I've been looking and haven't found a simple question and answer on stack overflow looking into finding the average of an array.

This is the array that I have

const grades = [80, 77, 88, 95, 68];

I first thought that the answer to this problem would be something like this:

let avg = (grades / grades.length) * grades.length
console.log(avg)

However, this gave me an output of NaN.

So then I tried this:

for (let grade of grades)
	avg = (grade / grades.length) * grades.length
console.log(avg)

This gave me an output of 68. (I'm not sure why).

So with this I have two questions. 1. Why was my output 68? and 2. Could somebody help me out with actually finding the average of an array?

With ES6 you can turn Andy's solution into as a one-liner:

const average = (array) => array.reduce((a, b) => a + b) / array.length; console.log(average([1,2,3,4,5]));

You calculate an average by adding all the elements and then dividing by the number of elements.

var total = 0;
for(var i = 0; i < grades.length; i++) {
    total += grades[i];
}
var avg = total / grades.length;

The reason you got 68 as your result is because in your loop, you keep overwriting your average, so the final value will be the result of your last calculation. And your division and multiplication by grades.length cancel each other out.

For the second part of your question you can use reduce to good effect here:

const grades = [80, 77, 88, 95, 68];

function getAvg(grades) {
  const total = grades.reduce((acc, c) => acc + c, 0);
  return total / grades.length;
}

const average = getAvg(grades);
console.log(average);

The other answers have given good insight into why you got 68, so I won't repeat it here.

The MacGyver way, just for lulz

var a = [80, 77, 88, 95, 68];

console.log(eval(a.join('+'))/a.length)

There's no built in function, but you can use this to get the sum,

Array.prototype.sum = function() {
    return this.reduce(function(a,b){return a+b;});
};

then divide by the arrays length, e.g.:

var arr = [1,2,3,4,5]
console.log(arr.sum() / arr.length)

It can simply be done with a single reduce operation as follows;

var avg = [1,2,3,4].reduce((p,c,_,a) => p + c/a.length,0);
console.log(avg)

var total = 0
grades.forEach(function (grade) {
    total += grade        
});
console.log(total / grades.length)

The average function you can do is:

const getAverage = (arr) => arr.reduce((p, c) => p + c, 0) / arr.length

Also, I suggest that use the popoular open source tool, eg. Lodash:

const _ = require('lodash')
const getAverage = (arr) => _.chain(arr)
 .sum()
 .divide(arr.length)
 .round(1)
 .value()

You can use map/reduce functions of javascript to find average. Reduce will sum them up, and map will find average.

var avg = grades.map((c, i, arr) => c / arr.length).reduce((p, c) => c + p);