[javascript] 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

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

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

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

However this gave me an output of NaN.

So then I tried this:

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

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);

answer to your 1. question:

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

avg is declared in each loop. Thus, at the end of the loop, avg has the value of the last item in the array : 68 *5 / 5

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()

One liner challange accepted

const average = arr => arr.reduce((sume, el) => sume + el, 0) / arr.length;

and then

average([1,2,3,4]); // 2.5

Writing the function for average yourself is crazy. That's why modules exist. I found math.js

In my HTML file:

  <script src="http://cdnjs.cloudflare.com/ajax/libs/mathjs/3.2.1/math.min.js"></script>

(or you can download it)

In my JS code:

math.mean([1,2,3]) // =2
math.mean(1,2,3) // the same
math.mean(grades)

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);_x000D_
console.log(avg)

The MacGyver way,just for lulz

var a = [80, 77, 88, 95, 68];_x000D_
_x000D_
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)

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

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

let average = (array) => array.reduce((a, b) => a + b) / array.length;_x000D_
console.log(average([1,2,3,4,5]));

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

const grades = [80, 77, 88, 95, 68];_x000D_
_x000D_
function getAvg(grades) {_x000D_
  const total = grades.reduce((acc, c) => acc + c, 0);_x000D_
  return total / grades.length;_x000D_
}_x000D_
_x000D_
const average = getAvg(grades);_x000D_
console.log(average);

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