Can someone please explain to me what I am doing wrong here... This code is from eloquent javascript and it works fine

function sum(array) {
  let total = 0;
  for (let value of array) {
    total += value;
  }
  return total;
}

Can someone please explain to me what I am doing wrong here... This code is from eloquent javascript and it works fine

function sum(array) {
  let total = 0;
  for (let value of array) {
    total += value;
  }
  return total;
}

And this is what I wrote for the exercise but returns NaN..

function sum(numArray) { 
  let add = 0;
  for (let a = 0; a <= numArray.length; a++) {
  let addIndex = numArray[a];
    add += addIndex;
  } 
  return add;
}

• javascript
• arrays
• loops

• What values are actually in that array? I suspect that not everything in there is a 'number'. – ouflak Commented Sep 19, 2018 at 16:27

5 Answers 5

Your for loop goes out of array indexes. You have to use:

a < numArray.length

Instead of:

a <= numArray.length

You simply add undefined to add, because you run the index count to long.

for (let a = 0; a <= numArray.length; a++) {
//                 ^                         wrong, takes last index + 1

function sum(numArray) { 
  let add = 0;
  for (let a = 0; a < numArray.length; a++) {
    let Addindex = numArray[a];
    add += Addindex;
  } 
  return add;
}

console.log(sum([1, 2, 3, 4]));

The issue is because of this a <= numArray.length. Change it to a < numArray.length. This case a[5] that is the 6th element or the element at 5th index is undefined as the array starts from 0 index. So it will add an undefined with previously added number and hence it will be NaN

function sum(numArray) {
  let add = 0;
  for (let a = 0; a < numArray.length; a++) {
    let Addindex = numArray[a];
    add += Addindex;
  }
  return add;
}

console.log(sum([1, 2, 3, 4, 5]))

You're getting an out-of-bounds error. In your for loop, you can change it to:

for (let a = 0; a < numArray.length; a++) {

OR

for (let a = 0; a <= numArray.length - 1; a++) {

The latter works too, but is harder to read.

You can also write a function that has two parameters, an array and a callback function that adds the values of the array like

function forEach(array, arrayAdder){
    for (var i = 0; i < array.length; i ++)
    arrayAdder(array[i]) ;
}

We can now initialize both the array and the sum like

var array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], sum = 0;

After that we pass it into the function like this

forEach(array, function(number){
    sum += number ;
});

Then print the answer

console.log(sum);