Trying to summate array

I have the following payload and I am trying to summate all the used: items

My code is:

var total=0;
var i=0;

for (i=0; i< msg.payload.ScannedCount; i++) {

total = total + msg.payload.Items[i].used;

msg.payload = total;
return msg;

But I keep getting 0?

The length on an array in JavaScript is accessed via .length not .ScannedCount

There is also no Items property in a JavaScript array.

msg.payload[i].used is how you would access the used property in the array element object.

Reference: Array - JavaScript | MDN

You can use the Array.reduce() function. Something like this.

msg.payload = msg.payload.reduce( accumulate, 0)
return msg;

// add this value to the total
function accumulate(total, thisOne) {
    return total + thisOne.used

See JavaScript Array reduce() Method

You can do this in a change node to, using JSONata. Feed your array into the change node to see the result.