Environment
From tns info:
✔ Component nativescript has 4.2.4 version and is up to date.
✔ Component tns-core-modules has 4.2.1 version and is up to date.
✔ Component tns-ios has 4.2.0 version and is up to date.

The bug
The reduce function form ObservableArray doesn't handle initial value correctly. The first value will always be the first item in the list and not the initial value provided as a parameter.

To Reproduce

let products = new ObservableArray([{price:10},{price:20},{price:30}]);

const sum = products.reduce((sum, p, index) => {
  console.log('Iteration', index, 'Sum:', sum, 'Type of sum:', typeof(sum), 'Price:', p.price);
  return sum + p.price;
}, 0);

console.log('Final sum', sum, typeof(sum));

// Output
// Iteration 1 Sum: { "price": 10 } Type of sum: object Price: 20
// Iteration 2 Sum: "[object Object]20" Type of sum: string Price: 30
// Final sum "[object Object]2030" string

Run the code and see that sum will be {price:10} on the first iteration. This is wrong. It should be 0. However, if we didn't pass an initial value as a parameter this would be the expected behaviour.

Expected behavior
On the first iteration the sum should be 0 as it is provided as the initial value.