Node.js Data-Completion script
Just a few things


To start out, the return statement in your function completeCustomers should/could be changed.


The way the function is defined should also be changed. But we'll get to that in a second.

return (updatedCustomers = customers.map((customer, index) =&gt; {
  const updatedCustomer = Object.assign(customer, addresses[index]);

  return updatedCustomer;
}));


I would replace with:

return (updatedCustomers = customers.map((customer, index) =&gt; ({
  ...customer,
  ...addresses[index]
})))


Note, you don't have to make it take up multiple lines if you don't want.


Use const when you don't modify a variable, or arrow function


This means that the two functions below need to change:

completeCustomers = (filePathCustomers, filePathAddresses) =&gt; {


Would become:

const completeCustomers = (filePathCustomers, filePathAddresses) =&gt; {




let execCompleteCustomer = new Promise(resolve =&gt; {


And this would become:

const execCompleteCustomer = new Promise(resolve =&gt; {


That's all I have for now, but I'm not getting what part you are saying is redundant. And I think you handled the errors well.