reduce method fails unit test

I have the following method within an Angular component:

public mapInvoices(invoices: any[]): Record<string, any> {
    return invoices.reduce((map, obj) => {
      map[obj.letterType] = obj;
      return map;
    }, {});
  }

When I run the unit test for this component, I’m getting the following issue:

APAUInvoicesDocumentsComponent › should create

    TypeError: Cannot read property 'reduce' of undefined

      37 |
      38 |   public mapInvoices(invoices: any[]): Record<string, any> {
    > 39 |     return invoices.reduce((map, obj) => {
         |                     ^
      40 |       map[obj.letterType] = obj;
      41 |       return map;
      42 |     }, {});

I could bypass this issue by assigning a default value to the parameter like:

public mapInvoices(invoices: any[] = [])

but I would like to understand why this happens.

>Solution :

To answer your question, it is because mapInvoices is being called with an undefined parameter. By setting a default value you are actually overriding initial state.

public mapInvoices(invoices?: any[] = []) {}
mapInvoices(); // call bombs without a default value