TypeScript Makes Writing Cleaner Code Easier

Clean Coders know that Boolean Arguments can be dangerous and as a general rule, should be avoided.  I had this thought in mind when writing a TypeScript unit test recently.  I had this bit of code which is rather ugly and not immediately obvious what it was doing.

getPaymentMethodsSpy.and.callFake(() => ActionsStubs.GetPaymentMethods.fakeResponseWithPaymentCount(1));
await Payment.setPayments("");
CommonLib.setValue(Payment.fields.paymentMethod, 1);

The fakeResponseWithPaymentCount sets the getPaymentMethodsSpy to generate the given number of fake existing payments for a count (1 in this case) as a response from the custom action, GetPaymentMethods.  Each odd numbered payment is a bank, and each even numbered one is a credit card.   So for this snippet, line 1 fakes a response of 1 Bank Payment, line 2 sets the Payment on the form, and line 3 selects that payment as the payment to use.  Simple right?  Not.  Good luck remembering this and re-creating it for processing a Credit Card.
So how does TypeScript here help make this cleaner?  Before we get there, lets do the obvious first step and refactor these statements out to a method:

async function selectExistingPayment(isBank: boolean) {
    getPaymentMethodsSpy.and.callFake(() => ActionsStubs.GetPaymentMethods.fakeResponseWithPaymentCount(2));
    await Payment.setPayments("");
    CommonLib.setValue(Payment.fields.paymentMethod, isBank ? 1 : 2);
}

This is way more usable, except when I’m looking at a calls site for it, it’s still really not clear:

selectExistingPayment(true);
selectExistingPayment(false);

What does false mean?  Don’t select it?  Sure, you could create an enum and define “credit card”, and “bank” as the two options, but this is just a unit test helper method.  I’m not exposing it anywhere, and even if I did, that’s a lot of extra work (Ok, so it’s a tiny bit of work, but it can feel like a lot #1stWorldProblems).  The simple solution TypeScript offers is string Union Types.

async function selectExistingPayment(type: "CC" | "BANK") {
    getPaymentMethodsSpy.and.callFake(() => ActionsStubs.GetPaymentMethods.fakeResponseWithPaymentCount(2));
    await Payment.setPayments("");
    CommonLib.setValue(Payment.fields.paymentMethod, type === "BANK" ? 1 : 2);
}

So now the calls sites look like this:

Extremely readable as to what I’m doing, and you get compile time error checking that "Dad" (or any other values besides "CC" and "BANK" ) is not a valid Existing Payment Method!