As I look at this chunk, I conclude that it’s calculating the charge for one per-
formance. That conclusion is a piece of insight about the code. But as Ward
Cunningham puts it, this understanding is in my head—a notoriously volatile
form of storage. I need to persist it by moving it from my head back into the
code itself. That way, should I come back to it later, the code will tell me what
it’s doing—I don’t have to figure it out again.
The way to put that understanding into code is to turn that chunk of code into
its own function, naming it after what it does—something like amountFor(aPerformance).
When I want to turn a chunk of code into a function like this, I have a procedure
for doing it that minimizes my chances of getting it wrong. I wrote down this
procedure and, to make it easy to reference, named it Extract Function (106).
First, I need to look in the fragment for any variables that will no longer be in
scope once I’ve extracted the code into its own function. In this case, I have three:
perf, play, and thisAmount. The first two are used by the extracted code, but not
modified, so I can pass them in as parameters. Modified variables need more
care. Here, there is only one, so I can return it. I can also bring its initialization
inside the extracted code. All of which yields this:
function statement…
function amountFor(perf, play) {
let thisAmount = 0;
switch (play.type) {
case "tragedy":
thisAmount = 40000;
if (perf.audience > 30) {
thisAmount += 1000 * (perf.audience - 30);
}
break;
case "comedy":
thisAmount = 30000;
if (perf.audience > 20) {
thisAmount += 10000 + 500 * (perf.audience - 20);
}
thisAmount += 300 * perf.audience;
break;
default:
throw new Error(`unknown type: ${play.type}`);
}
return thisAmount;
}
When I use a header like “function someName…” in italics for some code, that means
that the following code is within the scope of the function, file, or class named in the
header. There is usually other code within that scope that I won’t show, as I’m not
discussing it at the moment.
The original statement code now calls this function to populate thisAmount:
7
Decomposing the statement Function
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