Expecting Bananas

"If a function expects a banana, don't hand it a coconut."

When you write a function, you're not simply saying "do this".

You're implicitly saying "I expect this to be true."

In the same way that a monkey expecting bananas will be disappointed by a coconut, a function expecting strings will be disappointed by a number.

Here's one way to express that expectation in TypeScript:

typescript
1function chimp(food: any) {
2  if (food !== "banana") {
3    throw new Feces("🐒 EXPECTED: 🍌  GOT: " + food + " 💩");
4  }
5  return "😋";
6}
7
8class Feces extends Error {
9  constructor(message: string) {
10    super(message);
11    this.name = "💩 UnexpectedFoodException 💩";
12  }
13}
14
15chimp("banana");  // "😋"
16chimp("coconut"); // 💩 UnexpectedFoodException 💩: 🐒 EXPECTED: 🍌  GOT: coconut 💩
17

The beauty of explicit expectations is that when they're violated, you get clear, actionable feedback instead of mysterious bugs downstream.