Lessons · TypeScript · types promise, tests prove

Types promise a shape; a test proves a value

The compiler checks that percent returns a number. Which number, for which input, is a claim only a test can make, and a claim that fails throws with your message.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

A typed codebase still ships wrong numbers. The type system cannot know that a quarter is 25, only that it is a number; a test is where that fact is written down.

How to think about it

Write the smallest assert you need: a check(ok, message) that throws. Claim the values that matter, with a message that says what was expected. Let the types handle shape and the tests handle truth.

Worked example

function check(ok: boolean, msg: string): void { if (!ok) throw new Error(msg); }

The smallest assert.

function share(part: number, whole: number): number { return whole === 0 ? 0 : Math.round((1000 * part) / whole) / 10; }

Typed: it returns a number.

check(share(1, 4) === 25, 'a quarter is 25');

Holds.

try { check(share(1, 3) === 33, 'a third rounds to 33.3'); } catch (e) { console.log('failed:', (e as Error).message); }

failed: a third rounds to 33.3

console.log('types said number; the tests say which number');

types said number; the tests say which number

Your turn

Claim the zero case.

check(share(5, 0) === , 'nothing out of nothing is 0');

The trap

Believing a green compile means the code is right. It means the shapes line up. The values are still yours to prove.

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