adds instructions for how to create tests

CONTRIBUTING.md

@@ -12,7 +12,41 @@ Article PRs should be reviewed using the [PR review process here](https://github
 
 ### Creating tests
 
-Test files must be within `/src`, with a file name ending in `.test.<ext>`, where `<ext>` is one of `js` `mjs` `jsx` `ts` `tsx`.
+1. For some module `foo.js`, create a file `foo.test.js` in the same directory.
+
+   - `.ts` / `.mjs` / `.js` are all valid extensions.
+   - `.tsx` / `.jsx` are permitted, though unless the XML syntax is necessary, use `.ts` / `.mjs` instead.
+   - Tests must be within `/src`.
+
+2. Import the module to be tested.
+
+   - For TypeScript, Jest utilities are not in global scope, so you will need to import these from `@jest/globals`.
+
+3. Use Jest's [`expect`](https://jestjs.io/docs/expect), [`it`](https://jestjs.io/docs/api#testname-fn-timeout) and [`describe`](https://jestjs.io/docs/api#describename-fn) to organise and assert the behaviour of the module. A test name should start with a verb, and descriptions should be used to indicate what component is being tested.
+
+```ts
+/** @file criticalMathModule.test.ts */
+import { describe, expect, it } from "@jest/globals"; // only needed for TypeScript
+import { isEven, log2 } from "./criticalMathModule";
+
+describe("isEven", () => {
+  it("checks if numbers are even", () => {
+    expect(isEven(1)).toBe(false);
+    expect(isEven(4)).toBe(true);
+  });
+});
+
+describe("log2", () => {
+  it("returns the base-2 logarithm of a number", () => {
+    expect(log2(1024)).toBeCloseTo(10);
+  });
+
+  it("throws an error for non-positive numbers", () => {
+    expect(log2(0)).toThrow();
+    expect(log2(-1)).toThrow();
+  });
+});
+```
 
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