Mocks in JavaScript

t.mock.fn records calls; t.mock.method replaces one on an object.

import test from "node:test";
import assert from "node:assert/strict";

function record(store, lang, tpm) {
  if (tpm <= 0) throw new RangeError("tpm must be positive");
  return store.save(lang, tpm);
}

test("saves through the store", (t) => {
  const save = t.mock.fn(() => "saved");
  const result = record({ save }, "go", 104);

  assert.equal(result, "saved");
  assert.equal(save.mock.callCount(), 1);
  assert.deepEqual(save.mock.calls[0].arguments, ["go", 104]);
});

test("replaces a method for one test", (t) => {
  const clock = { now: () => 0 };
  t.mock.method(clock, "now", () => 1234);
  assert.equal(clock.now(), 1234);
});

How it works

1. The mock's calls array holds the arguments and the result.
2. A replaced method is restored when the test ends.
3. Assert on the call, not on the implementation.

Keywords and builtins used here

• const
• from
• function
• if
• import
• return
• throw

The run, in numbers

Lines

22

Characters to type

620

Tokens

184

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 100 seconds.

Type this snippet

Step 1 of 2 in Mocks & time, step 5 of 9 in Testing with node:test.

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