snoop 🎵

Easy breezy test spies fo sheezy.

Create small, dependency-free function spies that record calls, results, errors, and relative call order. Snoop collaborates with everyone.

Installation

npm install --save-dev snoop

Other package managers:

yarn add --dev snoop
pnpm add --save-dev snoop
bun add --dev snoop

Usage

With uvu

import { snoop } from 'snoop';
import { test } from 'uvu';
import * as assert from 'uvu/assert';

const add = (a, b) => a + b;

test('add', () => {
  const addFn = snoop(add);
  const anotherAddFn = snoop(add);

  addFn.fn(1, 1);
  anotherAddFn.fn(1, 1);
  addFn.fn(2, 2);
  anotherAddFn.fn(2, 2);

  assert.ok(addFn.called);
  assert.not(addFn.notCalled);
  assert.not(addFn.calledOnce);
  assert.is(addFn.callCount, 2);
  assert.equal(addFn.calls[0].arguments, [1, 1]);
  assert.equal(addFn.calls[1].arguments, [2, 2]);
  assert.is(addFn.calls[0].result, 2);
  assert.is(addFn.calls[1].result, 4);
  assert.not(addFn.firstCall.error);
  assert.not(addFn.lastCall.error);
  assert.ok(addFn.calledBefore(anotherAddFn));
  assert.ok(addFn.calledAfter(anotherAddFn));
  assert.ok(addFn.calledImmediatelyBefore(anotherAddFn));
  assert.not(addFn.calledImmediatelyAfter(anotherAddFn));
});

test.run();

TypeScript declarations are included with the package.

API

snoop(fn)

Returns a spy object whose fn method invokes the supplied function and records each call.

The wrapper records thrown values in the call's error property and returns undefined instead of rethrowing them. This matches the package's existing behavior.

Property or method	Description
fn(...args)	Invokes the function and records its arguments and result.
testID	Internal numeric identifier used for global call ordering.
called	Whether the spy has been called at least once.
notCalled	Whether the spy has never been called.
calledOnce	Whether the spy has been called exactly once.
callCount	Number of recorded calls.
calls	All call records in invocation order.
firstCall	First call record, or undefined before the first call.
lastCall	Most recent call record, or undefined before the first call.
calledBefore(other)	Whether this spy was called before the other spy.
calledAfter(other)	Whether this spy was called after the other spy.
calledImmediatelyBefore(other)	Whether this spy's latest call immediately preceded the other.
calledImmediatelyAfter(other)	Whether this spy's latest call immediately followed the other.

Each call record contains arguments, result, and error.

Runtime requirements

• An ESM-compatible runtime or bundler
• No runtime dependencies

Development

Install dependencies:

bun install

Run the tests:

bun run test

Format the project:

bun run format

License

MIT