Type system can't save you from missing awaits

bad-locking.ts

/*
% npm install typescript
.. warnings blah blah..

+ typescript@3.2.2
updated 1 package and audited 2 packages in 0.538s
found 0 vulnerabilities

% node_modules/.bin/tsc --lib es2018,dom --target es2018 --strict --noImplicitReturns --noImplicitAny --out hello.js hello.ts

% node hello.js
buggy1: .- startup
buggy1: | → lock
buggy1: | ← unlock
buggy1: |  🡓 inside
buggy1: '- done
buggy2: .- startup
buggy2: | → lock
buggy2: | ← unlock
buggy2: |  🡓 inside
buggy2: '- done
buggy3: .- startup
buggy3: |  🡓 inside
buggy3: | → lock
buggy3: | ← unlock
buggy3: '- done
buggy4: .- startup
buggy4: | → lock
buggy4: |  🡓 inside
buggy4: '- done
buggy4: | ← unlock
buggy5: .- startup
buggy5: |  🡓 inside
buggy5: '- done
buggy5: | → lock
buggy5: | ← unlock
buggy6: .- startup
buggy6: | ← unlock
buggy6: |  🡓 inside
buggy6: '- done
buggy6: | → lock
okay1: .- startup
okay1: | → lock
okay1: |  🡓 inside
okay1: | ← unlock
okay1: '- done
okay2: .- startup
okay2: | → lock
okay2: |  🡓 inside
okay2: | ← unlock
okay2: '- done
app complete
*/

class Lock {
  name: string;

  constructor(name: string) {
    this.name = name;
  }

  async lock(): Promise<void> {
    await delay(30);
    console.log(`${this.name}: | → lock`);
  }

  async unlock(): Promise<void> {
    await delay(10);
    console.log(`${this.name}: | ← unlock`);
  }
}

async function buggy1<T>(l: Lock, cb: () => Promise<T>): Promise<T> {
  await l.lock();
  try {
    // no await, resolved in parent
    return cb();
  } finally {
    await l.unlock();
  }
}

async function buggy2<T>(l: Lock, cb: () => Promise<T>): Promise<T> {
  await l.lock();
  try {
    // still no await, explicit resolve does nothing
    return Promise.resolve(cb());
  } finally {
    await l.unlock();
  }
}

async function buggy3<T>(l: Lock, cb: () => Promise<T>): Promise<T> {
  // no waiting for lock
  l.lock();
  try {
    return await cb();
  } finally {
    await l.unlock();
  }
}

async function buggy4<T>(l: Lock, cb: () => Promise<T>): Promise<T> {
  await l.lock();
  try {
    return await cb();
  } finally {
    // no waiting for unlock
    l.unlock();
  }
}

async function buggy5<T>(l: Lock, cb: () => Promise<T>): Promise<T> {
  // no waiting for either
  l.lock();
  try {
    return await cb();
  } finally {
    l.unlock();
  }
}

async function buggy6<T>(l: Lock, cb: () => Promise<T>): Promise<T> {
  // no waiting for anything
  l.lock();
  try {
    return cb();
  } finally {
    l.unlock();
  }
}

// wait for everything
async function okay1<T>(l: Lock, cb: () => Promise<T>): Promise<T> {
  await l.lock();
  try {
    return await cb();
  } finally {
    await l.unlock();
  }
}

async function okay2<T>(l: Lock, cb: () => Promise<T>): Promise<T> {
  await l.lock();
  try {
    // wait for everything, with an explicit promise
    return Promise.resolve(await cb());
  } finally {
    await l.unlock();
  }
}

// driver
(async () => {
  for (const func of [buggy1, buggy2, buggy3, buggy4, buggy5, buggy6, okay1, okay2]) {
    const name = func.name;
    console.log(`${name}: .- startup`);

    await func(new Lock(name), async () => {
      await delay(20);
      console.log(`${name}: |  🡓 inside`)
    });
    console.log(`${name}: '- done`);
    await delay(70);
  }
})().then(() => console.log('app complete'));

function delay<T>(millis: number, value?: T): Promise<T> {
  return new Promise((resolve) => setTimeout(() => resolve(value), millis))
}