Catch and assert the value of any custom error thrown by a smart contract in Truffle unit testing by using just the built in web3.js module

expectCustomRevertError.js

const expectCustomRevertError = async function(promise, expectedErrorSignature) {
  try {
    await promise;
  } catch (error) {
    const encoded = web3.eth.abi.encodeFunctionSignature(expectedErrorSignature);
    const returnValue = Object.entries(error.data).filter(it=>it.length>1).map(it=>it[1]).find(it=>it!=null && it.constructor.name==="Object" && "return" in it).return
    assert.equal(returnValue, encoded);
    return;
  }
  expect.fail('Expected an exception but none was received');
}

// Usage
await expectCustomRevertError(
  contract.someMethod(),
  "TheCustomError()" // In Solidity: error TheCustomError();
);