Failing Transaction within Truffle Testing

Question

How does one test a transaction that is supposed to fail when testing with truffle?

For example, say I was testing an ERC token, in which accounts[0] had a balance of 20. Within a test, I call ERC.transfer(accounts[1], 25, {from: accounts[0]}), hoping to make sure that this would fail. How would I confirm that it did fail?

Sure, I could check that the balances of accounts[0] and accounts[1] do not change - but it seems like there may be a more elegant way to do so. Is there a way to do this?

Answer 1

Maybe this gist can help you

export default async promise => {
  try {
    await promise;
  } catch (error) {
    // TODO: Check jump destination to destinguish between a throw
    //       and an actual invalid jump.
    const invalidJump = error.message.search('invalid JUMP') >= 0;
    // TODO: When we contract A calls contract B, and B throws, instead
    //       of an 'invalid jump', we get an 'out of gas' error. How do
    //       we distinguish this from an actual out of gas event? (The
    //       testrpc log actually show an 'invalid jump' event.)
    const outOfGas = error.message.search('out of gas') >= 0;
    assert(
      invalidJump || outOfGas,
      "Expected throw, got '" + error + "' instead",
    );
    return;
  }
  assert.fail('Expected throw not received');
};

Answer 2

You can use my truffle-assertions library to test for reverts or other failures.

It can be installed with npm:

npm install truffle-assertions

And then imported at the top of your file:

const truffleAssert = require('truffle-assertions');

You can then use it inside your tests like this:

// when you want to test revert
truffleAssert.reverts(ERC.transfer(accounts[1], 25, {from: accounts[0]}));

// or general failure
truffleAssert.fails(ERC.transfer(accounts[1], 25, {from: accounts[0]}));

The full method documentation can be found on the GitHub repository.