How error propagate between contracts in Solidity?

Let's say we have 2 smart contracts. A and B. A calls B, and B throws an error.

What happen in A?

Not sure? haha, that's a tricky one!

Actually it depends on how B was called.

If B was called the normal way, the whole transaction get reverted:

contract A {
  function foo() {
   B b = new B();
   b.willThrow();
  }
}
contract B {
  function willThrow() {
    revert();
  }
}

BUT, if B was called with a low-level call() method, the error will NOT be propagated to A. Instead, A will received a false boolean value indicating there was an error in B. This way, A can recover from the error and handle the error itself.

Check out this video where I explain this in full details, as well as other error handling mechanisms in Solidity like assert, revert, require and throw.

By the way, in the Dapp 30 course, there is a bonus section dedicated to debugging in Solidity when I explain in details the different kind of errors in Solidity and how to debug them.