lesson-9 testing` questions about callStatic

[TimeKeyRoll]

Here is the testing.

    ` const { upkeepNeeded } = await lottery.callStatic.checkUpkeep([])//??
              assert(!upkeepNeeded)

`

Patrick said checkupkeep is a public function so we don't want to create a transaction but will use callStatic to simulate it.

1- I need to know why does not we want to use if its a public function and create a transaction? Why not we want a transaction for testing? and what happens when we create a transaction?

2- And If callstatic does not create a transaction and execute the function. Where does this function is coming from? AND how its is different/useful in this case other than the non-transaction case?

[krakxn]

1. "I need to know why does not we want to use if its a public function and create a transaction? Why not we want a transaction for testing? and what happens when we create a transaction?" Creating a transaction leads to a change in state. We do not want the state to change for checkUpkeep (where we check for the validity of the values, we do not want them to change--we want them to remain as is). Hence, we do not want the transaction to execute.

2. "Where does this function is coming from?" It is included in Ethers.js, the library we are using for JavaScript.

"AND how its is different/useful in this case other than the non-transaction case?" For the difference b/w the two part: as above, a transaction leads to a change in state, and we do not want that.

As for the usefulness part: it is useful because we are essentially "simulating" here. As transactions (they lead to a state-change) are processed by the nodes, what callStatic does is it tells us the nodes to "pretend" that the call is not state-changing. Consequently, they will return the result accordingly which does not lead to a change in state. Thus, it is very useful in this case, accomplishing our goal of "simulating" checkUpkeep (i.e. check all the conditionals in there) whilst not changing the state.