vm.prank( ) cannot understand .

[Debangshu97]

Can someone explain the use of vm.prank with an example.

[pacelliv]

Before speaking about prank, let me explain a little bit about what I've learned about Foundry.

In foundry everything is a conctract -- scripts, tests, libraries, imports, etc..., due to this reason Foundry has set up a serie of default addresses that acts as the msg.sender to execute these files and the functions in them.

When we execute a test, is not our private key that is being used, in fact is a key from Foundry.

function testConsoleCaller() public view {
    console.log(msg.sender); // is not you!
}

This behaviour could be problematic depending on the test, for example the withdraw function from the FundMe contract has a modifier that only allows the owner to withdraw the funds, but by default the msg.sender in the test file is a default key from Foundry not you.

function testCanWithdrawTheFund() public {
    fundMe.withdraw(); // the test fails because the default address that execute the file is not the owner
}

And for cases like this one prank comes into play. prank allows us to set the msg.sender for the next call.

function testCanWithdrawTheFund() public {
    vm.prank(USER); // your key will be used to withdraw the funds
    fundMe.withdraw();
}

prank only works one time with the first transaction it sees:

function testCanFundAndWithdraw() public {
    vm.prank(USER); // your key will be used to fund the contract
    fundMe.fund{value: 1 ether}(); // prank only works in this transaction and then it's over
    fundMe.withdraw(); // the `msg.sender` is again the key from Foundry
}

If you want to bundle transactions use startPrank and stopPrank:

function testCanWithdrawTheFund() public {
    vm.startPrank(USER); // your key will be used for all the next transactions until you call `stopPrank`
    fundMe.fund{value: 1 ether}();
    fundMe.withdraw();
    vm.stopPrank();
}

This is a recopilation of the behaviours I have noticed from prank.

[cromewar]

Amazing answer @pacelliv

[Debangshu97]

thanks man!