Need someone explain the execution order of transferFrom in ERC20

[arbitary]

I was looking at the implementation of transferFrom in ERC20.sol. I noticed that the _transfer function is called before allowance check: require(currentAllowance >= amount, "ERC20: transfer amount exceeds allowance");. Shouldn’t we check the allowance before calling the _transfer function. How does the execution order work?

     _transfer(sender, recipient, amount);

     uint256 currentAllowance = _allowances[sender][_msgSender()];
     require(currentAllowance >= amount, "ERC20: transfer amount exceeds allowance");
     _approve(sender, _msgSender(), currentAllowance - amount);

     return true;
 }

[waqas_wahid]

@arbitary

_approve(sender, _msgSender(), currentAllowance - amount);

you should reverse _approve and _transfer function in the contract.