Testing in Web3

Hardhat and Truffle can help you out with. They can create a testing network on localhost, then generate a bunch of wallet addresses loaded up with fake crypto that you can use for testing in the browser. You can connect MetaMask to your testing network instead of operating on the actual Ethereum network.

In addition if you're building a serious app you should consider writing automated tests for it. There's a library called Waffle that can help you do that.