This selection of tutorials will guide you on how to build smart contracts that interacts with the most popular decentralized finance (DeFi) protocols.
ERC20 Tokens
Understand the ERC20 token smart contract
Interact with ERC20 tokens with your Solidity smart contract: building a simple DEX/ICO
Reading the balance and making transfers of ERC20 tokens in JS
Reading Chainlink Oracles for currency prices using JavaScript.
Decentralized exchanges
Trading and Arbitrage on Ethereum DEXes: Get the rates for a token swap (part 1)
Making flash loans to access Ether or ERC20 inside of your transactions
Testing
Fork the Ethereum mainnet to test your code with live protocols
Data
Using The Graph to query indexed Ethereum application data
NFTs
Using NFT20 developer library to buy/sell NFTs programaticaly