Ref - https://solana.com/docs/core/pda

Video - https://www.youtube.com/watch?v=p0eD29d8JCM

Program Derived Addresses (PDAs) provide developers on Solana with two main use cases:

• Deterministic Account Addresses: PDAs provide a mechanism to deterministically derive an address using a combination of optional "seeds" (predefined inputs) and a specific program ID.
• Enable Program Signing: The Solana runtime enables programs to "sign" for PDAs which are derived from its program ID.

Properties

• PDAs are addresses derived deterministically using
  • a combination of user-defined seeds
  • a bump seed
  • and a program's ID.
• PDAs are addresses that fall off the Ed25519 curve and have no corresponding private key.
• Solana programs can programmatically "sign" for PDAs that are derived using its program ID.
• Deriving a PDA does not automatically create an on-chain account.
• An account using a PDA as its address must be explicitly created through a dedicated instruction within a Solana program.

Find the associated token account for a user and a mint

const { PublicKey } = require('@solana/web3.js');
const { ASSOCIATED_TOKEN_PROGRAM_ID, TOKEN_PROGRAM_ID } = require('@solana/spl-token');

// Replace these with your actual values
const userAddress = new PublicKey('5gjLjKtBhDxWL4nwGKprThQwyzzNZ7XNAVFcEtw3rD4i');
const tokenMintAddress = new PublicKey('6NeR2StEEb6CP75Gsd7ydbiAkabdriMdixPmC2U9hcJs');

// Derive the associated token address
const getAssociatedTokenAddress = (mintAddress, ownerAddress) => {
    return PublicKey.findProgramAddressSync(
        [
            ownerAddress.toBuffer(),
            TOKEN_PROGRAM_ID.toBuffer(),
            mintAddress.toBuffer(),
        ],
        ASSOCIATED_TOKEN_PROGRAM_ID
    );
};

const [associatedTokenAddress, bump] = getAssociatedTokenAddress(tokenMintAddress, userAddress);
console.log(`Associated Token Address: ${associatedTokenAddress.toBase58()}, bump: ${bump}`);

createProgramAddress vs findProgramAddress

const { PublicKey } = require('@solana/web3.js');
const { ASSOCIATED_TOKEN_PROGRAM_ID, TOKEN_PROGRAM_ID } = require('@solana/spl-token');

const PDA = PublicKey.createProgramAddressSync(
  [userAddress.toBuffer(), TOKEN_PROGRAM_ID.toBuffer(), tokenMintAddress.toBuffer(), Buffer.from([255])],
  ASSOCIATED_TOKEN_PROGRAM_ID,
);
 
console.log(`PDA: ${PDA}`);