Convert Solana Private Key to base58 Keypair Array

solana-privatekey-to-keypair-file.js

// npm install bs58
import bs58 from 'bs58';
import fs from 'fs';
import path from 'path';
// Replace with the private key string that was exported from your solana wallet provider (phantom, etc)
const privateKey = "abc123456def";
// bs58.decode will return a ByteArray.
const keypairFileData = bs58.decode(privateKey);
fs.writeFileSync(path.join(__dirname, 'new-keypair.json'), JSON.stringify(keypairFileData));