A simple phantom wallet base58 private key text to json convertor.

solana-secretbase58-to-json.js

/* 
  "dependencies": {
    "@solana/web3.js": "^1.95.4",
    "bip39": "^3.1.0",
    "bs58": "^6.0.0"
  }
*/
const bs58 = require("bs58");
const { Keypair } = require("@solana/web3.js");

const privateKey = process.env.KEY || "";

const keypairData = Keypair.fromSeed(
  Uint8Array.from(bs58.default.decode(privateKey).slice(0, 32))
);

console.log("Secret Key:", keypairData.secretKey.toString());

// writ eto file
const fs = require("fs");
fs.writeFileSync(
  "keypair.json",
  JSON.stringify(Array.from(keypairData.secretKey))
);

// to mnemonics
const bip39 = require("bip39");

const mnemonic = bip39.entropyToMnemonic(
  bs58.default.decode(privateKey).slice(0, 32)
);

console.log("Mnemonic:", mnemonic);