NEAR Protocol - Sign TX as another account using an access key added to that account

sign-for-user.js

import * as nearAPI from 'near-api-js';

const {
	Account,
  KeyPair,
} = nearAPI;

// near is your current near connection instance (somewhere in your app init)

export const getAccessKeyAccount = (near, accountId, secretKey) => {
    const keyPair = KeyPair.fromString(secretKey);
	  near.connection.signer.keyStore.setKey(near.config.networkId, accountId, keyPair);
	  const account = new Account(near.connection, accountId);
	  return account;
};