Put a signature in the output so that this output cannot be copied to other tx

OP_CODESEPARATOR.js

const { expect } = require('chai');
const { bsv, signTx, toHex } = require('scryptlib');
const crypto = require('crypto');
/**
 * an example test for contract containing signature verification
 */
const {  inputSatoshis,  dummyTxId } = require('../../helper');

const privateKey = bsv.PrivateKey.fromRandom('testnet')


describe('OP_CHECKSIG with OP_CODESEPARATOR', () => {
  
  it('Put a signature in the output so that this output cannot be copied to other tx', () => {

    const tx = new bsv.Transaction().from({
      txId: dummyTxId,
      outputIndex: 0,
      script: '',   // placeholder
      satoshis: inputSatoshis
    });

    const script = new bsv.Script("OP_CHECKSIGVERIFY");
    sig = signTx(tx, privateKey, script, inputSatoshis)  // if sign with fake key, it will fails
    const lockingScript = new bsv.Script();

    lockingScript.add(Buffer.from(sig, "hex"))
          .add(Buffer.from(toHex(privateKey.publicKey), "hex"))
          .add(bsv.Opcode.OP_CODESEPARATOR)
          .add(bsv.Opcode.OP_CHECKSIGVERIFY)
    
    // set ScriptPubKey
    tx.inputs[0].output.script.add(lockingScript)
    // set ScriptSig
    tx.inputs[0].script.add(bsv.Opcode.OP_TRUE)

    let result = tx.verifyInputScript(0); // should verify true
    expect(result.success,  result.error).to.be.true

    const faketx = new bsv.Transaction().from({
      txId: crypto.randomBytes(32).toString('hex'), //with a different txId
      outputIndex: 0,
      script: '',   // placeholder
      satoshis: inputSatoshis
    });

    // same ScriptPubKey
    faketx.inputs[0].output.script.add(lockingScript)
    // same ScriptSig
    faketx.inputs[0].script.add(bsv.Opcode.OP_TRUE)

    result = faketx.verifyInputScript(0); // should verify true
    expect(result.success, result.error).to.be.false

  });

});