Postman pre-request code that works with localhost server to include signed header in request

postman-signed-headers.js

// This script reqires a local server to be running at localhost:3000 
// to handle the cryptographic signature.  

// store your signing key in your enviroment
const signingKey = pm.environment.get("signing_key");

// javascript Date.now() returns unix timestamp in ms, convert to s
const timestamp = Math.floor(Date.now() / 1000)
const method = pm.request.method.toString();
// Postman does not substitute URL parameters before running this script
// this line performs the variable substitution
pm.collectionVariables.values.substitute(pm.request.url, null, true)
const path = '/api' + pm.request.url.getPath()

if (pm.request.body.raw) {
    // remove all spaces from body THIS IS REQUIRED
    pm.request.body.raw = pm.request.body.raw.replace(/\s/g, '')
    var message = `${timestamp}${method}${path}${pm.request.body.raw}`;
} else {
    var message = `${timestamp}${method}${path}`;
}

try {
    const response = await pm.sendRequest({
      url: "http://localhost:3000",
      method: "POST",
      header: {
        'Content-Type': 'application/json',
        'Accept': 'application/json'
      },
      body: {
        mode: 'raw',
        raw: JSON.stringify({
          m: message,
          pkey: signingKey,
        })
      }
    });

    var jsonResponse = response.json()
    pm.request.headers.add({
        key: 'x-timestamp',
        value: timestamp
    });
    pm.request.headers.add({
        key: 'x-signature',
        value: jsonResponse.s
    });

} catch (err) {
    if (err.code === 'ECONNREFUSED') {
        console.error(
            "Please ensure you have a signature server running on localhost:3000"
        )
    }
    console.error(err.message);
}