Google Cloud Function

In GCP (not firebase) I have a bucket and a function which will be called when a new file was created in the bucket. Works great.

/**
 * Triggered from a change to a Cloud Storage bucket.
 */
exports.mycompany_upload_file = (event, context) => {
    const gcsEvent = event;
    const filename = gcsEvent.name;
    const bucketName = event.bucket;
    console.log(`=====================================================`);
    console.log(`Event Type: ${context.eventType}`);
    console.log(`Bucket:     ${bucketName}`);
    console.log(`Datei:      ${filename}`);
    console.log(`=====================================================`);

    // now I would like to open that file and read it line-by-line 

How do I address that file? What's the path of that file? Can I use standard node libraries like 'fs'