Porting the Backup Check

Porting the Backup Check

In this lesson, you ported the sysadmin’s Python check to JavaScript, line by line, keeping its logic and changing its declarations and types. Then you ran both versions three ways and compared their output and exit status.

• In Node, process.env plays the part of os.environ, and it needs no import. You read each setting as a property, with a dot, not with a get() call. An unset variable is undefined, not None, and every set one is a string.
• Each read becomes a declaration with a camelCase name. A setting the check never changes is a const. The region is reassigned further down, when its default is filled in, so it’s a let.
• The flag compares with === "1", so dryRun holds true or false. DRY_RUN=false is the text "false", which isn’t "1", so the check runs live.
• The placeholder lastSuccess is a let, because the job reassigns it when a backup finishes, and it starts as null, where Python uses None.
• Reading process.env.BACKUP_RETRIES is a plain property read, with nowhere to pass a default the way you pass one to os.environ.get(). So the port fills in the retry count’s default of "3" the way it fills in the region’s, then converts the text once with Number(retriesText). Without the conversion, retries + 1 would join text and promise 51 attempts.
• Unlike int(), Number() never raises: it gives NaN for text that isn’t a number, such as "abc". The port doesn’t check for that, so it runs on and exits with status 0, where Python’s int() stops the check with status 1.
• is None becomes === undefined. A === null check would wave an unset bucket through.
• sys.exit(1) becomes process.exit(1), which ends the program with an exit code, as sys.exit() does. An exit code of 0 means success and anything else means failure, which is what cron checks, so cron sees the failure.
• The mode is declared with let mode; before its if/else, so the print lines can reach it. A const inside each branch would end at its closing brace.
• node --check backup-check.js reads the whole file without running it, and prints nothing when there’s no syntax error, such as a second let for the same name.

Here’s the port, backup-check.js, without the warning and the debug line:

const bucket = process.env.BACKUP_BUCKET;
let region = process.env.BACKUP_REGION;
const notifyEmail = process.env.NOTIFY_EMAIL;
const dryRun = process.env.DRY_RUN === "1";
let retriesText = process.env.BACKUP_RETRIES;
if (retriesText === undefined) {
  retriesText = "3";
}
const retries = Number(retriesText);
let lastSuccess = null;

if (bucket === undefined) {
  console.log("No BACKUP_BUCKET set: refusing to start.");
  process.exit(1);
}

if (region === undefined) {
  region = "eu-west-1";
}

let mode;
if (dryRun) {
  mode = "dry run";
} else {
  mode = "live";
}

console.log(`Backing up to ${bucket} in ${region} (${mode})`);
console.log(`Up to ${retries + 1} attempts`);