Declaring Names Once With const and let

Declaring Names Once With const and let

In this lesson, you ported the top of the backup check to JavaScript and found that JavaScript declares each name once, then assigns it.

• In Python, a name exists as soon as you assign to it, and assigning again points it at a new value.
• The port is backup-check.js in the folder backup-check/, whose package.json holds { "type": "module" }, so the file runs as a module, in strict mode.
• Like Python, JavaScript reads the whole file before it runs any of it. A let declares a name, and a name can be declared only once, so a second let for it is a SyntaxError that stops every line, even the ones above it.
• JavaScript keeps declaring and assigning apart: you declare a name once, with const or let, then assign it new values with no keyword.
• Assigning to a name that was never declared stops a module with a ReferenceError when that line runs, where Python would create the name. That’s strict mode at work.
• += works as in Python. warnings += 1; is short for warnings = warnings + 1;, so the name gets a new value.
• That reassignment is why warnings stays a let in the finished port below, even though the lines that add to it come later.
• Start every name as const, and use let only when the code really reassigns it. The keyword is a promise to the reader: the lets show at a glance which names change.
• Python marks a constant with an UPPER_CASE name, but nothing at runtime stops a reassignment. A const is enforced.
• Watch out: unlike the second let, which stopped the whole file, a const that’s reassigned gives a TypeError only when that line runs.
• Watch out: a const fixes what the name points at, not the contents, so an array or object in it can still change, as a Python list can.
• In older code, you’ll meet var, an earlier way to declare names. Write let and const instead.

Here’s the line-by-line port, where the second status line got its own let:

let bucket = "kestrel-db-backups";
let region = "eu-west-1";
let notifyEmail = "[email protected]";
let warnings = 0;

let status = "checking settings";
console.log(`Status: ${status}`);

let status = "ready to back up";
console.log(`Status: ${status}`);