Checking for Empty Values

Checking for Empty Values

In this lesson, you ported the two checks that warn when nobody will hear about a failed backup: the one for an empty NOTIFY_EMAIL, and the one for an empty recipients list in the new server’s settings file.

• Where Python writes not, JavaScript writes ! in front of the value. Like not, it answers true or false, in lowercase. Inside the if, console.log() does print()’s job.
• For strings, null, and undefined, truthiness carries over: an empty string and undefined count as false, as an empty string and None do in Python. So the notify check warns with NOTIFY_EMAIL empty or unset, as the Python does, and the message’s “empty” covers unset too.
• Values that count as false in a condition are falsy, and every other value is truthy. MDN’s Falsy page keeps the full list.
• JavaScript’s falsy values are false, 0, the empty string "", null and undefined, and NaN, the “not a number” value you get by adding one to a missing retry count. Two rare ones also count: negative zero (-0) and the zero of bigint (0n), a type for huge whole numbers you won’t need here.
• Any text that isn’t empty is truthy, even "false", in both languages.
• On JavaScript’s falsy values, Python agrees on all but one: its float("nan") is truthy, but NaN is falsy in JavaScript. Python’s own falsy values also include its empty containers, such as an empty list or dict, and that’s where the two languages part.
• An array is an object in JavaScript, empty or not, and every object is truthy. Python asks a list or a dict whether it’s empty, but JavaScript’s falsy list is a fixed set of primitive values, plain values like strings and numbers, and it never looks inside an object. So !job.recipients is false for an empty array, and the line-by-line port of if not job["recipients"]: prints nothing.
• Ask the question you mean instead: an array’s .length counts its items, like Python’s len(), so job.recipients.length === 0 is true for an empty array. Where Python’s habit is if not, a JavaScript developer checks the length.
• An empty object is truthy too, and its .length is undefined, so checking it never warns either. Count its keys instead, as len() does for a dict: Object.keys(obj).length === 0, where Object.keys() gives you an array of an object’s keys, and you check the array’s length is zero. Objects get a course of their own later on.
• The rule: for a string or a missing value, if (!notifyEmail) works as Python’s if not notify_email: does. For an array, check its length: job.recipients.length === 0. For an object, count its keys: Object.keys(obj).length === 0.

Here’s the notify check in the port:

const notifyEmail = process.env.NOTIFY_EMAIL;

if (!notifyEmail) {
  console.log(
    "Warning: NOTIFY_EMAIL is empty,",
    "so nobody hears about failures.",
  );
}