Asking a Value's Type With typeof

Asking a Value’s Type With typeof

In this lesson, you ported two lines of the backup check that ask about a value’s type: the debug line, and the check that makes sure the settings file’s recipients are a list, not one address typed as a lone string.

• JavaScript has no type() function. You write typeof in front of a value instead. It’s an operator rather than a function, like Python’s not, so it needs no parentheses.
• Python’s type() gives you the class, so the debug line asks for its .__name__. typeof skips the class and answers with the name as a string, and the names are JavaScript’s own: string where Python says str.
• The type names: int and float are both "number", so typeof 2.5 is "number" too. str is "string", and bool is "boolean", whose value is lowercase true. Where Python has None, JavaScript has undefined and null. Two more, bigint and symbol, complete the list of primitive types.
• To check a type, compare the answer with the name in quotes: typeof retries === "number" is true, as isinstance(retries, int) is True in Python. Python writes a type bare, but typeof’s answer is a string: without the quotes, number is an unknown name, and JavaScript stops with a reference error, its version of a NameError.
• A primitive is a basic value rather than an object, where in Python even 3 is an int object. JavaScript lends a number methods such as .toFixed() when you call one.
• typeof has an answer for each primitive type, and says "object" for almost everything else, arrays included. So typeof job.recipients is "object", for an array as for a plain object like job, and the "array" check never passes. Functions get an answer of their own, "function".
• typeof null is "object" too, a long-standing quirk, so a check for "object" lets null through.
• To ask whether a value is an array, use Array.isArray(), a method of JavaScript’s built-in Array. For the recipients, it says true, as isinstance() does in Python.
• typeof is like type() only partway: type() and isinstance() work with any class, your own included, while typeof only picks a name from its fixed list.

Here’s the debug line in the port, with the settings held in constants for now:

const bucket = "kestrel-db-backups";
const retries = 3;
const dryRun = true;

console.log(`Debug: bucket is ${typeof bucket}`);