Js: The Weird Parts

console.log(typeof NaN); // "number" Yes. The representation of "Not a Number" is technically a number. And as we saw, it refuses to be friends with anyone, including itself. The only reliable way to check for NaN is:

console.log(isNaN(NaN)); // true // But wait... console.log(isNaN("hello")); // true (because "hello" can't be a number) That’s why ES6 gave us Number.isNaN() which actually behaves. In many languages, if you forget to declare a variable, you get an error. In JavaScript (non-strict mode), you get a present : js the weird parts

Welcome to the weird parts. Let’s start with the most infamous party trick. console

You are not alone. JavaScript is the quirky, misunderstood genius of the programming world. It was built in 10 days, it drives the modern web, and it has a list of "features" that look more like bugs. The only reliable way to check for NaN is: console

console.log([] + []); // "" (empty string) console.log([] + {}); // "[object Object]" console.log({} + []); // 0 (wait, WHAT?) The last one is a parsing quirk. In some engines, {} at the start of a line is treated as an empty block, not an object. So {} + [] becomes + [] which coerces to 0. Never, ever trust == . It’s like asking a toddler if two things are the same.

const arr = [1, 2, 3]; arr["foo"] = "bar"; console.log(arr); // [1, 2, 3, foo: "bar"] console.log(arr.length); // 3 (still!) The length property only counts numeric indices. The string key "foo" is there, but the array pretends it isn’t. JavaScript tries to be "helpful" with Automatic Semicolon Insertion (ASI). But sometimes, it helps too much.

And arrow functions? They don’t have their own this at all—they inherit from the surrounding scope. Arrays in JS are just objects with numeric keys and a special length property. That means you can do... questionable things.