Js The Weird Parts [verified] -

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.

function oops() { accidentalGlobal = "I'm everywhere now"; } oops(); console.log(window.accidentalGlobal); // "I'm everywhere now" You didn't use var , let , or const ? Congratulations, you just polluted the global object. This is why we have linters and "use strict" . The this keyword is like a chameleon on caffeine. Its value depends entirely on how a function is called.

function showThis() { console.log(this); } showThis(); // window (or undefined in strict mode) js the weird parts

console.log(0 == false); // true console.log("" == false); // true console.log(null == undefined); // true console.log(NaN == NaN); // false (yes, NaN is not equal to itself) The rule is simple: . It compares both value and type. The == operator tries to be "helpful" by converting types behind your back. That "help" is the source of countless bugs. NaN : The Loneliest Number Speaking of NaN (Not a Number), it has a personality disorder.

If you’ve spent more than five minutes writing JavaScript, you’ve probably had a moment where you stared at your screen and whispered, “...why?” const arr = [1, 2, 3]; arr["foo"] = "bar"; console

console.log(1 + "1"); // "11" (string) console.log(1 - "1"); // 0 (number) Why? Because + is overloaded. If either operand is a string, it prefers string concatenation. But - doesn’t have a string version, so it coerces everything to numbers. Fun, right?

const obj = { showThis: showThis }; obj.showThis(); // obj But sometimes, it helps too much

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.