JavaScript: the language and the browser, the inPace way

Seven primitives and objects

string, number, bigint, boolean, undefined, symbol, null, and then objects (arrays, functions, dates, maps... are objects). Primitives are copied by value; objects by reference.

const a = { n: 1 }; const b = a; b.n = 2;
console.log(a.n);           // 2: same object
typeof null                 // "object" (a famous old bug)
typeof []                   // "object"; use Array.isArray

== vs ===

=== compares without converting types. == converts first, with surprising rules ("" == 0 is true). Use === everywhere; the one common exception is x == null, which checks for both null and undefined.

Truthy and falsy

Falsy: false, 0, -0, 0n, "", null, undefined, NaN. Everything else is truthy, including "0", [] and {}.

Modern helpers you'll see in app.js

const name = user?.profile?.name ?? "friend";   // optional chaining + nullish coalescing
const { x, y } = point;                          // destructuring
const all = [...list, item];                     // spread
const n = +localStorage.getItem("inpace-plan-at") || 0;   // unary + converts to a number

?? only falls back on null/undefined; || also falls back on 0 and "". Interviewers love that difference.