Operators and conditionals

if (age >= 18) {
  console.log("Adult");
} else if (age >= 13) {
  console.log("Teenager");
} else {
  console.log("Child");
}

Use === and !== (strict equality), not ==/!=. The strict versions don't silently convert types, which avoids a whole category of confusing bugs:

0 == false     // true  -- surprising
0 === false    // false -- correct, different types

The ternary operator is a compact if/else for simple cases:

const status = age >= 18 ? "adult" : "minor";

Truthy and falsy values

An if condition doesn't have to be a strict boolean -- JavaScript converts whatever you give it to true or false. Only a handful of values are "falsy": false, 0, "" (empty string), null, undefined, and NaN. Everything else -- including "0" as a string, and any non-empty string or object -- is truthy:

if ("") {
  console.log("won't run -- empty string is falsy");
}
if ("0") {
  console.log("this runs -- non-empty string is truthy, even '0'");
}
if ([]) {
  console.log("this runs too -- even an empty array is truthy");
}

This is why you'll often see a simple existence check written as if (user) { ... } rather than if (user !== null && user !== undefined) { ... }.

Short-circuiting with && and ||

const name = userName || "Guest";        // fallback if userName is falsy
isLoggedIn && console.log("Welcome back"); // only runs if isLoggedIn is truthy

|| returns its first truthy value, which makes it a common (if slightly old-fashioned) way to supply a default. Newer code often uses ?? (nullish coalescing) instead, which only falls back on null/undefined rather than every falsy value -- important if 0 or "" are valid values you don't want overridden:

const count = quantity ?? 1; // only falls back if quantity is null/undefined
// with ||, a quantity of 0 would incorrectly fall back to 1

switch: an alternative to long if/elif chains

switch (status) {
  case "active":
    console.log("Account is active");
    break;
  case "suspended":
    console.log("Account is suspended");
    break;
  default:
    console.log("Unknown status");
}

Each case needs its own break, or execution "falls through" into the next case -- usually not what you want, and a common source of bugs when a break gets forgotten.