Operators and Conditionals

Operators combine or compare values into a new result. You already know arithmetic operators (+, -, *, /). Two other families matter just as much:

Comparison operators produce a boolean: === (strictly equal), !== (strictly not equal), <, >, <=, >=. Always prefer ===/!== over ==/!= — the loose versions silently convert types before comparing ("5" == 5 is true), which causes subtle bugs. The strict versions require both type and value to match.

Logical operators combine booleans: && (and — both sides must be true), || (or — at least one side must be true), ! (not — flips a boolean).

const age = 20;
const hasTicket = true;
const canEnter = age >= 18 && hasTicket;

Conditionals let your program take different paths depending on a condition:

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

Only the first matching branch runs. else if chains are checked top to bottom, and else catches everything else.

JavaScript also has the idea of truthy and falsy values: outside of actual booleans, values are automatically treated as true or false in a boolean context (like an if condition). Exactly these values are falsy: false, 0, "" (empty string), null, undefined, and NaN. Everything else — including "0" (a non-empty string!) and empty arrays/objects — is truthy. This is why if (username) is a common, convenient way to check "is this non-empty," but it can also trip you up if a legitimate value like the number 0 is falsy in a context where you meant something else.