Conditions & Switches

const value: number = 10;

if (value > 10) {
  console.log("big");
} else if (value <= 10 && value !== 0) {
  console.log("medium");
} else {
  console.log("small");
}

const result = value > 1 ? "amazing" : "not amazing";

const name = input ?? "default";    // null or undefined only
const label = input || "fallback";  // any falsy value

function process(value: string | number) {
  if (typeof value === "string") {
    // value: string
    console.log(value.toUpperCase());
  } else {
    // value: number
    console.log(value.toFixed(2));
  }
}

if (error instanceof TypeError) { ... }

if ("name" in obj) {
  console.log(obj.name);  // narrowed
}

type Result =
  | { status: "ok"; data: string }
  | { status: "error"; message: string };

function handle(result: Result) {
  switch (result.status) {
    case "ok":
      console.log(result.data);    // narrowed to ok
      break;
    case "error":
      console.log(result.message); // narrowed to error
      break;
  }
}

switch (command) {
  case "start":
    start();
    break;
  case "stop":
  case "quit":
    stop();
    break;
  default:
    unknown();
}