switch, the ternary operator and lookup tables

switch: one value, many cases

When you compare one value against many fixed options, a long if / else if chain repeats the same variable again and again. switch reads the value once and jumps to the matching case. It compares with strict equality (===).

function statusMessage(status) {
  switch (status) {
    case 'PLACED':
      return 'We have your order';
    case 'SHIPPED':
      return 'On the way';
    case 'DELIVERED':
      return 'Delivered. Enjoy!';
    default:
      return 'Unknown status';
  }
}
console.log(statusMessage('SHIPPED')); // On the way

break and fall-through

Inside a function, return ends the switch. Otherwise you must write break at the end of each case. Without break, JavaScript keeps running the next case's code too. This is called fall-through. It is sometimes used on purpose to group cases.

const day = 'Sat';
let kind;
switch (day) {
  case 'Sat':
  case 'Sun':
    kind = 'weekend';   // both cases share this code on purpose
    break;
  default:
    kind = 'weekday';
}
console.log(kind); // weekend

Because switch uses ===, case '1' does not match the number 1. Convert input first.

The ternary operator

The ternary (conditional) operator is the only JavaScript operator with three parts: condition ? valueIfTrue : valueIfFalse. It is an expression, so it produces a value you can store or put inside a template literal. Use it for short either/or choices.

const age = 20;
const label = age >= 18 ? 'adult' : 'minor';
const items = 1;
console.log(`You have ${items} ${items === 1 ? 'item' : 'items'}`); // You have 1 item

|| versus ?? for defaults

Both give a fallback, but differently. a || b uses b whenever a is falsy (0, '', false…). a ?? b uses b only when a is null or undefined. Pick ?? when 0 or an empty string is a valid answer.

Lookup tables: data instead of branches

When each case just maps a key to a value, store the mapping in an object and read from it. Adding a new case means adding a line of data, not a new branch of code.

const SHIPPING_DAYS = { metro: 2, city: 4, rural: 7 };
function deliveryDays(area) {
  return SHIPPING_DAYS[area] ?? 10;  // 10 for unknown areas
}
console.log(deliveryDays('city'));   // 4
console.log(deliveryDays('island')); // 10