if, else, and switch

Programs rarely follow a single path. Control flow statements let your code respond differently based on conditions.

if runs code when a condition is true

The simplest conditional:

const temperature = 32;

if (temperature <= 32) {
  console.log("Water freezes at this temperature.");
}

The expression inside the parentheses is evaluated. If it is truthy, the block runs. If it is falsy, the block is skipped.

else provides an alternative

const age = 15;

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

Exactly one branch runs. There is no third option.

else if chains multiple conditions

const score = 75;

if (score >= 90) {
  console.log("A");
} else if (score >= 80) {
  console.log("B");
} else if (score >= 70) {
  console.log("C");
} else {
  console.log("Below C");
}

Conditions are checked top to bottom. The first true condition runs and the rest are skipped. The else at the end catches everything that did not match.

The order matters. Reversing the conditions would produce wrong results:

// Wrong order — everything matches the first condition
if (score >= 70) {
  console.log("C");      // catches 75, 85, 95 — all wrong
} else if (score >= 90) {
  console.log("A");      // never reached
}

Ternary operator for simple choices

When you need a value (not just a statement), use the ternary operator:

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

This is an expression — it produces a value you can assign, return, or pass to a function. Use it for simple two-way choices. Nesting ternaries makes code hard to read:

// Avoid this
const label = score >= 90 ? "A" : score >= 80 ? "B" : "C";

Prefer if/else if/else when there are more than two branches.

switch for multi-way branching

When you compare one value against many possibilities, switch is clearer than a long else if chain:

const day = "monday";

switch (day) {
  case "monday":
  case "tuesday":
  case "wednesday":
  case "thursday":
  case "friday":
    console.log("Weekday");
    break;
  case "saturday":
  case "sunday":
    console.log("Weekend");
    break;
  default:
    console.log("Invalid day");
}

Each case tests the value with strict equality (===). The break statement exits the switch block. Without it, execution falls through to the next case.

Fall-through is intentional

Missing break is one of the most common switch bugs:

switch (value) {
  case 1:
    console.log("one");
    // missing break — falls through to case 2
  case 2:
    console.log("two");
    break;
}
// If value is 1, this prints both "one" and "two"

Fall-through can be useful when multiple cases share behavior (as in the weekday example above), but it must be intentional and clear.

When to use if vs switch

Use if/else when:

• conditions are complex expressions, not simple equality
• you need to compare different variables
• you need range checks (>, <)

Use switch when:

• you are comparing one value against several known values
• multiple cases share the same behavior (fall-through)
• the list of cases might grow (easier to scan than else if)

Pattern matching with objects

For more complex branching, some code uses an object as a lookup table:

const handlers = {
  mon: () => "Monday",
  tue: () => "Tuesday",
  wed: () => "Wednesday",
};

const getDay = (abbr) => {
  const handler = handlers[abbr];
  return handler ? handler() : "Unknown";
};

getDay("mon");  // "Monday"

This works well when every case maps to a result. It does not replace if for conditions that require logic or comparisons.

What to carry forward

• if runs a block when a condition is truthy
• else if chains multiple conditions, checked top to bottom
• else catches everything that did not match
• ternary (? :) is an expression — use it when you need a value
• switch compares one value against many possibilities
• always use break in switch cases unless fall-through is intentional
• choose if for complex conditions and switch for known value matching

Control flow determines which code runs. The next lesson covers how to repeat code with loops.