Conditionals

JavaScript · Reference cheat sheet

📋 Overview

Conditionals branch control flow with if / else if / else, switch, and short-circuit logic. Prefer clear boolean expressions, early returns, and exhaustive switches for unions. Truthiness rules matter — know what counts as falsy.

🔧 Core concepts

• Falsy: false, 0, -0, 0n, "", null, undefined, NaN.
• Truthy: everything else (including [], \{\}, "0").
• if (expr): coerces with ToBoolean.
• else if / else: chained alternatives.
• switch (x): strict equality (===) match; remember break.
• Guards: early return / throw to flatten nesting.

if (user?.isAdmin) {
  allow();
} else if (user) {
  deny();
} else {
  login();
}

💡 Examples

// Early return style
function canEdit(doc, user) {
  if (!user) return false;
  if (doc.locked) return false;
  return doc.ownerId === user.id || user.role === "admin";
}

// switch with fall-through (intentional)
switch (code) {
  case 200:
  case 201:
    return "ok";
  case 404:
    return "missing";
  default:
    return "error";
}

// switch (true) pattern
switch (true) {
  case score >= 90:
    return "A";
  case score >= 80:
    return "B";
  default:
    return "C";
}

// Short-circuit
isReady && start();
value || fallback; // prefer ?? for nullish-only
exists ? use(exists) : create();

// Numeric range
if (n >= 1 && n <= 10) {
  /* ... */
}

// Nullish check without truthiness
if (count != null) {
  // allows 0
}

⚠️ Pitfalls

• if (arr) is always true for empty arrays — check arr.length.
• Assignment in condition if (x = y) is legal and often a bug — use ===.
• switch without break falls through — use break or return.
• switch uses === — no coercion between "1" and 1.
• Deep nesting hurts readability — extract functions / use lookup maps.

🔗 Related

• ternary.md — conditional expressions
• boolean.md — booleans
• equality.md — === vs ==
• switch_case.md — switch deep dive
• nullish_coalescing.md — defaults