JavaScript Control Flow¶

Conditional statements, comparison operators, logical operators, and loops for controlling program execution.

Conditional Statements¶

if (condition) {
  // truthy
} else if (other) {
  // second check
} else {
  // fallback
}

switch¶

switch (role) {
  case "admin":  console.log("Full access"); break;
  case "editor": console.log("Edit access"); break;
  default:       console.log("Unknown");
}

Uses strict equality (===). Fall-through without break is sometimes intentional for grouping.

Ternary¶

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

Comparison Operators¶

5 === "5"    // false (strict - no coercion)
5 == "5"     // true  (loose - type coercion!)
5 !== "5"    // true  (strict inequality)

Always use === and !==. Loose equality surprises:

0 == false       // true
"" == false      // true
null == undefined // true
null == 0        // false (special case!)
NaN == NaN       // false

Logical Operators¶

// AND: returns first falsy, or last if all truthy
"hello" && 42       // 42
0 && "hello"        // 0

// OR: returns first truthy, or last if all falsy
false || "hello"    // "hello"
0 || "" || null     // null

// NOT
!true               // false
!!value             // Convert to boolean

// Nullish Coalescing: right side only if left is null/undefined
null ?? "default"   // "default"
0 ?? "default"      // 0 (0 is NOT null/undefined!)
"" ?? "default"     // "" (empty string is NOT null/undefined!)

|| vs ??: || replaces ANY falsy value. ?? only replaces null/undefined. Use ?? when 0 or "" are valid.

Short-circuit¶

isLoggedIn && showDashboard();         // Conditional execution
const name = input || "Anonymous";      // Default for falsy
const port = config.port ?? 3000;       // Default for null/undefined

Optional Chaining¶

const city = user?.address?.city;       // undefined if any part null/undefined
const first = arr?.[0];                  // Safe array access
const result = obj?.method?.();          // Safe method call

Loops¶

for¶

for (let i = 0; i < 10; i++) { console.log(i); }

for...of (values - arrays, strings, iterables)¶

for (const fruit of ["apple", "banana"]) { console.log(fruit); }
for (const [i, v] of arr.entries()) { console.log(i, v); }
for (const char of "hello") { console.log(char); }

for...in (keys - objects)¶

for (const key in { name: "Alice", age: 25 }) { console.log(key); }

Don't use for...in on arrays - iterates all enumerable properties, order not guaranteed.

while / do...while¶

while (condition) { /* may never execute */ }
do { /* executes at least once */ } while (condition);

break / continue¶

for (let i = 0; i < 100; i++) {
  if (i === 5) break;      // Exit loop
  if (i === 2) continue;   // Skip iteration
}

// Labeled break for nested loops
outer: for (let i = 0; i < 3; i++) {
  for (let j = 0; j < 3; j++) {
    if (i === 1 && j === 1) break outer;
  }
}

Gotchas¶

== coercion traps: "" == false is true, null == 0 is false - use ===
|| replacing valid 0: count || 10 returns 10 when count is 0; use count ?? 10
for...in on arrays: includes inherited properties, use for...of instead
Switch without break: fall-through to next case silently
Optional chaining + nullish: user?.name ?? "Anonymous" combines both safely