Control Flow

JavaScript · 9 entries

if / else if / else

syntax

if (condition) { ... }
else if (condition) { ... }
else { ... }

example

function getDiscount(memberLevel) {
  if (memberLevel === "gold") {
    return 0.20;
  } else if (memberLevel === "silver") {
    return 0.10;
  } else {
    return 0;
  }
}
console.log(getDiscount("gold")); // 0.20

Note Conditions are coerced to boolean. Watch out for truthy/falsy gotchas: if (arr.length) works because 0 is falsy.

if elseconditionalbranchingif statement

Ternary Operator

syntax

condition ? valueIfTrue : valueIfFalse

example

const age = 20;
const category = age >= 18 ? "adult" : "minor";
console.log(category); // "adult"

// Nested (use sparingly)
const tier = age >= 65 ? "senior" : age >= 18 ? "adult" : "minor";

Note Great for simple inline conditions. Avoid deeply nested ternaries -- they quickly become unreadable. Use if/else for complex logic.

ternaryinline ifconditional expression? : operator

switch Statement

syntax

switch (expression) {
  case value: ... break;
  default: ...
}

example

function getStatusText(code) {
  switch (code) {
    case 200: return "OK";
    case 301: return "Moved Permanently";
    case 404: return "Not Found";
    case 500: return "Internal Server Error";
    default:  return `Unknown (${code})`;
  }
}
console.log(getStatusText(404));

output

"Not Found"

Note Uses strict equality (===). Forgetting break causes fall-through to the next case. Using return instead of break is a clean pattern in functions.

switch caseswitch statementmultiple conditionsselect case

for Loop

syntax

for (init; condition; update) { ... }

example

const items = ["apple", "banana", "cherry"];
for (let i = 0; i < items.length; i++) {
  console.log(`${i + 1}. ${items[i]}`);
}

output

"1. apple"
"2. banana"
"3. cherry"

Note Classic loop when you need the index. For simple iteration, prefer for...of. Cache .length in the initializer if the array is very large and not changing.

for looploop with indexiterate with counterclassic loop

for...of Loop

syntax

for (const element of iterable) { ... }

example

const scores = [88, 92, 75, 100];
let sum = 0;
for (const score of scores) {
  sum += score;
}
console.log(`Average: ${sum / scores.length}`);

// Works with strings too
for (const char of "hello") {
  process(char);
}

output

"Average: 88.75"

Note Works on any iterable: arrays, strings, Maps, Sets, generators. Does NOT work on plain objects -- use Object.entries() or for...in for those.

for of loopiterate arrayloop iterablefor of

for...in Loop

syntax

for (const key in object) { ... }

example

const theme = { primary: "#3498db", secondary: "#2ecc71", accent: "#e74c3c" };
for (const colorName in theme) {
  console.log(`${colorName}: ${theme[colorName]}`);
}

output

"primary: #3498db"
"secondary: #2ecc71"
"accent: #e74c3c"

Note Iterates over enumerable string properties, including inherited ones. Avoid for arrays (use for...of). Use Object.hasOwn() to filter inherited keys.

for in looploop object keysiterate propertiesfor in

while and do...while

syntax

while (condition) { ... }
do { ... } while (condition);

example

// Retry until success (with limit)
let attempts = 0;
let success = false;
while (!success && attempts < 5) {
  attempts++;
  success = Math.random() > 0.7;
}
console.log(`Succeeded after ${attempts} attempt(s): ${success}`);

// do...while always runs at least once
let input;
do {
  input = prompt("Enter a number > 10:");
} while (Number(input) <= 10);

Note do...while guarantees at least one iteration. Useful for retry logic and input validation loops.

while loopdo whileloop until conditionrepeat until

break and continue

syntax

break;     // exit loop
continue;  // skip to next iteration

example

const data = [3, -1, 7, 0, 5, -3, 9];
const positives = [];

for (const n of data) {
  if (n < 0) continue;  // skip negatives
  if (positives.length >= 3) break;  // stop after 3
  positives.push(n);
}
console.log(positives); // [3, 7, 5]

Note break exits the innermost loop. Use labeled breaks to exit outer loops: outer: for (...) { for (...) { break outer; } }

break loopcontinue loopskip iterationexit loop early

Labeled Statements

syntax

label: for (...) { break label; }

example

const matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9],
];

let found = null;
search: for (const row of matrix) {
  for (const cell of row) {
    if (cell === 5) {
      found = cell;
      break search;
    }
  }
}
console.log(found); // 5

Note Labels let you break/continue a specific outer loop from inside a nested loop. Rarely needed but invaluable for searching nested structures.

labeled statementbreak outer loopnested loop breaklabel break