Control Flow & Loops

# Control Flow & Loops

if / else if / else

const score = 78;
if (score >= 90)       grade = "A";
else if (score >= 75)  grade = "B";
else if (score >= 60)  grade = "C";
else                   grade = "F";

Truthy / falsy

Conditions don't need to be booleans — they're coerced. The 6 falsy values:

false   0   ""   null   undefined   NaN

Everything else is truthy — including "0", "false", [], {}.

if (user) { … }           // user is set
if (arr.length) { … }     // not empty
if (!str) return;          // early-exit guard

Ternary — small inline if

const label = isAdmin ? "admin" : "user";

switch

switch (status) {
  case "open":
  case "in-review":            // fall-through — both share the body
    color = "blue";
    break;
  case "merged":
    color = "green";
    break;
  case "closed":
    color = "red";
    break;
  default:
    color = "gray";
}

• Uses strict equality (===) for matching.
• Don't forget break — without it, execution falls through to the next case.

while / do-while

while (n > 0) {
    console.log(n--);
}
do {
    answer = prompt("y/n");
} while (answer !== "y" && answer !== "n");    // runs at least once

Classic for

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

for…of — iterate VALUES (preferred for arrays + iterables)

const fruits = ["apple", "banana", "cherry"];
for (const f of fruits) {
    console.log(f);
}
// works on strings, Maps, Sets, NodeLists, generators
for (const ch of "hello") console.log(ch);

for…in — iterate KEYS (objects)

const user = { id: 1, name: "alice", admin: true };
for (const key in user) {
    console.log(key, user[key]);
}

> ⚠️ Don't use for…in on arrays — it walks all enumerable string keys, including inherited ones. Use for…of (or .forEach / .entries()).

entries / keys / values

for (const [i, value] of arr.entries()) { … }
for (const key   of Object.keys(user))   { … }
for (const value of Object.values(user)) { … }
for (const [k,v] of Object.entries(user)){ … }

break / continue (and labels)

for (let i = 0; i < 10; i++) {
    if (i === 3) continue;     // skip
    if (i === 7) break;        // exit loop
    console.log(i);            // 0 1 2 4 5 6
}
outer:
for (const row of grid) {
    for (const cell of row) {
        if (cell === target) break outer;   // exit BOTH loops
    }
}

A note on forEach

arr.forEach((item, i) => console.log(i, item));

• Cannot break or continue. Returning from the callback only exits *that* iteration.
• For early-exit, use for…of or .some() / .find().