if/else, switch, the ternary, five kinds of loop, and break/continue with labels.

Conditionals

JavaScript

const grade = (score) => {
  if (score >= 90) return "A";
  else if (score >= 75) return "B";
  else return "C";
};
console.log(grade(95), grade(80), grade(10));
const age = 20;
console.log(age >= 18 ? "adult" : "minor");

Output

A B C
adult

`switch` uses strict equality and falls through

JavaScript

function kind(day) {
  switch (day) {
    case "sat":
    case "sun":
      return "weekend";
    case 1:
      return "number one";
    default:
      return "weekday";
  }
}
console.log(kind("sun"), kind("mon"), kind(1), kind("1"));

let log = [];
switch (2) {
  case 1: log.push(1);
  case 2: log.push(2);   // no break → falls through
  case 3: log.push(3);
    break;
  case 4: log.push(4);
}
console.log(log);

Output

weekend weekday number one weekday
[ 2, 3 ]

Loops

Loop	Iterates over	Use for
`for (init; cond; step)`	a counter	index-based work, early exit
`while (cond)`	until the condition fails	unknown number of iterations
`do { } while (cond)`	at least once	prompts / retries
`for (const x of iterable)`	values of arrays, strings, maps, sets, generators	the default for arrays
`for (const k in object)`	enumerable keys (incl. inherited) as strings	plain objects only
`arr.forEach(fn)`	array elements	side effects; cannot `break`

JavaScript

const arr = ["a", "b"];
arr.extra = "!";
for (const i in arr) console.log("in:", i, typeof i);
for (const v of arr) console.log("of:", v);
for (const [i, v] of arr.entries()) console.log(i, v);
for (const ch of "hi") console.log(ch);

Output

in: 0 string
in: 1 string
in: extra string
of: a
of: b
0 a
1 b
h
i

while / do…while

JavaScript

let n = 1;
while (n < 100) n *= 3;
console.log(n);
let tries = 0;
do {
  tries++;
} while (false);
console.log(tries);

Output

243
1

break, continue and labels

JavaScript

for (let i = 0; i < 6; i++) {
  if (i === 1) continue;
  if (i === 4) break;
  console.log(i);
}
outer: for (let r = 0; r < 3; r++) {
  for (let c = 0; c < 3; c++) {
    if (c === 1) continue outer;
    if (r === 2) break outer;
    console.log(`r${r}c${c}`);
  }
}

Output

0
2
3
r0c0
r1c0

What is the difference between for...in and for...of?

Can you break out of forEach?

Conditionals

switch uses strict equality and falls through

Loops

while / do…while

break, continue and labels

`switch` uses strict equality and falls through