1.19 -- Conditionals and Loops: Quick Revision

Compact cheat sheet. Print-friendly.

How to use this material (instructions)

1. Skim before labs or interviews.
2. Drill gaps -- reopen README.md then 1.19.a...1.19.f.
3. Practice -- 1.19-Exercise-Questions.md.
4. Polish answers -- 1.19-Interview-Questions.md.

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Core vocabulary

Term	One-liner
Truthy	Any value that coerces to true in a boolean context
Falsy	false, 0, -0, 0n, "", null, undefined, NaN
Guard clause	Early if + return/throw to reject invalid input immediately
Fall-through	Execution continues to next case when break is missing
Sentinel	Special value that signals loop termination
Iterable	Object with [Symbol.iterator]() -- arrays, strings, Maps, Sets

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If / Else If / Else

if (condition1) {
  // first truthy branch wins
} else if (condition2) {
  // checked only if condition1 was falsy
} else {
  // fallback
}

Ternary: const x = cond ? valTrue : valFalse; -- single-expression only, avoid nesting.

Guard clause pattern:

function f(x) {
  if (!x) return;          // guard
  // happy path (flat)
}

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Switch

switch (expr) {           // uses === (strict equality)
  case val1:
    // ...
    break;                // required -- prevents fall-through
  case val2: case val3:   // grouped cases
    // ...
    break;
  default:
    // no match
}

Remember: Wrap case bodies in { } when declaring let/const.

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For loop

for (init; condition; update) { }
//   once   before     after
//          each iter  each iter

Variant	Iterates	Best for
for (let i = 0; i < n; i++)	Index	Arrays (when index needed), counted loops
for (const val of iterable)	Values	Arrays, strings, Maps, Sets
for (const key in obj)	Keys (strings)	Plain objects only (NOT arrays)

Closure trap: var in for + closure = all see final value. Fix: use let.

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While / Do-While

while (condition) { }          // 0+ iterations (checks BEFORE body)

do { } while (condition);      // 1+ iterations (checks AFTER body)

Use while when iteration count is unknown. Use do...while when at least one pass is needed.

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Break / Continue / Return

Statement	Effect
break	Exit innermost loop (or switch)
continue	Skip to next iteration of innermost loop
return	Exit entire function
break label	Exit the labeled loop
continue label	Skip to next iteration of the labeled loop

Warning: break inside a switch inside a for exits the switch, not the loop.

Warning: continue in while that skips the update variable = infinite loop.

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Falsy values (memorize)

false | 0 | -0 | 0n | "" | null | undefined | NaN

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

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Common pitfalls cheat sheet

Pitfall	Fix
if (x = 5) (assignment)	if (x === 5)
if (x === 1 || 2)	if (x === 1 || x === 2)
Missing break in switch	Add break/return to every case
for...in on array	Use for...of or classic for
var in for loop closures	Use let
continue skipping update in while	Put update before continue
Nested ternaries	Use if/else or extract function
while (x !== 10) with step > 1	Use while (x < 10)

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Loop selection flowchart

Do you know the number of iterations?
  YES --> for loop
  NO  --> Must body run at least once?
            YES --> do...while
            NO  --> while

Iterating values of array/string?  --> for...of
Iterating keys of an object?       --> for...in (or Object.keys + for...of)
Need the index?                    --> classic for (or for...of + .entries())

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Pattern quick-reference

FizzBuzz

for (let i = 1; i <= n; i++) {
  let s = "";
  if (i % 3 === 0) s += "Fizz";
  if (i % 5 === 0) s += "Buzz";
  console.log(s || i);
}

Digit extraction

while (n > 0) {
  const digit = n % 10;
  n = Math.floor(n / 10);
}

Prime check

function isPrime(n) {
  if (n < 2) return false;
  for (let i = 2; i <= Math.sqrt(n); i++) {
    if (n % i === 0) return false;
  }
  return true;
}

Search with early exit

for (const item of items) {
  if (item.id === target) { result = item; break; }
}

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Master workflow

1. Choose the right conditional (if for ranges/booleans, switch for discrete values).
2. Choose the right loop (for = known count, while = unknown count, for...of = iterables).
3. Flatten with guard clauses and early returns.
4. Control flow with break (exit) and continue (skip) sparingly.
5. Test edge cases: zero iterations, one iteration, empty input, off-by-one.

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End of 1.19 quick revision.