Episode 1 — Fundamentals / 1.18 — Operators and Type System

1.18 — Exercise Questions: Operators and the Type System

Practice questions for all seven subtopics in Section 1.18. Mix of code prediction, short-answer, debugging, and reasoning tasks.

How to use this material (instructions)

Read lessons in order — README.md, then 1.18.a → 1.18.g.
Answer closed-book first — then compare to the matching lesson.
Use DevTools Console — paste code snippets to verify predictions.
Interview prep — 1.18-Interview-Questions.md.
Quick review — 1.18-Quick-Revision.md.

1.18.a — Arithmetic Operators (Q1–Q10)

Q1. What is the result of 10 % 3? What does the % operator return?

Q2. Predict the output:

let x = 5;
console.log(x++);
console.log(x);
console.log(++x);
console.log(x);

Q3. What is 2 ** 3 ** 2? Explain why (hint: associativity).

Q4. Predict:

console.log(1 + 2 + "3");
console.log("1" + 2 + 3);

Q5. What does 0 / 0 return? What about 1 / 0?

Q6. Why does 0.1 + 0.2 !== 0.3? How would you fix this comparison?

Q7. Predict:

console.log(typeof NaN);
console.log(NaN === NaN);

Q8. Write a function isEven(n) using the % operator.

Q9. What is the result of "hello" * 2?

Q10. Predict:

console.log(2 + 3 * 4);
console.log((2 + 3) * 4);
console.log(2 ** 3 * 2);

1.18.b — Comparison Operators (Q11–Q20)

Q11. What is the difference between == and ===?

Q12. Predict the output of each:

console.log(5 == "5");
console.log(5 === "5");
console.log(0 == false);
console.log(0 === false);
console.log(null == undefined);
console.log(null === undefined);

Q13. Why does "" == false return true? Walk through the coercion steps.

Q14. Predict:

console.log(NaN === NaN);
console.log(NaN == NaN);
console.log(NaN !== NaN);

Q15. What are the results of these three expressions?

console.log(null > 0);
console.log(null == 0);
console.log(null >= 0);

Explain the inconsistency.

Q16. Predict:

console.log("10" < "9");
console.log("10" < 9);

Why are they different?

Q17. How does Object.is() differ from ===? Give two examples.

Q18. What does [] == false return? Walk through the coercion steps.

Q19. Write a comparison that checks if a variable is null or undefined using a single ==.

Q20. Debug this code: The function should return true only for the number 0, but it has a bug:

function isZero(val) {
  return val == 0;
}
console.log(isZero(0));     // true (correct)
console.log(isZero(""));    // ? (should be false)
console.log(isZero(false)); // ? (should be false)

1.18.c — Logical Operators (Q21–Q32)

Q21. Predict the output:

console.log(true && "hello");
console.log(false && "hello");
console.log("a" && "b" && "c");
console.log("a" && 0 && "c");

Q22. Predict:

console.log(false || "hello");
console.log("" || "default");
console.log(0 || null || "found");
console.log("first" || "second");

Q23. What is the result of !!"hello"? What about !!0? What about !![]?

Q24. What is the difference between || and ??? Give an example where they produce different results.

Q25. Predict:

console.log(0 || 42);
console.log(0 ?? 42);
console.log("" || "default");
console.log("" ?? "default");

Q26. Why does this code fail?

const result = null || undefined ?? "default";

Q27. Predict:

console.log(!true || false);
console.log(!(true || false));
console.log(true || false && false);

Q28. What does short-circuit evaluation mean? Write code that demonstrates && short-circuiting.

Q29. Rewrite this using ?? and optional chaining:

const city = user && user.address && user.address.city || "Unknown";

Q30. What is the output?

let x = 0;
const result = x || 10;
console.log(result);

Is this always the intended behavior?

Q31. Predict:

console.log(null && "hello");
console.log(undefined && "hello");
console.log(0 && "hello");
console.log("" && "hello");
console.log(1 && 0);

Q32. Write a function getDisplayName(user) that returns user.nickname, falling back to user.name, falling back to "Anonymous". Use ??.

1.18.d — Assignment Operators (Q33–Q40)

Q33. What is the value of score after:

let score = 100;
score += 50;
score -= 30;
score *= 2;
score /= 4;
score %= 7;

Q34. Predict the output:

let x = 0;
x ||= 10;
console.log(x);

let y = 0;
y ??= 10;
console.log(y);

Q35. What is the difference between x ||= 5 and x ??= 5?

Q36. What does x &&= x.trim() do? When does the assignment NOT happen?

Q37. Debug: Why does this create a global variable?

let x = y = 10;

Q38. Predict:

let str = "Hello";
str += " ";
str += "World";
console.log(str);

Q39. Write code that initializes config.retries to 3 only if it is currently null or undefined, using a logical assignment operator. It should NOT overwrite 0.

Q40. Predict:

const obj = { a: null, b: 0, c: "", d: false };
obj.a ??= "default";
obj.b ??= "default";
obj.c ??= "default";
obj.d ??= "default";
console.log(obj);

1.18.e — typeof Operator (Q41–Q48)

Q41. What does typeof null return? Why?

Q42. Predict the typeof result for each:

typeof 42
typeof "hello"
typeof true
typeof undefined
typeof null
typeof []
typeof {}
typeof function(){}
typeof NaN
typeof Symbol()
typeof 10n

Q43. Why does typeof on an undeclared variable NOT throw an error? Give a practical use case.

Q44. How would you check if a value is an array? Why does typeof fail here?

Q45. What is the difference between typeof x === "function" and x instanceof Function?

Q46. Write a function getType(value) that returns the accurate type name, including "null" for null and "array" for arrays.

Q47. Predict:

console.log(typeof class Foo {});
console.log(typeof Math);
console.log(typeof JSON);
console.log(typeof console);

Q48. Debug: This validation function has a bug. Identify and fix it:

function isValidNumber(val) {
  return typeof val === "number";
}
console.log(isValidNumber(42));   // true (correct)
console.log(isValidNumber(NaN));  // ? (should be false)

1.18.f — Truthy and Falsy Values (Q49–Q56)

Q49. List all 8 falsy values in JavaScript.

Q50. Are these truthy or falsy?

[]
{}
"0"
"false"
" "
new Boolean(false)
-1
Infinity

Q51. Explain why [] == false is true even though [] is truthy.

Q52. Debug: This function has a bug when score is 0:

function displayScore(score) {
  if (!score) {
    return "No score yet";
  }
  return `Score: ${score}`;
}

Fix it so that displayScore(0) returns "Score: 0".

Q53. What is the difference between !!value and Boolean(value)?

Q54. Why is if (arr) NOT a reliable way to check if an array has elements?

Q55. Predict:

console.log(Boolean([]));
console.log(Boolean(""));
console.log(Boolean("0"));
console.log(Boolean(0));
console.log(Boolean(new Number(0)));

Q56. Write code that checks if user.email exists and is a non-empty string.

1.18.g — Type Coercion (Q57–Q67)

Q57. Predict:

console.log("5" + 3);
console.log("5" - 3);
console.log("5" * "2");
console.log(true + true);
console.log(null + 1);
console.log(undefined + 1);

Q58. What is the result of 1 + 2 + "3" + 4? Walk through each step.

Q59. What is the difference between Number("42px") and parseInt("42px", 10)?

Q60. Predict:

console.log(Number(""));
console.log(Number(" "));
console.log(Number(null));
console.log(Number(undefined));
console.log(Number([]));
console.log(Number([5]));
console.log(Number([1, 2]));
console.log(Number({}));

Q61. What is the result of [] + []? What about [] + {}?

Q62. Why does {} + [] evaluate to 0 in the browser console but "[object Object]" when wrapped in console.log()?

Q63. What does the unary + operator do? How is +"42" different from parseInt("42")?

Q64. Predict:

const obj = {};
obj[1] = "one";
obj[true] = "yes";
console.log(obj["1"]);
console.log(obj["true"]);
console.log(Object.keys(obj));

Q65. Debug: This function sometimes returns NaN. Why?

function add(a, b) {
  return a + b;
}
console.log(add(5, 3));          // 8
console.log(add("5", 3));       // ?
console.log(add(undefined, 3)); // ?

Q66. Write a function that safely converts a string to a number, returning null if the string is not a valid number (including empty strings).

Q67. Predict:

console.log([] - []);
console.log({} - {});
console.log([] == 0);
console.log("" == 0);
console.log("" == false);
console.log("0" == false);
console.log("" == "0");

Answer hints

Q	Hint
Q2	Post-increment returns old value; pre-increment returns new
Q3	`` is right-associative: `2 (3 2)` = `2 9` = 512
Q10	`` before `+`; `` before ``
Q15	`>` and `>=` use numeric coercion; `==` has special null rules
Q16	Both strings → lexicographic; string vs number → numeric
Q20	Use `===` to fix: `return val === 0`
Q25	`
Q26	Cannot mix `??` with `
Q34	`
Q40	`??=` only assigns for null/undefined; 0, "", false are kept
Q41	Historical bug — null's type tag was 0 (same as objects)
Q48	`typeof NaN === "number"` — add `!Number.isNaN(val)` check
Q51	`[] → "" → 0`, `false → 0`, `0 == 0`
Q52	Use `if (score == null)` or `if (score === null \|\| score === undefined)`
Q58	`1+2=3`, `3+"3"="33"`, `"33"+4="334"`
Q61	`[] → "" + "" = ""`; `[] → "" + "[object Object]"`
Q67	`0`, `NaN`, `true`, `true`, `true`, `true`, `false`

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