Quiz: Predict the Output (Basics)

Time to Put It All Together

You've worked through variables, operators, functions, arrays, objects, destructuring, strings, error handling, and modules. Now let's see how well those concepts actually stuck.

This isn't about memorizing syntax. It's about having a mental model of how JavaScript evaluates code, step by step. Each question below is something you could genuinely encounter in real code. No trick questions for the sake of being tricky — every one tests a concept that matters.

How to use this quiz: Read each snippet, trace the execution in your head (or on paper), pick your answer, then read the explanation carefully — even if you got it right. The explanations connect back to the "why" behind the behavior.

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Question 1: The Warm-Up

let x = 10;
let y = "5";
console.log(x + y);
console.log(x - y);

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Question 2: Hoisting Surprise

console.log(a);
console.log(b);
var a = 5;
let b = 10;

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Question 3: Function Hoisting

console.log(greet());
console.log(hello());

function greet() {
  return "Hi!";
}

var hello = function () {
  return "Hello!";
};

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Question 4: Scope Boundaries

for (var i = 0; i < 3; i++) {
  // loop body
}
console.log(i);

for (let j = 0; j < 3; j++) {
  // loop body
}
console.log(j);

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Checkpoint: How's It Going?

Four questions down. If you got at least 3 right, your fundamentals are solid. If hoisting tripped you up, that's completely normal — it trips up experienced developers too. The key insight is: declarations are hoisted, assignments are not, and let/const have a temporal dead zone that var does not.

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Question 5: Array Methods and Return Values

const nums = [1, 2, 3, 4, 5];
const result = nums.filter(n => n > 2).map(n => n * 10);
console.log(result);
console.log(nums);

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Question 6: Object References

const person = { name: "Alice", age: 25 };
const copy = person;
copy.name = "Bob";
console.log(person.name);
console.log(copy.name);

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Question 7: Destructuring with Defaults

const config = { theme: "dark", lang: undefined };
const { theme, lang = "en", debug = false } = config;
console.log(theme);
console.log(lang);
console.log(debug);

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Question 8: Template Literal Expressions

const a = 10;
const b = 20;
console.log(`Sum: ${a + b}`);
console.log("Sum: " + a + b);

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Checkpoint: Halfway There

Eight questions in. If you nailed 6 or more, you're building strong instincts. The next four questions ramp up the difficulty. They combine multiple concepts — exactly the kind of code you will see in production.

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Question 9: The typeof Quirk

console.log(typeof null);
console.log(typeof undefined);
console.log(typeof NaN);

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Question 10: Error Handling Flow

function divide(a, b) {
  try {
    if (b === 0) throw new Error("Cannot divide by zero");
    return a / b;
  } catch (err) {
    console.log(err.message);
    return -1;
  } finally {
    console.log("done");
  }
}

console.log(divide(10, 0));

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Question 11: Spread and Shallow Copy

const original = { a: 1, b: { c: 2 } };
const clone = { ...original };
clone.a = 99;
clone.b.c = 99;
console.log(original.a);
console.log(original.b.c);

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Question 12: Putting It All Together

var count = 0;

function increment() {
  count++;
  return count;
}

const results = [increment(), increment(), increment()];
const doubled = results.map(n => n * 2);
const [first, ...rest] = doubled;

console.log(first);
console.log(rest);
console.log(count);

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Scoring Guide

No matter your score, remember: understanding the why behind the behavior matters more than getting the right answer by instinct. Every question you missed is a concept you just learned more deeply by reading the explanation.