Test: Module 01 — Introduction to JavaScript¶

Instructions: Answer all questions. Write your answers directly below each question in the Answer: field. Bonus questions are optional and can raise your score above 25 points.

Total Points: 25 pts (Easy: 8 + Medium: 9 + Hard: 6 + Expert: 2) Bonus Available: 6 pts

Section 1: Easy Questions (2 pts each)¶

Q1. What year was JavaScript created, who created it, and at which company?

Q2. What is the difference between null and undefined? Give one example of a situation where you would encounter each.

Q3. List the seven primitive types in JavaScript.

Q4. What output does the following code produce? Explain why.

let x;
console.log(typeof x);
console.log(x === undefined);

const y = null;
console.log(typeof y);
console.log(y === null);

Section 2: Medium Questions (3 pts each)¶

Q5. Explain the difference between == (loose equality) and === (strict equality). Give two examples where == returns true but === returns false. When is it acceptable to use ==?

Q6. What is hoisting in JavaScript? Explain how var, let, and function declarations are hoisted differently. What happens if you access a let variable before its declaration line?

Q7. Explain what a closure is in your own words. Look at the following code — what does it output, and why?

function outer() {
  let secret = 42;
  return function inner() {
    secret++;
    return secret;
  };
}

const fn = outer();
console.log(fn()); // ?
console.log(fn()); // ?
console.log(fn()); // ?

Section 3: Hard Questions (3 pts each)¶

Q8. The following code has a bug. Identify the bug, explain why it occurs, and write the corrected version.

const results = [];
for (var i = 0; i < 3; i++) {
  results.push(function() { return i; });
}
console.log(results[0]()); // Expected: 0, Actual: ?
console.log(results[1]()); // Expected: 1, Actual: ?
console.log(results[2]()); // Expected: 2, Actual: ?

Q9. Write a function flatten(arr) that takes a nested array (up to 2 levels deep) and returns a new flat array. Do not use Array.prototype.flat(). Your function must handle arrays like [1, [2, 3], [4, [5, 6]]] and return [1, 2, 3, 4, 5, 6].

// Write your function here:

// Expected outputs:
// flatten([1, [2, 3], [4, [5, 6]]]) → [1, 2, 3, 4, 5, 6]
// flatten([1, 2, 3])               → [1, 2, 3]
// flatten([[1, 2], [3, 4]])         → [1, 2, 3, 4]

Section 4: Expert Question (2 pts)¶

Q10. The following code is intended to print 0 through 4, with each number logged after 1 second. It does not work as intended. Describe all the problems with it, explain the underlying JavaScript concepts that cause each problem, and provide a working rewrite that achieves the intended behaviour.

for (var i = 0; i < 5; i++) {
  setTimeout(function() {
    console.log(i);
  }, 1000 * i);
}

Bonus Questions (2 pts each)¶

Bonus 1. What is the ECMAScript specification and how does it relate to JavaScript? What is the difference between ES5, ES2015/ES6, and ES2022? (+2 pts)

Bonus 2. Explain the difference between a function declaration and a function expression with regard to hoisting. What is the difference in behaviour between these two? (+2 pts)

// Version A
console.log(add(2, 3));
function add(a, b) { return a + b; }

// Version B
console.log(multiply(2, 3));
const multiply = (a, b) => a * b;

Bonus 3. What does the rest parameter syntax (...args) do? How is it different from the arguments object available in traditional function declarations? Why can't arrow functions use arguments? (+2 pts)