The following questions are intended to be challenging and instructive. If you know exactly how to answer each one, that's great, but if you get some wrong and learn why you got it wrong, I contend that's even better!

Let me know in the comments if you learn anything from the quiz!

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Question 1: IIFE, HOF, or Both

Does the following snippet illustrate an Immediately-Invoked Function Expression (IIFE), a Higher-Order Function (HOF), both, or neither?

((fn, val) => {
  return fn(val);
})(console.log, 5);

Answer and Explanation

Answer: Both IIFE and HOF

The snippet clearly illustrates an IIFE as we immediately invoke a function by passing console.log and 5 to it. Additionally, we find that this is a HOF as fn is a function, and a HOF is defined as any function that takes another function as a parameter or returns a function.

Question 2: Array-to-Object Efficiency

Both a and b are objects with the same properties and values. Which is created more efficiently?

const arr = [1, 2, 3];

const a = arr.reduce(
  (acc, el, i) => ({ ...acc, [el]: i }),
  {}
);

const b = {};
for (let i = 0; i < arr.length; i++) {
  b[arr[i]] = i;
}

Answer and Explanation

Answer: b

When b is being set, the b[arr[i]] property is set to the current index on each iteration. When a is being set, the spread syntax (...) will create a shallow copy of the accumulator object (acc) on each iteration and additionally set the new property. This shallow copy is more expensive than not performing a shallow copy; a requires the construction of 2 intermediate objects before the result is achieved, whereas b does not construct any intermediate objects. Therefore, b is being set more efficiently.

Question 3: Batman v. Superman

Consider the following superheroMaker function. What gets logged when we pass the following two inputs?

const superheroMaker = a => {
  return a instanceof Function ? a() : a;
};

console.log(superheroMaker(() => 'Batman'));
console.log(superheroMaker('Superman'));

Answer and Explanation

Answer: "Batman" "Superman"

When passing () => 'Batman' to superheroMaker, a is an instance of Function. Therefore, the function gets called, returning the string "Batman". When passing "Superman" to superheroMaker, a is not an instance of Function and therefore the string "Superman" is just returned. Therefore, the output is both "Batman" and "Superman".

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Question 4: Object Keys, Object Values

Consider the following object.

const obj = {
  1: 1,
  2: 2,
  3: 3
};

Is Object.keys equal to Object.values?

console.log(Object.keys(obj) == Object.values(obj));

Answer and Explanation

Answer: false

In this case, Object.keys converts the keys to be string ["1", "2", "3"] and Object.values gives [1, 2, 3]. Even if the values turn out to be the same type, the two arrays are both different objects in memory, so the equality comparison will return false. You will see a lot of quiz questions here drilling into the concepts of object and array comparison!

Question 5: Basic Recursion

Consider the following recursive function. If we pass the string "Hello World" to it, what gets logged?

const myFunc = str => {
  if (str.length > 1) {
    return myFunc(str.slice(1));
  }

  return str;
};

console.log(myFunc('Hello world'));

Answer and Explanation

Answer: "d"

The first time we call the function, str.length is greater than 1 ("Hello World" is 11 characters), so we return the same function called on str.slice(1), which is the string "ello World". We repeat this process until the string is only one character long: the character "d", which gets returned to the initial call of myFunc. We then log that character.

Question 6: Function Equality

What gets logged when we test the following equality scenarios?

const a = c => c;
const b = c => c;

console.log(a == b);
console.log(a(7) === b(7));

Answer and Explanation

Answer: false true

In the first test, a and b are different objects in memory; it doesn't matter that the parameters and return values in each function definition are identical. Therefore, a is not equal to b. In the second test, a(7) returns the number 7 and b(7) returns the number 7. These primitive types are strictly equal to each other.

In this case, the equality (==) vs identity (===) comparison operators don't matter; no type coercion will affect the result.

Question 7: Object Property Equality

a and b are different objects with the same firstName property. Are these properties strictly equal to each other?

const a = {
  firstName: 'Bill'
};

const b = {
  firstName: 'Bill'
};

console.log(a.firstName === b.firstName);

Answer and Explanation

Answer: true

The answer is yes, they are. a.firstName is the string value "Bill" and b.firstName is the string value "Bill". Two identical strings are always equal.

Question 8: Function Function Syntax

Let's say myFunc is a function, val1 is a variable, and val2 is a variable. Is the following syntax allowed in JavaScript?

myFunc(val1)(val2);

Answer and Explanation

Answer: yes

This is a common pattern for a higher-order function. If myFunc(val1) returns a function, then that function will be called with val2 as an argument. Here's an example of this in action that you can try out:

const timesTable = num1 => {
  return num2 => {
    return num1 * num2;
  };
};

console.log(timesTable(4)(5));
// 20

Question 9: Object Property Mutation

Consider objects a and b below. What gets logged?

const a = { firstName: 'Joe' };
const b = a;
b.firstName = 'Pete';
console.log(a);

Answer and Explanation

Answer: { firstName: 'Pete' }

When we set b = a in the second line, b and a are pointing to the same object in memory. Changing the firstName property on b will therefore change the firstName property on the only object in memory, so a.firstName will reflect this change.

Question 10: Greatest Number in an Array

Will the following function always return the greatest number in an array?

function greatestNumberInArray(arr) {
  let greatest = 0;
  for (let i = 0; i < arr.length; i++) {
    if (greatest < arr[i]) {
      greatest = arr[i];
    }
  }
  return greatest;
}

Answer and Explanation

Answer: no

This function will work fine for arrays where at least one value is 0 or greater; however, it will fail if all numbers are below 0. This is because the greatest variable starts at 0 even if 0 is greater than all array elements.

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