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JavaScript MCQ Practice Part 5


101. What's the value of output?
const one = false || {} || null;
const two = null || false || '';
const three = [] || 0 || true;

console.log(one, two, three);
  • A: false null []
  • B: null "" true
  • C: {} "" []
  • D: null null true
Answer

Answer: C

With the || operator, we can return the first truthy operand. If all values are falsy, the last operand gets returned.

(false || {} || null): the empty object {} is a truthy value. This is the first (and only) truthy value, which gets returned. one is equal to {}.

(null || false || ""): all operands are falsy values. This means that the last operand, "" gets returned. two is equal to "".

([] || 0 || ""): the empty array[] is a truthy value. This is the first truthy value, which gets returned. three is equal to [].


102. What's the value of output?
const myPromise = () => Promise.resolve('I have resolved!');

function firstFunction() {
  myPromise().then(res => console.log(res));
  console.log('second');
}

async function secondFunction() {
  console.log(await myPromise());
  console.log('second');
}

firstFunction();
secondFunction();
  • A: I have resolved!, second and I have resolved!, second
  • B: second, I have resolved! and second, I have resolved!
  • C: I have resolved!, second and second, I have resolved!
  • D: second, I have resolved! and I have resolved!, second
Answer

Answer: D

With a promise, we basically say I want to execute this function, but I'll put it aside for now while it's running since this might take a while. Only when a certain value is resolved (or rejected), and when the call stack is empty, I want to use this value.

We can get this value with both .then and the await keyword in an async function. Although we can get a promise's value with both .then and await, they work a bit differently.

In the firstFunction, we (sort of) put the myPromise function aside while it was running, but continued running the other code, which is console.log('second') in this case. Then, the function resolved with the string I have resolved, which then got logged after it saw that the callstack was empty.

With the await keyword in secondFunction, we literally pause the execution of an async function until the value has been resolved before moving to the next line.

This means that it waited for the myPromise to resolve with the value I have resolved, and only once that happened, we moved to the next line: second got logged.


103. What's the value of output?
const set = new Set();

set.add(1);
set.add('Lydia');
set.add({ name: 'Lydia' });

for (let item of set) {
  console.log(item + 2);
}
  • A: 3, NaN, NaN
  • B: 3, 7, NaN
  • C: 3, Lydia2, [object Object]2
  • D: "12", Lydia2, [object Object]2
Answer

Answer: C

The + operator is not only used for adding numerical values, but we can also use it to concatenate strings. Whenever the JavaScript engine sees that one or more values are not a number, it coerces the number into a string.

The first one is 1, which is a numerical value. 1 + 2 returns the number 3.

However, the second one is a string "Lydia". "Lydia" is a string and 2 is a number: 2 gets coerced into a string. "Lydia" and "2" get concatenated, which results in the string "Lydia2".

{ name: "Lydia" } is an object. Neither a number nor an object is a string, so it stringifies both. Whenever we stringify a regular object, it becomes "[object Object]". "[object Object]" concatenated with "2" becomes "[object Object]2".


104. What's its value?
Promise.resolve(5);
  • A: 5
  • B: Promise {<pending>: 5}
  • C: Promise {<resolved>: 5}
  • D: Error
Answer

Answer: C

We can pass any type of value we want to Promise.resolve, either a promise or a non-promise. The method itself returns a promise with the resolved value. If you pass a regular function, it'll be a resolved promise with a regular value. If you pass a promise, it'll be a resolved promise with the resolved value of that passed promise.

In this case, we just passed the numerical value 5. It returns a resolved promise with the value 5.


105. What's its value?
function compareMembers(person1, person2 = person) {
  if (person1 !== person2) {
    console.log('Not the same!');
  } else {
    console.log('They are the same!');
  }
}

const person = { name: 'Lydia' };

compareMembers(person);
  • A: Not the same!
  • B: They are the same!
  • C: ReferenceError
  • D: SyntaxError
Answer

Answer: B

Objects are passed by reference. When we check objects for strict equality (===), we're comparing their references.

We set the default value for person2 equal to the person object, and passed the person object as the value for person1.

This means that both values have a reference to the same spot in memory, thus they are equal.

The code block in the else statement gets run, and They are the same! gets logged.


106. What's its value?
const colorConfig = {
  red: true,
  blue: false,
  green: true,
  black: true,
  yellow: false,
};

const colors = ['pink', 'red', 'blue'];

console.log(colorConfig.colors[1]);
  • A: true
  • B: false
  • C: undefined
  • D: TypeError
Answer

Answer: D

In JavaScript, we have two ways to access properties on an object: bracket notation, or dot notation. In this example, we use dot notation (colorConfig.colors) instead of bracket notation (colorConfig["colors"]).

With dot notation, JavaScript tries to find the property on the object with that exact name. In this example, JavaScript tries to find a property called colors on the colorConfig object. There is no property called colors, so this returns undefined. Then, we try to access the value of the first element by using [1]. We cannot do this on a value that's undefined, so it throws a TypeError: Cannot read property '1' of undefined.

JavaScript interprets (or unboxes) statements. When we use bracket notation, it sees the first opening bracket [ and keeps going until it finds the closing bracket ]. Only then, it will evaluate the statement. If we would've used colorConfig[colors[1]], it would have returned the value of the red property on the colorConfig object.


107. What's its value?
console.log('❤️' === '❤️');
  • A: true
  • B: false
Answer

Answer: A

Under the hood, emojis are unicodes. The unicodes for the heart emoji is "U+2764 U+FE0F". These are always the same for the same emojis, so we're comparing two equal strings to each other, which returns true.


108. Which of these methods modifies the original array?
const emojis = ['✨', '🥑', '😍'];

emojis.map(x => x + '✨');
emojis.filter(x => x !== '🥑');
emojis.find(x => x !== '🥑');
emojis.reduce((acc, cur) => acc + '✨');
emojis.slice(1, 2, '✨');
emojis.splice(1, 2, '✨');
  • A: All of them
  • B: map reduce slice splice
  • C: map slice splice
  • D: splice
Answer

Answer: D

With splice method, we modify the original array by deleting, replacing or adding elements. In this case, we removed 2 items from index 1 (we removed '🥑' and '😍') and added the ✨ emoji instead.

map, filter and slice return a new array, find returns an element, and reduce returns a reduced value.


109. What's the output?
const food = ['🍕', '🍫', '🥑', '🍔'];
const info = { favoriteFood: food[0] };

info.favoriteFood = '🍝';

console.log(food);
  • A: ['🍕', '🍫', '🥑', '🍔']
  • B: ['🍝', '🍫', '🥑', '🍔']
  • C: ['🍝', '🍕', '🍫', '🥑', '🍔']
  • D: ReferenceError
Answer

Answer: A

We set the value of the favoriteFood property on the info object equal to the string with the pizza emoji, '🍕'. A string is a primitive data type. In JavaScript, primitive data types act by reference

In JavaScript, primitive data types (everything that's not an object) interact by value. In this case, we set the value of the favoriteFood property on the info object equal to the value of the first element in the food array, the string with the pizza emoji in this case ('🍕'). A string is a primitive data type, and interact by value (see my blogpost if you're interested in learning more)

Then, we change the value of the favoriteFood property on the info object. The food array hasn't changed, since the value of favoriteFood was merely a copy of the value of the first element in the array, and doesn't have a reference to the same spot in memory as the element on food[0]. When we log food, it's still the original array, ['🍕', '🍫', '🥑', '🍔'].


110. What does this method do?
JSON.parse();
  • A: Parses JSON to a JavaScript value
  • B: Parses a JavaScript object to JSON
  • C: Parses any JavaScript value to JSON
  • D: Parses JSON to a JavaScript object only
Answer

Answer: A

With the JSON.parse() method, we can parse JSON string to a JavaScript value.

// Stringifying a number into valid JSON, then parsing the JSON string to a JavaScript value:
const jsonNumber = JSON.stringify(4); // '4'
JSON.parse(jsonNumber); // 4

// Stringifying an array value into valid JSON, then parsing the JSON string to a JavaScript value:
const jsonArray = JSON.stringify([1, 2, 3]); // '[1, 2, 3]'
JSON.parse(jsonArray); // [1, 2, 3]

// Stringifying an object  into valid JSON, then parsing the JSON string to a JavaScript value:
const jsonArray = JSON.stringify({ name: 'Lydia' }); // '{"name":"Lydia"}'
JSON.parse(jsonArray); // { name: 'Lydia' }


111. What's the output?
let name = 'Lydia';

function getName() {
  console.log(name);
  let name = 'Sarah';
}

getName();
  • A: Lydia
  • B: Sarah
  • C: undefined
  • D: ReferenceError
Answer

Answer: D

Each function has its own execution context (or scope). The getName function first looks within its own context (scope) to see if it contains the variable name we're trying to access. In this case, the getName function contains its own name variable: we declare the variable name with the let keyword, and with the value of 'Sarah'.

Variables with the let keyword (and const) are hoisted, but unlike var, don't get initialized. They are not accessible before the line we declare (initialize) them. This is called the "temporal dead zone". When we try to access the variables before they are declared, JavaScript throws a ReferenceError.

If we wouldn't have declared the name variable within the getName function, the javascript engine would've looked down the scope chain. The outer scope has a variable called name with the value of Lydia. In that case, it would've logged Lydia.

let name = 'Lydia';

function getName() {
  console.log(name);
}

getName(); // Lydia


112. What's the output?
function* generatorOne() {
  yield ['a', 'b', 'c'];
}

function* generatorTwo() {
  yield* ['a', 'b', 'c'];
}

const one = generatorOne();
const two = generatorTwo();

console.log(one.next().value);
console.log(two.next().value);
  • A: a and a
  • B: a and undefined
  • C: ['a', 'b', 'c'] and a
  • D: a and ['a', 'b', 'c']
Answer

Answer: C

With the yield keyword, we yield values in a generator function. With the yield* keyword, we can yield values from another generator function, or iterable object (for example an array).

In generatorOne, we yield the entire array ['a', 'b', 'c'] using the yield keyword. The value of value property on the object returned by the next method on one (one.next().value) is equal to the entire array ['a', 'b', 'c'].

console.log(one.next().value); // ['a', 'b', 'c']
console.log(one.next().value); // undefined

In generatorTwo, we use the yield* keyword. This means that the first yielded value of two, is equal to the first yielded value in the iterator. The iterator is the array ['a', 'b', 'c']. The first yielded value is a, so the first time we call two.next().value, a is returned.

console.log(two.next().value); // 'a'
console.log(two.next().value); // 'b'
console.log(two.next().value); // 'c'
console.log(two.next().value); // undefined


113. What's the output?
console.log(`${(x => x)('I love')} to program`);
  • A: I love to program
  • B: undefined to program
  • C: ${(x => x)('I love') to program
  • D: TypeError
Answer

Answer: A

Expressions within template literals are evaluated first. This means that the string will contain the returned value of the expression, the immediately invoked function (x => x)('I love') in this case. We pass the value 'I love' as an argument to the x => x arrow function. x is equal to 'I love', which gets returned. This results in I love to program.


114. What will happen?
let config = {
  alert: setInterval(() => {
    console.log('Alert!');
  }, 1000),
};

config = null;
  • A: The setInterval callback won't be invoked
  • B: The setInterval callback gets invoked once
  • C: The setInterval callback will still be called every second
  • D: We never invoked config.alert(), config is null
Answer

Answer: C

Normally when we set objects equal to null, those objects get garbage collected as there is no reference anymore to that object. However, since the callback function within setInterval is an arrow function (thus bound to the config object), the callback function still holds a reference to the config object. As long as there is a reference, the object won't get garbage collected. Since it's not garbage collected, the setInterval callback function will still get invoked every 1000ms (1s).


115. Which method(s) will return the value 'Hello world!'?
const myMap = new Map();
const myFunc = () => 'greeting';

myMap.set(myFunc, 'Hello world!');

//1
myMap.get('greeting');
//2
myMap.get(myFunc);
//3
myMap.get(() => 'greeting');
  • A: 1
  • B: 2
  • C: 2 and 3
  • D: All of them
Answer

Answer: B

When adding a key/value pair using the set method, the key will be the value of the first argument passed to the set function, and the value will be the second argument passed to the set function. The key is the function () => 'greeting' in this case, and the value 'Hello world'. myMap is now { () => 'greeting' => 'Hello world!' }.

1 is wrong, since the key is not 'greeting' but () => 'greeting'. 3 is wrong, since we're creating a new function by passing it as a parameter to the get method. Object interact by reference. Functions are objects, which is why two functions are never strictly equal, even if they are identical: they have a reference to a different spot in memory.


116. What's the output?
const person = {
  name: 'Lydia',
  age: 21,
};

const changeAge = (x = { ...person }) => (x.age += 1);
const changeAgeAndName = (x = { ...person }) => {
  x.age += 1;
  x.name = 'Sarah';
};

changeAge(person);
changeAgeAndName();

console.log(person);
  • A: {name: "Sarah", age: 22}
  • B: {name: "Sarah", age: 23}
  • C: {name: "Lydia", age: 22}
  • D: {name: "Lydia", age: 23}
Answer

Answer: C

Both the changeAge and changeAgeAndName functions have a default parameter, namely a newly created object { ...person }. This object has copies of all the key/values in the person object.

First, we invoke the changeAge function and pass the person object as its argument. This function increases the value of the age property by 1. person is now { name: "Lydia", age: 22 }.

Then, we invoke the changeAgeAndName function, however we don't pass a parameter. Instead, the value of x is equal to a new object: { ...person }. Since it's a new object, it doesn't affect the values of the properties on the person object. person is still equal to { name: "Lydia", age: 22 }.


117. Which of the following options will return 6?
function sumValues(x, y, z) {
  return x + y + z;
}
  • A: sumValues([...1, 2, 3])
  • B: sumValues([...[1, 2, 3]])
  • C: sumValues(...[1, 2, 3])
  • D: sumValues([1, 2, 3])
Answer

Answer: C

With the spread operator ..., we can spread iterables to individual elements. The sumValues function receives three arguments: x, y and z. ...[1, 2, 3] will result in 1, 2, 3, which we pass to the sumValues function.


118. What's the output?
let num = 1;
const list = ['🥳', '🤠', '🥰', '🤪'];

console.log(list[(num += 1)]);
  • A: 🤠
  • B: 🥰
  • C: SyntaxError
  • D: ReferenceError
Answer

Answer: B

With the += operand, we're incrementing the value of num by 1. num had the initial value 1, so 1 + 1 is 2. The item on the second index in the list array is 🥰, console.log(list[2]) prints 🥰.


119. What's the output?
const person = {
  firstName: 'Lydia',
  lastName: 'Hallie',
  pet: {
    name: 'Mara',
    breed: 'Dutch Tulip Hound',
  },
  getFullName() {
    return `${this.firstName} ${this.lastName}`;
  },
};

console.log(person.pet?.name);
console.log(person.pet?.family?.name);
console.log(person.getFullName?.());
console.log(member.getLastName?.());
  • A: undefined undefined undefined undefined
  • B: Mara undefined Lydia Hallie undefined
  • C: Mara null Lydia Hallie null
  • D: null ReferenceError null ReferenceError
Answer

Answer: B

With the optional chaining operator ?., we no longer have to explicitly check whether the deeper nested values are valid or not. If we're trying to access a property on an undefined or null value (nullish), the expression short-circuits and returns undefined.

person.pet?.name: person has a property named pet: person.pet is not nullish. It has a property called name, and returns Mara. person.pet?.family?.name: person has a property named pet: person.pet is not nullish. pet does not have a property called family, person.pet.family is nullish. The expression returns undefined. person.getFullName?.(): person has a property named getFullName: person.getFullName() is not nullish and can get invoked, which returns Lydia Hallie. member.getLastName?.(): member is not defined: member.getLastName() is nullish. The expression returns undefined.


120. What's the output?
const groceries = ['banana', 'apple', 'peanuts'];

if (groceries.indexOf('banana')) {
  console.log('We have to buy bananas!');
} else {
  console.log(`We don't have to buy bananas!`);
}
  • A: We have to buy bananas!
  • B: We don't have to buy bananas
  • C: undefined
  • D: 1
Answer

Answer: B

We passed the condition groceries.indexOf("banana") to the if-statement. groceries.indexOf("banana") returns 0, which is a falsy value. Since the condition in the if-statement is falsy, the code in the else block runs, and We don't have to buy bananas! gets logged.


121. What's the output?
const config = {
  languages: [],
  set language(lang) {
    return this.languages.push(lang);
  },
};

console.log(config.language);
  • A: function language(lang) { this.languages.push(lang }
  • B: 0
  • C: []
  • D: undefined
Answer

Answer: D

The language method is a setter. Setters don't hold an actual value, their purpose is to modify properties. When calling a setter method, undefined gets returned.


122. What's the output?
const name = 'Lydia Hallie';

console.log(!typeof name === 'object');
console.log(!typeof name === 'string');
  • A: false true
  • B: true false
  • C: false false
  • D: true true
Answer

Answer: C

typeof name returns "string". The string "string" is a truthy value, so !typeof name returns the boolean value false. false === "object" and false === "string" both returnfalse.

(If we wanted to check whether the type was (un)equal to a certain type, we should've written !== instead of !typeof)


123. What's the output?
const add = x => y => z => {
  console.log(x, y, z);
  return x + y + z;
};

add(4)(5)(6);
  • A: 4 5 6
  • B: 6 5 4
  • C: 4 function function
  • D: undefined undefined 6
Answer

Answer: A

The add function returns an arrow function, which returns an arrow function, which returns an arrow function (still with me?). The first function receives an argument x with the value of 4. We invoke the second function, which receives an argument y with the value 5. Then we invoke the third function, which receives an argument z with the value 6. When we're trying to access the value x, y and z within the last arrow function, the JS engine goes up the scope chain in order to find the values for x and y accordingly. This returns 4 5 6.


124. What's the output?
async function* range(start, end) {
  for (let i = start; i <= end; i++) {
    yield Promise.resolve(i);
  }
}

(async () => {
  const gen = range(1, 3);
  for await (const item of gen) {
    console.log(item);
  }
})();
  • A: Promise {1} Promise {2} Promise {3}
  • B: Promise {<pending>} Promise {<pending>} Promise {<pending>}
  • C: 1 2 3
  • D: undefined undefined undefined
Answer

Answer: C

The generator function range returns an async object with promises for each item in the range we pass: Promise{1}, Promise{2}, Promise{3}. We set the variable gen equal to the async object, after which we loop over it using a for await ... of loop. We set the variable item equal to the returned Promise values: first Promise{1}, then Promise{2}, then Promise{3}. Since we're awaiting the value of item, the resolved promsie, the resolved values of the promises get returned: 1, 2, then 3.


125. What's the output?
const myFunc = ({ x, y, z }) => {
  console.log(x, y, z);
};

myFunc(1, 2, 3);
  • A: 1 2 3
  • B: {1: 1} {2: 2} {3: 3}
  • C: { 1: undefined } undefined undefined
  • D: undefined undefined undefined
Answer

Answer: D

myFunc expects an object with properties x, y and z as its argument. Since we're only passing three separate numeric values (1, 2, 3) instead of one object with properties x, y and z ({x: 1, y: 2, z: 3}), x, y and z have their default value of undefined.


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