JavaScript MCQ: JavaScript Multiple Choice Questions and Answers

Which class-based lifecycle method would be called at the same time as this effect Hook?

useEffect(() => {
// do things
}, []);

What two values will this code print?
function printA() {
console.log(answer);
var answer = 1;
}
printA();
printA();

0 && hi

Why is it usually better to work with Objects instead of Arrays to store a collection of records?

Which of the following values is not a Boolean false?

Which Variable-defining keyword allows its variable to be accessed (as undefined) before the line that defines it?

What is the result of running this statement?
console.log(typeof(42));

Review the code below. Which statement calls the addTax function and passes 50 as an argument?
function addTax(total) {
return total * 1.05;
}

What is the result in the console of running this code?
function logThis() {
console.log(this);
}
logThis();

Why would you include a "use strict" statement in a JavaScript file?

Which statement is the correct way to create a variable called rate and assign it the value 100?

What is the output of this code?
var obj;
console.log(obj);

How do you import the lodash library making it top-level Api available as the "_" variable?

What is the HTTP verb to request the contents of an existing resource?

Your code is producing the error: TypeError: Cannot read property 'reduce' of undefined. What does that mean?

Which variable is an implicit parameter for every function in JavaScript?

What is the result of running the statement shown?
let a = 5;
console.log(++a);

After the following code, what is the value of a.length?
var a = ['dog', 'cat', 'hen'];
a[100] = 'fox';
console.log(a.length);

What is the result in the console of running this code?
'use strict';
function logThis() {
this.desc = 'logger';
console.log(this);
}
new logThis();

Which statement is used to skip iteration of the loop?

Which choice is valid example for an arrow function?

Which choice is not a unary operator?

Which expression evaluates to true?

This program has a problem. What is it?

var a;
var b = (a = 3) ? true : false;

Which statement creates a new function called discountPrice?

You've written the code shown to log a set of consecutive values, but it instead results in the value 5, 5, 5, and 5 being logged to the console. Which revised version of the code would result in the value 1, 2, 3 and 4 being logged?
for (var i = 1; i <= 4; i++) {
setTimeout(function () {
console.log(i);
}, i * 10000);
}

Which statement selects all img elements in the DOM tree?

Which statement represents the starting code converted to an IIFE?

For the following class, how do you get the value of 42 from an instance of X?
class X {
get Y() {
return 42;
}
}

What is the result in the console of running the code shown?
var start = 1;
function setEnd() {
var end = 10;
}
setEnd();
console.log(end);

If you attempt to call a value as a function but the value is not a function, what kind of error would you get?

What is the result of running this code?
sum(10, 20);
diff(10, 20);
function sum(x, y) {
return x + y;
}

let diff = function (x, y) {
return x - y;
};

Which method is called automatically when an object is initialized?

Which collection object allows unique value to be inserted only once?

Which statement references the DOM node created by the code shown?
<p class="pull">lorem ipsum</p>

What type of scope does the end variable have in the code shown?
var start = 1;
if (start === 1) {
let end = 2;
}

Why would you choose an asynchronous structure for your code?

What will this code log in the console?
function sayHello() {
console.log('hello');
}

console.log(sayHello.prototype);

Which statement creates a new object using the Person constructor?

. What is the value of dessert.type after executing this code?
const dessert = { type: 'pie' };
dessert.type = 'pudding';

Why might you choose to make your code asynchronous?

Which keyword is used to create an error?

You're adding error handling to the code shown. Which code would you include within the if statement to specify an error message?
function addNumbers(x, y) {
if (isNaN(x) || isNaN(y)) {
}
}

What is the result in the console of running the code shown?
var Storm = function () {};
Storm.prototype.precip = 'rain';
var WinterStorm = function () {};
WinterStorm.prototype = new Storm();
WinterStorm.prototype.precip = 'snow';
var bob = new WinterStorm();
console.log(bob.precip);

When would 'results shown' be logged to the console?

let modal = document.querySelector('#results');
setTimeout(function () {
modal.classList.remove('hidden');
}, 10000);

For the following class, how do you get the value of 42 from "X" ?
class X {
get Y() {
return 42;
}
}
var x = new X();

What will be logged to the console?
var a = ['dog', 'cat', 'hen'];
a[100] = 'fox';
console.log(a.length);

What value does this code return?
let answer = true;
if (answer === false) {
return 0;
} else {
return 10;
}

When would you use a conditional statement?

Which operator returns true if the two compared values are not equal?

. How many prototype objects are in the chain for the following array?
let arr = [];

. You've written the event listener shown below for a form button, but each time you click the button, the page reloads. Which statement would stop this from happening?
button.addEventListener(
'click',
function (e) {
button.className = 'clicked';
},
false,
);

What will this code print?
var v = 1;
var f1 = function () {
console.log(v);
};

var f2 = function () {
var v = 2;
f1();
};

f2();

How would you reference the text 'avenue' in the code shown?
let roadTypes = ['street', 'road', 'avenue', 'circle'];

Which statement is true about the "async" attribute for the HTML script tag?

What is the name of a function whose execution can be suspended and resumed at a later point?

Which method cancels event default behavior?

How does the forEach() method differ from a for statement?

How does a function create a closure?

. How would you use the TaxCalculator to determine the amount of tax on $50?
class TaxCalculator {
static calculate(total) {
return total * 0.05;
}
}

Which property references the DOM object that dispatched an event?

What does the following expression evaluate to?

[] == [];

What will the value of y be in this code:
const x = 6 % 2;
const y = x ? 'One' : 'Two';

. Which Object method returns an iterable that can be used to iterate over the properties of an object?

How do you add a comment to JavaScript code?

Which of these is a valid variable name?

Which method converts JSON data to a JavaScript object?

What will be logged to the console?
'use strict';
function logThis() {
this.desc = 'logger';
console.log(this);
}
new logThis();

Which of the following operators can be used to do a short-circuit evaluation?

You need to match a time value such as 12:00:32. Which of the following regular expressions would work for your code?

Which concept is defined as a template that can be used to generate different objects that share some shape and/or behavior?

Which choice is an incorrect way to define an arrow function that returns an empty object?

Which method do you use to attach one DOM node to another?

What would be the result in the console of running this code?
for (var i = 0; i < 5; i++) {
console.log(i);
}

Which statement is true about Functional Programming?

What's one difference between the async and defer attributes of the HTML script tag?

Which statement sets the Person constructor as the parent of the Student constructor in the prototype chain?

How is a forEach statement different from a for statement?

What is one difference between collections created with Map and collections created with Object?

Which event is fired on a text field within a form when a user tabs to it, or clicks or touches it?

When would the final statement in the code shown be logged to the console?
let modal = document.querySelector('#result');
setTimeout(function(){
modal.classList.remove('hidden);
}, 10000);
console.log('Results shown');

Which of the following is not a keyword in JavaScript?