JavaScript quiz 1

Solution

1. To tell parsers to interpret your JavaScript syntax loosely.
2. To tell parsers to enforce all JavaScript syntax rules when processing your code.
3. To instruct the browser to automatically fix any errors it finds in the code.
4. To enable ES6 features in your code.

Relevant resource(s):

• Strict mode.

Solution

1. this
2. catch
3. function
4. array

Relevant resource(s):

• JavaScript keywords and identifiers.

Solution

1. 5thItem
2. firstName
3. grand total
4. function

Relevant resource(s):

• Identifier.
• JavaScript Valid Variable Names.

Solution

1. ! This is a comment
2. # This is a comment
3. \\ This is a comment
4. // This is a comment

Relevant resource(s):

• How To Write Comments in JavaScript.

Solution

1. let rate = 100;
2. let 100 = rate;
3. 100 = let rate;
4. rate = 100;

Relevant resource(s):

• Statements: declaring variables.

Solution

1. 'float'
2. 'value'
3. 'number'
4. 'integer'

Relevant resource(s):

• Data types.
• The typeof operator.

Solution

1. const
2. var
3. let
4. var and let

Relevant resource(s):

• Hoisting.
• The flaws of the old var.

Solution

1. Boolean(0)
2. Boolean('')
3. Boolean(NaN)
4. Boolean('false')

Relevant resource(s):

• The Boolean constructor and function.
• Understanding Javascript's Boolean object constructor.

Solution

1. ReferenceError: obj is not defined
2. {}
3. undefined
4. null

Relevant resource(s):

• let.

Solution

1. ReferenceError
2. 'decimal'
3. float
4. 'number'

Relevant resource(s):

• Data types.
• typeof.

Solution

1. 'object'.
2. Number.
3. String.
4. None of the other solutions.

Explanation

This question is a bit tricky. The typeof operator always return a string. So the initial code would return the string 'number' and not Number.

Relevant resource(s):

• typeof.

Solution

1. conditional
2. block
3. global
4. function>

Relevant resource(s):

• Scope.
• Demystifying JavaScript variable scope and hoisting.

Solution

1. '93'
2. 93
3. '633'
4. '66'

Relevant resource(s):

• JavaScript type coercion explained.

Solution

1. 'string'
2. String
3. [String]
4. ['string']

Relevant resource(s):

• typeof.

Solution

1. line 5, because lion cannot be reassigned a value.
2. line 6, because the += operator cannot be used with the undefined variable bear.
3. line 5, because the prefix (++) operator does not exist in JavaScript.
4. line 3, because the variable bear is left undefined.

Relevant resource(s):

• const.

Solution

1. It will throw a ReferenceError about x.
2. It will print 18.
3. It will print undefined.
4. It will throw a ReferenceError about y.

Relevant resource(s):

• Scope.
• Demystifying JavaScript variable scope and hoisting.

Explanation

The var statement declares a variable x that is not block-scoped. So, x will be accessible within the console.log() statement. Its value will be 5.

On the other hand, the let statement declares a variable y that is block-scoped. So, it won't be accessibe from outside the curly braces of the if statement. Within the console.log() statement, the JavaScript parser will throw a ReferenceError when trying to get the value of y, since y doesn't exist in the current scope (global scope).

Solution

1. undefined
2. 'Wild'
3. 'Tabby'
4. 'Athena'

Explanation

When swap(cat); is executed, the local variable feline, scoped to the function swap, is assigned the object { name: 'Athena' }.

Therefore, the variables feline and cat reference the same object.

On the first line of the function, the name property of feline is changed to 'Wild'. Since feline and cat reference the same object, the name of the object referenced by the variable cat is also changed to 'Wild'.

On the following line of the function, the local variable feline is assigned a new object. The variables feline and cat stop referencing the same object. The object referenced by the variable cat stays as it is. It is the object { name: 'Wild' }.

Therefore, the console.log(cat.name) prints 'Wild'.

Solution

1. a is 1.
2. a is undefined.
3. It won't print anything.
4. a is 2.

Relevant resource(s):

• Scope.

Solution

1. 'pot'.indexOf('potato') !== -1
2. 'potato'.includes('Pot')
3. 'potato'.includes('pot')
4. 'potato'.contains('pot')

Relevant resource(s):

• String.prototype.includes().

Solution

1. []
2. undefined
3. 0
4. null

Relevant resource(s):

• Falsy.

Solution

1. TypeError
2. SystemError
3. SyntaxError
4. LogicError

Relevant resource(s):

• TypeError: "x" is not a function.

Solution

1. for (var i = 1; i <= 4; i++) {
     setTimeout(function () {
       console.log(i);
     }, i * 10000);
   }

2. for (var i = 1; i <= 4; i++) {
     (function (i) {
       setTimeout(function () {
         console.log(j);
       }, j * 1000);
     })(j);
   }

3. for (var i = 1; i <= 4; i++) {
     setTimeout(function () {
       console.log(i);
     }, i * 1000);
   }

4. for (var i = 1; i <= 4; i++) {
     (function (j) {
       setTimeout(function () {
         console.log(j);
       }, j * 1000);
     })(i);
   }

5. for (var j = 1; j <= 4; j++) {
     setTimeout(function () {
       console.log(j);
     }, j * 1000);
   }

Relevant resource(s):

• Immediately Invoked Function Expression (IIFE).
• The specific case scenario of using a for statement with var and setTimeout():
  • Thrown for a loop: understanding for loops and timeouts in JavaScript.
  • How to resolve setTimeout() inside for loop.

This question is quite tricky, and the concepts behind the solutions are challenging. Take the time to read the resources I mention. Don't worry if you don't get it straight away, these are quite advanced concepts.

Solution

1. 1
2. 10
3. ReferenceError
4. undefined

Relevant resource(s):

• var.
• Demystifying JavaScript variable scope and hoisting.

Solution

1. 1 then 1.
2. 1 then undefined.
3. undefined then undefined.
4. undefined then 1.

Relevant resource(s):

• Understanding Hoisting in JavaScript.
• Demystifying JavaScript variable scope and hoisting.

Solution

1. <>
2. ~
3. ==!
4. !==

Relevant resource(s):

• comparison operators.

Solution

1. ReferenceError
2. true
3. 0
4. false

Relevant resource(s):

• Logical AND (&&).

Solution

1. ++
2. --
3. ==
4. ||

Relevant resource(s):

• Short-circuit evaluation.

Solution

1. true
2. undefined
3. []
4. false

Relevant resource(s):

• Why are two identical objects not equal to each other?

Note: in JavaScript, arrays are objects.

Solution

1. typeof
2. delete
3. instanceof
4. void

Relevant resource(s):

• How to use JavaScript unary operators.
• typeof.
• delete.
• instanceof.
• void.

Solution

1. 'One'
2. undefined
3. true
4. 'Two'

Explanation

The expression 6 % 2 produces (or evalutes to) 0. Therefore, the variable x is assigned the integer 0.

0 is a falsy value in JavaScript. Therefore, the second part of the ternary operator applies. The variable y is assigned the string 'Two'.

Relevant resource(s):

• Remainder (%).
• Conditional (ternary) operator.
• Falsy.

Solution

1. The condition in the ternary is using the assignment operator.
2. You can't define a variable without initializing it.
3. You can't use a ternary in the right-hand side of an assignment operator.
4. The variable a is not initialized with a value.

Explanation

There is no point in using a ternary operator if the condition always evaluates to true. The assignment expression (a = 3) always evaluates to 3 (the assigned value), which is a truthy value.

Note that the assigment expression (a = 0) always evaluate to false, since 0 is a falsy value.

Relevant resource(s):

• Assignment (=).
• Conditional (ternary) operator.
• Falsy.

Solution

1. [3] == [3]
2. 3 == '3'
3. 3 != '3'
4. 3 === '3'

Relevant resource(s):

• Strict vs loose equality explained.

Solution

1. 4
2. 10
3. 6
4. 5

Relevant resource(s):

• The increment operator.

My advice: don't use the increment operator in your code. It's best to use +=. It clarifies the intent of your code and prevents from any undesired or forgotten side-effect.

Solution

1. a['x'] === b['x']
2. a != b
3. a === b
4. a.x === b.x

Relevant resource(s):

• Object equality in JavaScript.

Solution

1. conservation && !deforestation
2. !deforestation && !conservation
3. !conservation || deforestation
4. deforestation && conservation || deforestation

Relevant resource(s):

• Logical AND (&&).
• Logical OR (||).
• Logical NOT (!).

Solution

1. 18
2. 10
3. 9
4. 20

Explanation

First, the expression a++ is evaluated. It evaluates to the integer 5.

Then, the expression (+(+(+b))) is evaluated, from the most inner parenthesis to the most outer ones.

(+b) evaluates to the positive integer +4, which is the same as 4.

Similarly, (+(4)) evaluates to +4, which is the same as 4.

Finally, the expression (5(+4)) evaluates to (5 + 4), which is 9.

Relevant resource(s):

• Grouping operator ( ).

Solution

1. It is used to spread iterables to individual elements.
2. It describe the datatype of undefined.
3. No such operator exists.

Relevant resource(s):

• The 3 dots syntax.

Solution

1. test1 == test2;
2. return test1 === test2;
3. return test1 == test2;
4. return test1 != test2;

Relevant resource(s):

• Equality (==).

Solution

1. exception('One or both parameters are not numbers');
2. catch('One or both parameters are not numbers');
3. error('One or both parameters are not numbers');
4. throw('One or both parameters are not numbers');

Relevant resource(s):

• The throw statement.

When using throw, it is common to throw an Error object rather than a string:

throw new Error('One or both parameters are not numbers');

However, the syntax with the string as used above is correct:

throw('One or both parameters are not numbers');

Solution

1. When you want to reuse a set of statements multiple times.
2. When you want your code to choose between multiple options.
3. When you want to group data together.
4. When you want to loop through a group of statement.

Relevant resource(s):

• Making decisions in your code — conditionals.

Solution

1. 1 2 3 4 5
2. 1 2 3 4
3. 0 1 2 3 4
4. 0 1 2 3 4 5

Relevant resource(s):

• The for statement.

Solution

1. throw
2. exception
3. catch
4. error

Relevant resource(s):

• throw.

Solution

1. 10
2. true
3. false
4. 0

Relevant resource(s):

• if...else.

Solution

1. break
2. pass
3. skip
4. continue

Relevant resource(s):

• continue.

Solution

1. else
2. when
3. if
4. switch

Relevant resource(s):

• switch.

Solution

1. do…while
2. forEach
3. while
4. for

Relevant resource(s):

• do...while.

Solution

1. const vowels = 'aeiou'.toArray();
2. const vowels = Array.of('aeiou');
3. const vowels = {'a', 'e', 'i', 'o', 'u'};
4. const vowels = 'aeiou';

Relevant resource(s):

• How to index, split, and manipulate strings in JavaScript.

Solution

1. 2
2. 4
3. 6
4. 8

Explanation

The key is to know how many iterations the while loop allows. This depends on the values of rainForestAcres and animals before each iteration:

1. Before 1st iteration. rainForestAcres: 10, animals: 0.
2. Before 2nd iteration. rainForestAcres: 11, animals: 2.
3. Before 3rd iteration. rainForestAcres: 12, animals: 4.
4. Before theoritical 4th iteration. rainForestAcres: 13, animals: 6. Since rainForestAcres < 13 is false, there is no 4th iteration.

Solution

1. Object.get()
2. Object.loop()
3. Object.each()
4. Object.keys()

Relevant resource(s):

• Object methods.

Solution

1. 'pie'
2. The code will throw an error.
3. 'pudding'
4. undefined

Relevant resource(s):

• Working with objects.

Solution

1. function
2. undefined
3. Function.prototype
4. The global object

Relevant resource(s):

• Demystifying the JavaScript this keyword.
• Global object.

Solution

1. The function bar needs to be defined as a key/value pair.
2. Trailing commas are not allowed in JavaScript.
3. Functions cannot be declared as properties of objects.
4. Nothing, everything is fine.

Relevant resource(s):

• Javascript object methods.

Solution

1. delete name from foo;
2. delete foo.name;
3. del foo.name;
4. remove foo.name;

Relevant resource(s):

• delete.

Solution

1. Nothing, it throws an error.
2. 0 2
3. undefined 2
4. 1 2

Relevant resource(s):

• Spread syntax (...).
• Duplicate property names.

Solution

1. Objects are more efficient in terms of storage.
2. Adding a record to an object is significantly faster than pushing a record into an array.
3. Most operations involve looking up a record, and objects are more adpated than arrays to do this.
4. Working with objects makes the code more readable.

Explanation

Records in an object can be retrieved using their key which can be any given value (e.g. an employee ID, a city name, etc.), whereas to retrieve a record from an array we need to know its index.

Solution

1. It will throw a TypeError.
2. ["Name", "Age", "HairColor"]
3. ["DAVE", 40, "BLUE"]
4. ["NAME", "AGE", "HAIRCOLOR"]

Relevant resource(s):

• Object.keys().
• Array.prototype.map().
• String.prototype.toUpperCase()).

Solution

1. obj?.a.b
2. obj.a?.b
3. obj[a][b]
4. obj.?a.?b

Relevant resource(s):

• Optional chaining (?.).

Solution

1. Nothing is printed to the console.
2. 'grunt'
3. undefined
4. 'roar'

Explanation

When the function bearSound() is executed, the property sound of the object referred by this is printed to the screen.

In the global scope in which bearSound() is executed, this refers to the global object, either window or global depending on where you are running the code (in a browser or on Node).

Since the global object doesn't have a sound property, undefined is printed to the screen.

Relevant resource(s):

• Demystifying the JavaScript this keyword.
• Global object.

Solution

1. 'tiger' 'large'
2. 'lion' undefined
3. undefined 'large'
4. 'lion' 'large'

Explanation

On the first line, an object cat is created. Its prototype is the object { type: 'lion' }, i.e. the argument of Object.create().

This means that cat.type returns the string 'lion', thanks to prototypal inheritance. The JavaScript engine first looks for a property named type on the cat object. If it doesn't find one, it looks in the prototype. Here, the prototype happens to have a property named type, so the JavaScript engine stops climbing up the prototype chain. But note that the type property is not an own property of cat. It is "inherited" from its prototype. In other words, at this point of the code, the object cat doesn't have any own property, i.e. properties defined at its "level".

The second line of the code adds an own property named size to the cat object. At this point, the cat object looks like this: { size: 'large' }.

Then, a copyCat object is created by using the spread operator (...). At this point, the copyCat object looks like this: { size: 'large' }. Thanks to the use of the spread operator and to the fact that the size property has a primitive value (a string), the copyCat object happens to be a deep copy of the cat object. In other words, the 2 objects are totally independent from one another: an update of the value of the property size on 1 object doesn't affect the other object and vice versa.

Then, a property named type is added to the cat object. This doesn't change the copyCat object. Also, the cat object now has an own property named type. This own property overrides the type property defined on the prototype of cat. A this point, cat.type now returns the string 'tiger' instead of the string 'lion'.

To sum up, this is what the 2 objects look like:

1. cat: { size: 'large', type: 'tiger' }.
2. copyCat: { size: 'large' }.

copyCat doesn't have a type property, and none of its prototypes do. Note that the prototypes of copyCat are: Object.prototype then null. So copyCat.type returns the primitive value undefined.

copyCat has an own property size. copyCat.size returns the string 'large'.

Relevant resource(s):

• Object.create().
• Spread syntax (...).
• 3 ways to copy objects in JavaScript.

Solution

1. first: 30 , second: 50.
2. first: 50 , second: 30.
3. first: 30 , second: 20.
4. None of the other solutions.

Relevant resource(s):

• Duplicate property names.

Solution

1. You are calling a method named reduce on undefined.
2. You are calling a method named reduce on an object that does not exist.
3. You are calling a method named reduce on an empty array.
4. You are calling a method named reduce on null.

Relevant resource(s):

• TypeError: "x" has no properties.

Solution

1. Only a for statement uses a callback function.
2. A for statement is generic, but a forEach statement can be used only with an array.
3. Only a forEach statement lets you specify your own iterator.
4. A forEach statement is generic, but a for statement can be used only with an array.

Relevant resource(s):

• for.
• Array.prototype.forEach().

Solution

1. roadTypes.2
2. roadTypes[3]
3. roadTypes.3
4. roadTypes[2]

Relevant resource(s):

• Arrays.

Solution

1. 101
2. 3
3. 4
4. 100

Relevant resource(s):

• Dense and sparse arrays.

Solution

1. 1
2. undefined
3. NaN
4. This is not proper JavaScript syntax and an error is thrown.

Relevant resource(s):

• Destructuring assignment.

Solution

1. The forEach() method returns a new array with a transformation applied on each item in the original array, whereas the map() method iterates through an array with no return value.
2. The forEach() method returns a single output value, whereas the map() method performs operation on each value in the array.
3. The map() method returns a new array with a transformation applied on each item in the original array, whereas the forEach() method iterates through an array with no return value.
4. None of the other solutions.

Explanation

The following answer is almost right: The map() method returns a new array with a transformation applied on each item in the original array, whereas the forEach() method iterates through an array with no return value.

Almost, because the forEach() method, like every other methods in JavaScript, does return a value. In this case, it returns the primitive value undefined.

Relevant resource(s):

• Array.prototype.map().
• Array.prototype.forEach().

Solution

1. ["Amazon","Borneo","Cerrado","Congo"]
2. ["Cerrado", "Congo"]
3. ["Congo"]
4. ["Amazon","Borneo"]

Relevant resource(s):

• Array.prototype.splice().

Solution

1. const [one, two, three, four, five] = numbers;
2. const { one, two, three, four, five } = numbers;
3. const [one, two, three, four, five] = [numbers];
4. const { one, two, three, four, five } = { numbers };

Relevant resource(s):

• Destructuring assignment.

Solution

1. animals.filter(e => e === "jaguar");
2. animals.reverse();
3. animals.shift();
4. animals.pop();

Relevant resource(s):

• Array.prototype.pop().

Solution

1. matrix[2][2]
2. matrix[1][1]
3. matrix[1,2]
4. matrix[1][2]

Relevant resource(s):

• Array.

Solution

1. It adds 'Lizard' to the start of the animals array.
2. It adds 'Lizard' to the end of the animals array.
3. It replaces 'Rabbit' with 'Lizard' in the animals array.
4. It replaces 'Cat' with 'Lizard' in the animals array.

Relevant resource(s):

• Array.prototype.unshift().

Solution

1. [2, 3, 4, 5, 6, 7]
2. [3, 5, 7, 2, 4, 6]
3. [[2, 4, 6], [3, 5, 7]]
4. [2, 4, 6, 3, 5, 7]

Relevant resource(s):

• Spread syntax (...).

Solution

1. array.slice()
2. array.shift()
3. array.push()
4. array.replace()

Relevant resource(s):

• Array.

Solution

1. [4, 5, 6, 7, 8, 9, 10]
2. [4, 5, 6, 7]
3. [1, 2, 3, 4, 5, 6]
4. [4, 5, 6]

Relevant resource(s):

• Array.prototype.map().
• Array.prototype.filter().

Solution

1. for (let key of cipherText.keys()) {
     plainText += key % 2 === 0 ? key : ' ';
   }

2. for (let [index, value] of cipherText.entries()) {
     plainText += index % 2 !== 0 ? value : '';
   }

3. for (let [index, value] of cipherText.entries()) {
     plainText += index % 2 === 0 ? value : '';
   }

4. for (let value of cipherText) {
     plainText += value;
   }

Relevant resource(s):

• Array.prototype.entries().
• Iterator.

Solution

1. 'Snorlax'
2. 'Jigglypuff'
3. 'Charmander'
4. 'Squirtle'

Relevant resource(s):

• Array.prototype.pop().

Solution

1. animals.every(key)
2. animals.some(key).length === 1
3. animals.filter(key) === true
4. animals.some(key)

Relevant resource(s):

• Array.prototype.every().
• Array.prototype.filter().
• Array.prototype.some().

Solution

1. 3
2. 4
3. 6
4. 7

Relevant resource(s):

• Array.prototype.push().
• Array.prototype.pop().
• Array.prototype.reduce().

Solution

1. 'bear' 'bear' 'tiger' 'sheep'.
2. 'lion' 'bear' 'sheep' 'sheep'.
3. 'bear' 'bear' 'tiger' 'tiger'.
4. 'lion' 'bear' 'tiger' 'sheep'.

Explanation

On line 2, the method slice() creates a shallow copy of animals which is assigned to the variable clones.

animals[0] and clones[0] reference (or point to) the same object. In other words, any change of the object referenced by animals[0] or clones[0] will impact both arrays.

On the other hand, animals[1] and clones[1] point to 2 different primitive values. So index 1 of an array can be reassigned a new value with no consequence on index 1 of the other array.

Relevant resource(s):

• Shallow copy.

Solution

1. addTax = 50;
2. return addTax 50;
3. addTax(50);
4. addTax 50;

Relevant resource(s):

• Parameters and arguments in JavaScript.

Solution

1. It reloads the document whenever the value changes.
2. It returns a reference to a variable in its parent scope.
3. It completes execution without returning.
4. It copies a local variable to the global scope.

Relevant resource(s):

• Closures.
• How do JavaScript closures work?

Solution

1. let discountPrice = function (price) {
     return price * 0.85;
   };

2. let discountPrice(price) {
     return price * 0.85;
   };

3. let function = discountPrice(price) {
     return price * 0.85;
   };

4. discountPrice = function (price) {
     return price * 0.85;
   };

Relevant resource(s):

• Function expressions.

Solution

1. arguments
2. args
3. argsArray
4. argumentsList

Relevant resource(s):

• The arguments object.

Solution

1. 30, ReferenceError, 30, -10
2. 30, ReferenceError
3. 30, -10
4. ReferenceError, -10

Relevant resource(s):

• JavaScript hoisting.

Solution

1. Generator function
2. Async function
3. Arrow function
4. Promise function

Relevant resource(s):

• Generators.

Solution

1. 2
2. 1
3. Nothing - this code will throw an error.
4. undefined

Relevant resource(s):

• How do JavaScript closures work?

Solution

1. => ({})
2. => {}
3. => { return {};}
4. => (({}))

Relevant resource(s):

• JavaScript tip: return object literal from an arrow function.

Solution

1. (a, b) => c
2. a, b => {return c;}
3. a, b => c
4. { a, b } => c

Relevant resource(s):

• Arrow functions, the basics.

Solution

1. function() { console.log('lorem ipsum'); }()();
2. function() { console.log('lorem ipsum'); }();
3. (function() { console.log('lorem ipsum'); })();

Relevant resource(s):

• IIFE.

Solution

1. bear.bind(roar);
2. roar.bind(bear);
3. roar.apply(bear);
4. bear[roar]();

Relevant resource(s):

• Function.prototype.apply().
• Function.prototype.bind().

Solution

1. null 'no arg'
2. 'no arg' 'no arg'
3. null null
4. 'no arg' null

Relevant resource(s):

• Default parameters.

Solution

1. 8
2. 6
3. 2
4. 12

Relevant resource(s):

• Default parameters.
• Spread syntax (...).

Solution

1. const student = new Person();
2. const student = construct Person;
3. const student = Person();
4. const student = construct Person();

Relevant resource(s):

• The new operator.

Solution

1. a.prototype.belly[0]
2. Object.getPrototype0f (a).belly[0]
3. Animal.belly[0]
4. a.belly[0]

Relevant resource(s):

• JavaScript static methods.

Solution

1. Storm()
2. undefined
3. 'rain'
4. 'snow'

Explanation

The key for answering this question correctly is to have a clear view of the prototype chain. Here it is, from bottom to top:

1. the bob object, which happens to be a WinterStorm instance.
2. WinterStorm.prototype, which happens to be a Storm instance.
3. Storm.prototype.

To return the value of bob.precip, the JavaScript engine climbs up the prototype chain, checking at each "level" if the object has an own property named precip.

bob doesn't have an own property named precip, so the JavaScript goes 1 level up to look for it.

WinterStorm.prototype has an own property precip, with the value 'snow'. The JavaScript engine has found what it was looking for and stops going up the prototype chain.

Relevant resource(s):

• Inheritance and the prototype chain.

Solution

1. undefined
2. window
3. {desc: "logger"}
4. function

Relevant resource(s):

• The new operator.
• 'use strict'. Here, the 'use strict' doesn't impact the rest of the code.

Solution

1. Student.parent = Person;
2. Student.prototype = new Person();
3. Student.prototype = Person;
4. Student.prototype = Person();

Relevant resource(s):

• Inheritance and the prototype chain.

Solution

1. x.get('Y')
2. x.Y
3. x.Y()
4. x.get().Y

Relevant resource(s):

• get.
• JavaScript Getters and Setters.

Solution

1. undefined.
2. 'hello'.
3. an object.
4. an error message.

Explanation

The sayHello() function, like any other JavaScript function, is an object which is an instance of the Function object.

The prototype of sayHello() is Function.prototype, which is an object.

That is why the following statement prints true:

console.log(Object.getPrototypeOf(sayHello) === Function.prototype);
// prints true

Relevant resource(s):

• Object prototypes.

Solution

1. class
2. generator function
3. map
4. proxy

Relevant resource(s):

• Classes.

Solution

1. create()
2. new()
3. constructor()
4. init()

Relevant resource(s):

• constructor.

Solution

1. calculate(50);
2. new TaxCalculator.calculate(50);
3. TaxCalculator.calculate(50);
4. new TaxCalculator().calculate(50);

Relevant resource(s):

• static.

Solution

1. undefined
2. 60
3. 80
4. null

Relevant resource(s):

• static.

Solution

1. Object.prototype.
2. All objects have prototype.
3. None of the objects have prototype.
4. Object.

Relevant resource(s):

• Prototype chain.

Solution

1. 3
2. 2
3. 0
4. 1

Explanation

Here is the prototype chain for the array arr, from bottom to up:

1. Array.prototype. Since arr is an instance of the Array object, its prototype is Array.prototype.
2. Object.prototype. Since Array.prototype is an object, its prototype is Object.prototype
3. null, which is the prototype of Object.prototype, but it doesn't count as a prototype in the prototype chain.

So arr has 2 prototypes in its prototype chain.

Relevant resource(s):

• Prototypal inheritance.
• Understanding prototypes and inheritance in JavaScript.

Solution

1. a node.
2. an instance variable.
3. a prototype.
4. an accessor.

Relevant resource(s):

• Inheritance and the prototype chain.

Solution

1. /[0-9]{2,}:[0-9]{2,}:[0-9]{2,}/
2. /\d\d:\d\d:\d\d/
3. /[0-9]+:[0-9]+:[0-9]+/
4. / : : /

Explanation

The regular expression /\d\d:\d\d:\d\d/ is the most correct because it will only match 2 digit time values. For example: '12:00:32'.

The regular expression /[0-9]{2,}:[0-9]{2,}:[0-9]{2,}/ would have worked if the repetitions range looked like [0-9]{2}. However, because of the comma, it will select 2 or more digits. For example, '120:000:321' matches.

The regular expression /[0-9]+:[0-9]+:[0-9]+/ will match any range of time digits, single and multiple. For instance, '1:2:3' and '111:222:333' match.

Relevant resource(s):

• Regular expressions.

Solution

1. JSON.fromString();
2. JSON.parse()
3. JSON.toObject()
4. JSON.stringify()

Relevant resource(s):

• Working with JSON.
• JSON.parse().

Solution

1. You can iterate over values in a Map in their insertion order.
2. You can count the records in a Map with a single method call.
3. Keys in Maps can be strings.
4. You can access values in a Map without iterating over the whole collection.

Relevant resource(s):

• Maps. In particular, this article has a useful table comparing Objects and Maps.

Solution

1. Object
2. Set
3. Array
4. Map

Relevant resource(s):

• Set.

Solution

1. flagDatabase.countries[1].flag
2. flagDatabase.countries[0].flag
3. flagDatabase[1].flag
4. flagsJSON.countries[0].flag

Relevant resource(s):

• JSON.parse().
• Array.

Solution

1. cat.toJSON('type');
2. JSON.stringify(cat, ['type']);
3. JSON.stringify(cat);
4. JSON.stringify(cat, /type/);

Relevant resource(s):

• JSON.stringify().

Solution

1. Date.
2. FileUpload.
3. Function.
4. All of the other solutions.

Solution

1. clearInterval.
2. clearTimer.
3. intervalOver.
4. None of the other solutions.

Relevant resource(s):

• setInterval() global function.
• clearInterval() global function.

Solution

1. import _ from 'lodash';
2. import 'lodash' as _;
3. import '_' from 'lodash;
4. import lodash as _ from 'lodash';

Relevant resource(s):

• import.

Solution

1. import printMe from './some-file';
2. import { printMe } from './some-file';
3. import default as printMe from './some-file';
4. const printMe = import './some-file';

Relevant resource(s):

• export and import.

Solution

1. after 10 seconds and after 'Pending'
2. after 10 seconds but before 'Pending'
3. after 10000 seconds and after 'Pending'
4. immediately and before 'Pending'

Relevant resource(s):

• The setTimeout() global function.
• Blocking and non-blocking JavaScript. setTimeout() is a non-blocking (or asynchronous) function.

Solution

1. to start tasks that might take some time without blocking subsequent tasks from executing immediately.
2. to ensure that tasks further down in your code are not initiated until earlier tasks have completed.
3. to make your code faster.
4. It is not possible to make code asynchronous. Either it already is or it isn't. We have to deal with it as it is.

Relevant resource(s):

• JavaScript: blocking and non-blocking functions.

Solution

1. I
   JavaScript!
   love

2. love
   I
   JavasSript!

3. I
   love
   JavaScript!

4. The output may change with each execution of code and cannot be determined.

Relevant resource(s):

• The setTimeout() global function.
• Blocking and non-blocking JavaScript. setTimeout() is a non-blocking (or asynchronous) function.

Solution

1. done()
2. then()
3. finally()
4. catch()

Relevant resource(s):

• Fetch API.

Solution

1. Every object in the program has to be a function.
2. Code is grouped with the state it modifies.
3. Date fields and methods are kept in units.
4. Side effects are not allowed.

Relevant resource(s):

• Master the JavaScript interview: what is Functional Programming?

Solution

1. overloading
2. closure
3. currying
4. overriding

Relevant resource(s):

• Currying in JavaScript explained with examples.