JavaScript Essentials

Question 1

Event executed when the browser fully loaded HTML, and the DOM tree is built, but external resources like pictures <img></img> and stylesheets may be not yet loaded.

Answer 1

document.addEventListener(“DOMContentLoaded”, function() { … });

Question 2

Event executed when not only HTML is loaded, but also all the external resources: images, styles etc.

Answer 2

window.onload = function() { … }

Question 3

JavaScript programs can be inserted into any part of an HTML document. True or false?

Answer 3

True

Question 4

A single tag can’t have both the src attribute and code inside (If src is set, the script content is ignored). True or false?

Answer 4

True

Question 5

The type and language attributes are required in a tag. True or false?

Answer 5

False

Question 6

Will this code run correctly?:

alert(“There will be an error”)

[1, 2].forEach(alert)

Answer 6

No.

The error occurs because JavaScript does not assume a semicolon before square brackets […].

Question 7

Nested comments are supported (there may be /…/ inside another /…/). True or False

Answer 7

False

Question 8

The directive to set strict-mode looks like a string: “use strict” or ‘use strict’. True or False

Answer 8

True

Question 9

The “use strict” directive switches the engine to the “modern” mode, changing the behavior of some built-in features. True or False

Answer 9

True

Question 10

What are the naming rules for variables in JS?

Answer 10

The name must contain only letters, digits, or the symbols $ and _.
The first character must not be a digit.

Question 11

Variables are case sensitive. True or false?

Answer 11

True

Question 12

Are non-Latin letters are allowed in variables?

Answer 12

Yes, but they are not recommended

Question 13

In strict-mode, is it possible to assign or use a variable before defining it?

Answer 13

No, it’s not possible

Question 14

What is the difference between “var”, “let” and “const”?

Answer 14

let – is a modern variable declaration. It’s block-scoped.
var – is an old-school variable declaration. Normally we don’t use it at all. It’s function scoped.
const – is like let, but the value of the variable can’t be changed. It’s also block-scoped.

Question 15

A variable in JavaScript can contain any data. True of false?

Answer 15

True

Question 16

What are the basic data types in JS?

Answer 16

Number, String, Boolean, null value, undefined value, Object, Symbol

Question 17

How do you represent a float in JS?

Answer 17

With a Number variable

Question 18

Besides regular numbers, what are special values for a numeric variable?

Answer 18

Infinity, -Infinity and NaN

Question 19

What is the result of the following code?

alert( 1 / 0 );

Answer 19

Infinity

Question 20

What is the result of the following code?

alert( “not a number” / 2 + 5 );

Answer 20

NaN

Question 21

What is the result of the following code?

alert( the result is ${1 + 2} );

Answer 21

“the result is 3”

Question 22

What is the result of the following code?

alert( “the result is ${1 + 2}” );

Answer 22

“the result is ${1 + 2}”

Question 23

What is the difference between “null” and “undefined”?

Answer 23

Normally, we use null to assign an “empty” or “unknown” value to a variable, and we use undefined for checks like seeing if a variable has been assigned.

Question 24

Is “typeof x” the same as “typeof(x)”?

Answer 24

Yes, it’s the same

Question 25

What returns "typeof"?

Answer 25

A string with the name of the type of the evaluated variable

Question 26

What does this return? typeof undefined

Answer 26

"undefined"

Question 27

What does this return? typeof 0

Answer 27

"number"

Question 28

What does this return? typeof Symbol("id")

Answer 28

"symbol"

Question 29

What does this return? typeof Math

Answer 29

"object"

Question 30

What does this return? typeof null

Answer 30

"object" (That’s wrong. It is an officially recognized error in typeof, kept for compatibility. Of course, null is not an object. It is a special value with a separate type of its own. So, again, this is an error in the language.)

Question 31

What does this return? typeof alert

Answer 31

"function"

Question 32

What does this return? alert( "6" / "2" );

Answer 32

3

Question 33

How do you explicitly convert a value into a number?

Answer 33

let num = Number(str); // becomes a number

Question 34

How do you explicitly convert a value into a string?

Answer 34

let num = String(str); // becomes a number

Question 35

What is the value of age? let age = Number("an arbitrary string instead of a number");

Answer 35

NaN

Question 36

What does this return? alert( 1 + '2' );

Answer 36

"12"

Question 37

What does this return? Boolean(0)

Answer 37

false

Question 38

What does this return? Boolean('')

Answer 38

false

Question 39

What does this return? Boolean(null)

Answer 39

false

Question 40

What does this return? Boolean(undefined)

Answer 40

false

Question 41

What does this return? Boolean(NaN)

Answer 41

false

Question 42

What does this return? Boolean("0")

Answer 42

true

Question 43

What does this return? Boolean(" ")

Answer 43

true

Question 44

What happens if you concatenate a number and a string (or a string plus a number)?

Answer 44

It returns a string. it doesn’t matter whether the first operand is a string or the second one. The rule is simple: if either operand is a string, the other one is converted into a string as well.

Question 45

What does this return? alert(2 + 2 + '1' );

Answer 45

"41"

Question 46

What does this return? alert( 2 - '1' );

Answer 46

1

Question 47

What does this return? alert( '6' / '2' );

Answer 47

3

Question 48

What does this return? alert( +true );

Answer 48

1

Question 49

What does this return? alert( +"" );

Answer 49

0

Question 50

What does the unary (+) plus operator?

Answer 50

The unary plus or, in other words, the plus operator + applied to a single value, doesn’t do anything to numbers. But if the operand is not a number, the unary plus converts it into a number.

Question 51

What does this return? ``` let apples = "2"; let oranges = "3"; ``` alert( +apples + +oranges );

Answer 51

5

Question 52

In the expression "+apples + +oranges" what is executed first? The addition or the unary operators?

Answer 52

The unary operators

Question 53

What does this return? a = b = c = 2 + 2;

Answer 53

a=4, b=4. c=4

Question 54

``` What does this return? let x = 0; if (x=1) { return x; } ```

Answer 54

1

Question 55

What does this return? alert( 8 % 3 );

Answer 55

2

Question 56

What does this return? alert( 2 ** 3 );

Answer 56

8 (2 * 2 * 2)

Question 57

What is the value of "a"? ``` let counter = 1; let a = ++counter; ```

Answer 57

2

Question 58

What is the value of "a"? ``` let counter = 1; let a = counter++; ```

Answer 58

1

Question 59

What does this return? ``` let counter = 1; alert( 2 * ++counter ); ```

Answer 59

4

Question 60

What does this return? ``` let counter = 1; alert( 2 * counter++ ); ```

Answer 60

2

Question 61

What's the value of n? let n = 2;

Answer 61

7

Question 62

What's the value of n? ``` let n = 2; n *= 2; ```

Answer 62

14

Question 63

What will be printed? let a = (1 + 2, 3 + 4); alert( a );

Answer 63

7 (only the last element after the comma and the parenthesis have higher precedence)

Question 64

What will be printed? let a = 1 + 2, 3 + 4 alert( a );

Answer 64

3, it's like t’s like (a = 1 + 2), 3 + 4.

Question 65

How does JavaScript compare to strings in a n expression like this: 'Glow' > 'Glee'

Answer 65

JavaScript uses the so-called “dictionary” or “lexicographical” order (Unicode). In other words, strings are compared letter-by-letter. (It resolves to true)

Question 66

What does the following comparison returns? alert( '2' > 1 );

Answer 66

true, string '2' becomes a number 2

Question 67

What does the following comparison returns? alert( '01' == 1 );

Answer 67

true, string '01' becomes a number 1

Question 68

What does the following comparison returns? alert( true == 1 );

Answer 68

true

Question 69

What does the following comparison returns? alert( false == 0 );

Answer 69

true

Question 70

What happens when you compares values of different types

Answer 70

When comparing values of different types, JavaScript converts the values to numbers.

Question 71

What is the difference between == and === ?

Answer 71

Different types are converted to numbers by the equality operator ==. An empty string, just like false, becomes a zero. A strict equality operator === checks the equality without type conversion.

Question 72

Whats does this return? null === undefined

Answer 72

false, both have different types

Question 73

Whats does this return? null == undefined

Answer 73

true

Question 74

What is the unique behavior of comparing null and undefined?

Answer 74

There’s a special rule. These two are a “sweet couple”: they equal each other (in the sense of ==), but not any other value.

Question 75

Explain the following results: alert( null > 0 ); // (1) false alert( null == 0 ); // (2) false alert( null >= 0 ); // (3) true

Answer 75

Comparisons convert null to a number, treating it as 0. That’s why (3) null >= 0 is true and (1) null > 0 is false. For == (without any convertions), null is only equals to undefined

Question 76

What is the result of comparing NaN to anything else?

Answer 76

Comparing NaN to anything else will always return false

Question 77

Explain the following results: alert( undefined > 0 ); // false (1) alert( undefined < 0 ); // false (2) alert( undefined == 0 ); // false (3)

Answer 77

Comparisons (1) and (2) return false because undefined gets converted to NaN and NaN is a special numeric value which returns false for all comparisons. The equality check (3) returns false because undefined only equals null, undefined, and no other value.

Question 78

What type of variable does the comparison operators return?

Answer 78

boolean

Question 79

What is the difference between the functions "alert", "prompt" and "confirm"?

Answer 79

"alert" shows a message. "prompt" shows a message asking the user to input text. It returns the text or, if Cancel button or Esc is clicked, null. "confirm" shows a message and waits for the user to press “OK” or “Cancel”. It returns true for OK and false for Cancel/Esc.

Question 80

What happens with values inside parenthesis in "if" operators? if (x)

Answer 80

they are converted to boolean

Question 81

What does the following code do? (company == 'Netscape') ? alert('Right!') : alert('Wrong.');

Answer 81

It's a short-hand version of if: If company is equal to 'Netscape', the alert will display 'Right!', else it will display 'Wrong.'

Question 82

What is the value of "result"? result = a || b;

Answer 82

if a is true, a, else b

Question 83

What is the value of "result", if 'a', 'b', 'c', 'd' are a;; false? result = a || b || c || d;

Answer 83

The last value is returned, which is false

Question 84

What's the value of "x"? let x; true || (x = 1); alert(x);

Answer 84

undefined, because (x = 1) not evaluated due the short circuit

Question 85

How does the AND (&&) operator works?

Answer 85

AND returns the first falsy value or the last value if none were found. When all values are truthy, the last value is returned:

Question 86

What's the difference between "break" and "continue"?

Answer 86

break exits the loop, while continue skips to the next iteration

Question 87

What does the following do? (i > 5) ? alert(i) : continue;

Answer 87

Syntax error. Syntax constructs that are not expressions cannot be used with the ternary operator ?. In particular, directives such as break/continue aren’t allowed there.

Question 88

What does the following code do on break? ``` outer: for (let i = 0; i < 3; i++) { for (let j = 0; j < 3; j++) { let input = prompt(`Value at coords (${i},${j})`, ''); if (!input) break outer; // (*) // do something with the value... } } alert('Done!'); ```

Answer 88

if an empty string or canceled, then break out of both loops

Question 89

What's the final value of "userName"? let userName = 'John'; ``` function showMessage() { let userName = "Bob"; // declare a local variable let message = 'Hello, ' + userName; // Bob alert(message); } ``` // the function will create and use its own userName showMessage(); alert( userName );

Answer 89

"John" as the "userName" variable internal to the function shadowed the external one

Question 90

What will the alert print? function showMessage(from, text = "no text given") { alert( from + ": " + text ); } showMessage("Ann");

Answer 90

"Ann: no text given"

Question 91

What does a function without a "return" statement return?

Answer 91

A function with an empty return or without it returns undefined

Question 92

What does the alert print? ``` function sayHi() { alert( "Hello" ); } ``` alert( sayHi );

Answer 92

The function code

Question 93

A Function Expression is created when the execution reaches it and is usable only from that moment. True or false?

Answer 93

True. Once the execution flow passes to the right side of the assignment let sum = function… – here we go, the function is created and can be used (assigned, called, etc. ) from now on. Function Declarations are different. A Function Declaration can be called earlier than it is defined.

Question 94

Can a function declared inside an "if" block be accessed outside it?

Answer 94

In strict mode, when a Function Declaration is within a code block, it’s visible everywhere inside that block. But not outside of it.

Question 95

Is this a valid arrow function? let double = n => n * 2;

Answer 95

Yes, it is valid