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Question 1

var myArray = [];
myArray[0] = "Val1";
myArray[2] = "Val3";

What is the value of myArray[1]?

Answer 1

undefined

myArray[1] == undefined

Question 2

var customer = {
name: "Lidiya"
};

What is the value of customer.Age?

Answer 2

undefined

customer.Age == undefined

Question 3

What operator can be used to check the type of a variable?

Answer 3

typeof

Question 4

var myName = “Myro”;

console.log( _______ myName);

What operator should be used in front of myName in the second line to get the type of myName to show in the console?

Answer 4

typeof

console.log( typeof myName);

Question 5

var myAge = 38 ;

console.log(typeof myAge);

What will be displayed in the console?

Answer 5

number

Question 6

When is null used in JavaScript?

Answer 6

In places where the object should be but can’t be created of found. For example, if you try the following:

var myObj = document.getElementById(“elm1”);

and elm1 id does not exist, myObj will be set to null.

Question 7

When is undefined used in JavaScript?

Answer 7

For the variable that is not initialised, an object’s missing property or a missing value of an array.

Question 8

var test10 = null;
console.log(typeof test);

What will be output to console?

Answer 8

object

Question 9

var test = 0/0;
console.log(test);

What will be output to console?

Answer 9

NaN

Question 10

var test = "food" * 10;
console.log(test);

What will be output to console?

Answer 10

NaN

Question 11

How to check whether something has NaN value?

Answer 11

isNan() function

For example:

var myNum = 0;
if(isNaN(myNum)){
    //some code
}

Question 12

var test11 = 0 / 0;
console.log(typeof test11);

What will be output to console?

Answer 12

number

Question 13

What happens when you compare a string to a number?

99 == “bob”

Answer 13

The string is converted to a number and then two numbers are compared. In this case “bob” converted to a number is NaN and 99 not equals to NaN.

Question 14

What number does boolean value “true” convert to?

Answer 14

1

Question 15

What number does boolean value “false” convert to?

Answer 15

0

Question 16

Ho will this comparison proceed?

1 == true

Answer 16

“true” will be converted to number 1 and compared to 1. The result will be true

Question 17

How will this comparison proceed?

“1” == true

Answer 17

1. “true” will be converted to number 1
   “1” == 1
2. “1” will be converted to number
   1 == 1

Question 18

is this true?

undefined == null

Answer 18

Yes! That is the rule in JavaScript.

Question 19

What number is the empty string converted to in this case?

1 == “”

Answer 19

”” is converted to number 0

Question 20

How will this comparison proceed?

“true” == true

Answer 20

1. boolean true will be converted to a number 1
2. “true” will be attempted to be converted to a number and the left side will get a value of NaN
3. NaN == 1? false

Question 21

Is there a !== operator?

Answer 21

Yes!

Question 22

var add = "4" + 3;
console.log(add);

What will be output to console?

Answer 22

43

”+” converts the number into the string first, them concatenates

Question 23

var add = 78+ "2";
console.log(add);

What will be output to console?

Answer 23

782

”+” converts the number into the string first, them concatenates

Question 24

var multi= 4 * "2";
console.log(multi);

What will be output to console?

Answer 24

8

In case of all arithmetic operators besides “+”, the string is first converted to number and then the operation is performed.

Question 25

``` var minus= "7" - 2; console.log(minus); ``` What will be output to console?

Answer 25

5 In case of all arithmetic operators besides "+", the string is first converted to number and then the operation is performed.

Question 26

``` var order = 1 + 2 + " pizzas"; console.log(order); ``` What will be output to console?

Answer 26

3 pizzas "+" operator has left-to-right associativity

Question 27

``` var order = 1 + (2 + " pizzas"); console.log(order); ``` What will be output to console?

Answer 27

12 pizzas concatenating string with number always converts number to string first

Question 28

What function is used to convert string to number?

Answer 28

Number() For example: var num = 3 + Number("4"); num will be set to 7

Question 29

``` var res= "Result: " + 10/2; console.log(res); ``` What will be output to console?

Answer 29

Result: 5 ``` The division will happen first because of the operators' order of precedence: "Result: " + (10/2) | | V "Result: " + 5 ```

Question 30

What are 5 falsey values in JavaScript?

Answer 30

1. undefined 2. null 3. 0 (zero) 4. empty string 5. NaN

Question 31

``` if([]){ console.log("This is true"); } else{ console.log("This is false"); } ``` What will be output to console?

Answer 31

This is true. [] is an empty array and it is not one of the falsey values.

Question 32

``` if(" "){ console.log("This is true"); } else{ console.log("This is false"); } ``` What will be output to console? Note that this is a string with one space in it.

Answer 32

This is true. This is a non-empty string, therefore it is truthy.

Question 33

var uppCase = "HELLO"; How to put a text in variable uppCase into lower case?

Answer 33

use toLowerCase() methodof a string type, like so: alert(uppCase.toLowerCase());

Question 34

var emailVar = "hello@gmail.com"; How can we get a string containing "@" from variable emailVar?

Answer 34

var newStr = emailVar.charAt(5);

Question 35

``` var strVar = "I like movies"; console.log(strVar.indexOf("mov")); ``` What will be output to console?

Answer 35

7 indexOf() function of the string object returns the index of of the first character of the first occurrence of the argument.

Question 36

``` var strVar = "I like to move it move it."; console.log(strVar.indexOf("mov")); ``` What will be output to console?

Answer 36

10 Although, indexOf() returns the index of the first character of the argument, it only returns the index of the first character in the FIRST OCCURRENCE of the argument.

Question 37

var strVar = "I like to move it move it."; How can you get the index of the second occurrence of "move" in strVar?

Answer 37

Use indexOf() method of string object with the second parameter. Second parameter is an integer that indicates where in the string should the search begin. console.log(strVar.indexOf("mov", 11)); Guess what would be output to console?

Question 38

``` var strVar = "I like to move it move it."; console.log(strVar.indexOf("bleh")); ``` What will be output to console?

Answer 38

-1 indexOf() method returns -1 if the string is not found

Question 39

var strVar = "car/house/road"; How will you extract "house" string from strVar variable?

Answer 39

var extracted = strVar.substring(4,9);

Question 40

var strVar = "car/house/road"; How will you extract "house/road" string from strVar variable?

Answer 40

var extracted = strVar.substring(4); substring() method of string object, if used with only one parameter, returns the string that begins at the first parameter's index and ends at the end of the string.

Question 41

var people = "Jack,Jane,John"; What method of the sting object can we use to put each person into an element in array in one operation?

Answer 41

split() var personArr = people.split(",");

Question 42

What are two methods of string object that can be used to get a portion of the string from the original string?

Answer 42

1. slice() | 2. substring()

Question 43

What method of string object is used to get the last index of a substring in the original string?

Answer 43

lastIndexOf()

Question 44

What method of string object is used to add another string to the current one?

Answer 44

concat()

Question 45

How can you get a user input from a browser dialogue?

Answer 45

prompt() method. This is window's method. You can use either prompt() or window.prompt(). var userInput = prompt("What is your name?");