Javascript

Question 1

What are the differences between null and undefined?

Answer 1

JavaScript has two distinct values for nothing, null and undefined.

undefined means, value of the variable is not defined.

null means empty or non-existent value which is used by programmers to indicate “no value”.

Question 2

What are the differences between == and ===?

Answer 2

== will not check types and === will check whether both sides are of same type

=== compares the types and values

Question 3

How would you compare two objects in JavaScript?

Answer 3

JavaScript has two different approaches for testing equality. Primitives like strings and numbers are compared by their value, while objects like arrays, dates, and user defined objects are compared by their reference. This means it compares whether two objects are referring to the same location in memory.

Equality check will check whether two objects have same value for same property. To check that, you can get the keys for both the objects. If the number of properties doesn’t match, these two objects are not equal. Secondly, you will check each property whether they have the same value. If all the properties have same value, they are equal.

Question 4

Is ‘false’ is false?

Answer 4

No. Because, it’s a string with length greater than 0. Only empty string is false

Question 5

Is ‘ ‘ is false?

Answer 5

No. Because, it’s not an empty string. There is a white space in it.

Question 6

Is {} truthy or falsey

Answer 6

true. It’s an object. An object without any property is an object can’t be falsy.

Question 7

Is [] truthy or falsey

Answer 7

truthy. It’s an array object (array is child of object) is truthy.

Question 8

As [] is truthy, []==true should also be true. right?

Answer 8

Since left and right side of the equality are two different types, JavaScript can’t compare them directly . Hence, under the hood, JavaScript will convert them to compare. first right side of the equality will be cooereced to a number and number of true would be 1.

After that, JavaScript implementation will try to convert [] by usingtoPrimitive (of JavaScript implementation). since [].valueOf is not primitive will use toString and will get “”

Now you are comparing “” == 1 and still left and right is not same type. Hence left side will be converted again to a number and empty string will be 0.

Finally, they are of same type, you are comparing 0 === 1 which will be false.

Question 9

If you want to use an arbitrary object as value of this, how will you do that?

Answer 9

There are at least three different ways to doing this by using bind, call and apply.

Question 10

How can I tell if 2 is passed as a parameter of a function?

Answer 10

arguments is a local variable, available inside all functions that provides a collection of all the arguments passed to the function. arguments is not an array rather an array like object. It has length but doesn’t have the methods like forEach, indexOf, etc.

First convert arguments to an array by calling slice method on an array and pass arguments. After that simply use indexOf.

Question 11

What is typeof []

Answer 11

Object. Actually Array is derived from Object. If you want to check array use Array.isArray(arr)

Question 12

What is typeof arguments

Answer 12

Object. arguments are array like but not array. it has length, can access by index but can’t push pop, etc.

Question 13

What is 2+true

Answer 13

3. The plus operator between a number and a boolean or two boolean will convert boolean to number. Hence, true converts to 1 and you get result of 2+1

Question 14

What is ‘6’+9

Answer 14

69. If one of the operands of the plus (+) operator is string it will convert other number or boolean to string and perform a concatenation. For the same reason, “2”+true will return “2true”

Question 15

What is the value of 4+3+2+”1”

Answer 15

91 . The addition starts from the left, 4+3 results 7 and 7+2 is 9. So far, the plus operator is performing addition as both the operands are number. After that 9 + “1” where one of the operands is string and plus operator will perform concatenation.

Question 16

What is the value of “1”+2+4

Answer 16

“124”. For this one “1” + 2 will produce “12” and “12”+4 will generates “124”.

Question 17

What is the value of -‘34’+10

Answer 17

-24. minus(-) in front of a string is an unary operator that will convert the string to a number and will make it negative. Hence, -‘34’ becomes, -34 and then plus (+) will perform simple addition as both the operands are number.

Question 18

What is the value of +’dude’

Answer 18

NaN. The plus (+) operator in front of a string is an unary operator that will try to convert the string to number. Here, JavaScript will fail to convert the “dude” to a number and will produce NaN.

Question 19

If you have var y = 1, x = y = typeof x; What is the value of x?

Answer 19

“undefined”

Question 20

for var a = (2, 3, 5); what is the value of a?

Answer 20

5. The comma operator evaluates each of its operands (from left to right) and returns the value of the last operand.

Question 21

for var a = (1, 5 - 1) * 2 what is the value of a?

Answer 21

8

Question 22

What is the value of !’bang’

Answer 22

false. ! is NOT. If you put ! in front of truthy values, it will return false. Using !! (double bang) is a tricky way to check anything truthy or falsy by avoiding implicit type conversion of == comparison.

Question 23

What is the value of parseFloat(‘12.3.4’)

Answer 23

12.3

Question 24

What is the value of Math.max([2,3,4,5]);

Answer 24

NaN. Function should be written Math.max(…[2,3,4,5]);

Question 25

If var a = 2, b =3 what would be value of a && b

Answer 25

3

Question 26

Does JavaScript pass parameter by value or by reference?

Answer 26

Primitive type (string, number, etc.) are passed by value and objects are passed by reference. If you change a property of the passed object, the change will be affected. However, you assign a new object to the passed object, the changes will not be reflected.

Question 27

How would you implement currying for any functions?

Answer 27

Curring is partial invocation of a function. Currying means first few arguments of a function is pre-processed and a function is returned. The returning function can add more arguments to the curried function.

function addBase(base){
  return function(num){
    return base + num;
  }
}

var addTen = addBase(10);
addTen(5); //15
addTen(80); //90
addTen(-5); //5

Question 28

What’s the difference between undefined and not defined in JavaScript

Answer 28

In JavaScript if you try to use a variable that doesn’t exist and has not been declared, then JavaScript will throw an error var name is not defined and the script will stop execute thereafter. But If you use typeof undeclared_variable then it will return undefined.

var x is a declaration because you are not defining what value it holds yet, but you are declaring its existence and the need for memory allocation.

var x = 1 is both declaration and definition (also we can say we are doing initialisation)

Question 29

What is the drawback of declaring methods directly in JavaScript objects?

Answer 29

One of the drawback of declaring methods directly in JavaScript objects is that they are very memory inefficient. When you do that, a new copy of the method is created for each instance of an object.

Question 30

What is “closure” in javascript?

Answer 30

A closure is a function defined inside another function (called parent function) and has access to the variable which is declared and defined in parent function scope.

The closure has access to the variable in three scopes:

Variable declared in his own scope
Variable declared in parent function scope
Variable declared in the global namespace

Question 31

What is the DOM?

Answer 31

The Document Object Model (DOM) is a programming interface for HTML and XML documents. It represents the page so that programs can change the document structure, style, and content. The DOM represents the document as nodes and objects. That way, programming languages can connect to the page.

A Web page is a document. This document can be either displayed in the browser window or as the HTML source. But it is the same document in both cases. The Document Object Model (DOM) represents that same document so it can be manipulated. The DOM is an object-oriented representation of the web page, which can be modified with a scripting language such as JavaScript.