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Collection-of-Tricky-JS-Questlions.md

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1- Question: What is typeof []

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

Question: What is typeof arguments

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

Question: What is 2+true

Answer: 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: What is '6'+9

Answer: 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: What is the value of 4+3+2+"1"

Answer: 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: What is the value of "1"+2+4

Answer: "124". For this one "1" + 2 will produce "12" and "12"+4 will generates "124".

Question: What is the value of -'34'+10

Answer: -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: What is the value of +'dude'

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

Answer: "undefined"

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

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

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

Answer: 8

Question: What is the value of !'bang'

Answer: 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: What is the value of parseFloat('12.3.4')

Answer: 12.3

Question: What is value of 3 instanceof Number

Answer: false

Question:null == undefined

Answer: true

Question:What is the value of !!function(){};

Answer: true

Question: What is the value of typeof bar

Answer: "undefined"

Question: What is the value of typeof null

Answer: "object"

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

Answer: 3

Question: What would be consoled

var foo = 'outside';

function logIt() {
  console.log(foo); var foo = 'inside';
  }
logIt();

Answer: undefined

Question: What is -5%2

Answer:-1. the result of remainder always get the symbol of first operand

Question: Why .1+.2 != .3

Answer: This is not a javascript only limitation, it applies to all floating point calculations. The problem is that 0.1 and 0.2 and 0.3 are not exactly representable as javascript (or C or Java etc) floats. Thus the output you are seeing is due to that inaccuracy.

In particular only certain sums of powers of two are exactly representable. 0.5 = =0.1b = 2^(-1), 0.25=0.01b=(2^-2), 0.75=0.11b = (2^-1 + 2^-2) are all OK. But 1/10 = 0.000110001100011..b can only be expressed as an infinite sum of powers of 2, which the language chops off at some point. Its this chopping that is causing these slight errors.

Further Explanation

From The Floating-Point Guide:

Why don’t my numbers, like 0.1 + 0.2 add up to a nice round 0.3, and instead I get a weird result like 0.30000000000000004?

Because internally, computers use a format (binary floating-point) that cannot accurately represent a number like 0.1, 0.2 or 0.3 at all. When the code is compiled or interpreted, your “0.1” is already rounded to the nearest number in that format, which results in a small rounding error even before the calculation happens.

The site has detailed explanations as well as information on how to fix the problem (and how to decide whether it is a problem at all in your case).

For a very detailed ans to this above question, collected from various blogs - see this file

why-does-adding-two-decimals-in-javascript-produce-a-wrong-result

Question: 42..toString()

Answer: "42"

Question: 4.2..toString

Answer: //SyntaxError: Unexpected token .

Question:42 . toString()

Answer: "42"

Question: What is typeof(NaN) ? And why is NaN === NaN or NaN == NaN returning false.

Answer:"number"

NaN != NaN because they are not necessary the SAME non-number. Thus it makes a lot of sense... Also why floats have both +0.00 and -0.00 that are not the same. Rounding may do that they are actually not zero.

As for typeof, that depends on the language. And most languages will say that NaN is a float, double or number depending on how they classify it... I know of no languages that will say this is an unknown type or null.

More Explanations

It is not a peculiarity of javascript but common computer science principle.

From http://en.wikipedia.org/wiki/NaN:

There are three kinds of operation which return NaN:

Operations with a NaN as at least one operand

Indeterminate forms

  • The divisions 0/0, ∞/∞, ∞/−∞, −∞/∞, and −∞/−∞
  • The multiplications 0×∞ and 0×−∞
  • The power 1^∞
  • The additions ∞ + (−∞), (−∞) + ∞ and equivalent subtractions.

Real operations with complex results:

  • The square root of a negative number
  • The logarithm of a negative number
  • The tangent of an odd multiple of 90 degrees (or π/2 radians)
  • The inverse sine or cosine of a number which is less than −1 or greater than +1.

All these values may not be the same. A simple test for a NaN is to test value == value is false.

Question: What is 2 in [1,2]

Answer: false. Because "in" returns whether a particular property/index available in the Object. In this case object has index 0 and 1 but don't have 2. Hence you get false.