Collection-of-Tricky-JS-Questlions.md

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