Answer: Object. Actually Array is derived from Object. If you want to check array use Array.isArray(arr)
Answer: Object. arguments are array like but not array. it has length, can access by index but can't push pop, etc.
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
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"
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.
Answer: "124". For this one "1" + 2 will produce "12" and "12"+4 will generates "124".
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.
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.
Answer: "undefined"
Answer: 5. The comma operator evaluates each of its operands (from left to right) and returns the value of the last operand. ref: MDN
Answer: 8
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.
Answer: 12.3
Answer: false
Answer: true
Answer: true
Answer: "undefined"
Answer: "object"
Answer: 3
var foo = 'outside';
function logIt() {
console.log(foo); var foo = 'inside';
}
logIt();
Answer: undefined
Answer:-1. the result of remainder always get the symbol of first operand
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).
why-does-adding-two-decimals-in-javascript-produce-a-wrong-result
Answer: "42"
Answer: //SyntaxError: Unexpected token .
Answer: "42"
Question: typeof(NaN)
Answer:"number"
Question: 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.