Today #Day133 of #365DaysOfCode, Learning Essential JavaScript Interview Questions.
33. What is NaN? What is its type? How can you reliably test if a value is equal to NaN?
The NaN property represents a value that is “not a number”. This special value results from an operation that could not be performed either because one of the operands was non-numeric (e.g., "abc" / 4), or because the result of the operation is non-numeric.
While this seems straightforward enough, there are a couple of somewhat surprising characteristics of NaN that can result in hair-pulling bugs if one is not aware of them.
For one thing, although NaN means “not a number”, its type is, believe it or not, Number:
console.log(typeof NaN === "number"); // logs "true"
Additionally, NaN compared to anything – even itself! – is false:
console.log(NaN === NaN); // logs "false"
A semi-reliable way to test whether a number is equal to NaN is with the built-in function isNaN(), but even using isNaN() is an imperfect solution.
A better solution would either be to use value !== value, which would only produce true if the value is equal to NaN.
Also, ES6 offers a new Number.isNaN() function, which is a different and more reliable than the old global isNaN() function.
console.log(typeof NaN); //number
console.log(typeof typeof NaN); // string
console.log(typeof 'NaN'); // string
console.log(NaN == NaN); // false
console.log(typeof NaN == typeof NaN); // true
console.log(NaN > NaN); // false
console.log(NaN < NaN); // false
console.log(isNaN(1)); // false
console.log(isNaN(NaN)); // true
console.log(isNaN()); // true
console.log(isNaN(null)); // false
console.log(isNaN(undefined)); // true
console.log(isNaN(Array)); // true
console.log(isNaN({})); // true
console.log(isNaN({NaN})); // true
console.log(NaN !== NaN); // true
console.log(Number.isNaN(5)); // false
console.log(Number.isNaN(NaN)); // true
console.log(Number.isNaN()); // false
console.log(Number.isNaN(undefined)); // false
console.log(Number.isNaN({})); // false
34. What will the following code output and why?
var b = 1;
function outer(){
var b = 2
function inner(){
b++;
var b = 3;
console.log(b)
}
inner();
}
outer();
Output to the console will be “3”.
There are three closures in the example, each with it’s own var b declaration.
When a variable is invoked closures will be checked in order from local to global until an instance is found.
Since the inner closure has a b variable of its own, that is what will be output.
Furthermore, due to hoisting the code in inner will be interpreted as follows:
function inner () {
var b; // b is undefined
b++; // b is NaN
b = 3; // b is 3
console.log(b); // output "3"
}
Conclusion
Learned and practiced Essential JavaScript Interview Questions involving topics like NaN & closures.
Code
code
