Types Flashcards
Define primitive types in javascript & list them?
Primitive types are those which are immutable. They are the most basic data types. Following are primitive types in javascript
- undefined
- null
- boolean
- number
- string
- bigint
- symbol
Define reference types in javascript & give example?
Reference types are those which are mutable and there is only one such type in javascript called ‘object’.
What is typeof operator?
typeof operator is used for checking primitive type of a value and some extra types like object and function. It returns a string containing type of value.
Quote
In javascript, variables dont have types like in other languages but values have types
What is the type returned by typeof when the value is not of primitive type?
If its a callable object, then it returns “function”, else “object”
Why does typeof null returns “object” when it is a primitive type?
Its a bug in the earlier version of javascript. Basically each value is stored in bits and the lower bits of each value is a type tag. The type tag of object is 000. The bitwise representation of null value is all zeros. since the lower bits are 000, it is considered as object by typeof operator.
More info: https://2ality.com/2013/10/typeof-null.html
What does typeof Function return?
*Function is some function object.
“function”
Why does typeof Function return “function” when it isnt a primitive type?
Because the specification says if object is callable then typeof operator should return “function”
More info: https://262.ecma-international.org/5.1/#sec-11.4.3
What does typeof doesntExist return?
*doesntExist is a variable that is not declared.
“undefined”. It is again from specification.
It is considered weird because undefined is something that is assigned to a variable by javascript that got declared but not defined with any value. Here variable itself is not declared.
How do you define a bigint value?
With n at the end of a number like 12n
Mention some special values in javascript
- NaN
- -0
- Infinity
- -Infinity
Why NaN === NaN returns false?
It is not a bug. It is intentionally made so based on IEEE 754 spec which is a specification for numeral representation which mentions NaN is not equal to any other value including itself.
How do you check if a value is NaN?
There are three ways to do it:
- isNaN
- Number.isNaN
- Object.is
Difference between isNaN and Number.isNaN?
isNaN: It will first coerce the value passed to it into number and then checks if the resulting value is NaN
Eg:
isNaN(NaN) // true
isNaN(“Hey!!”) // true (Here, “Hey!!” isnt actually NaN, it is string)
Number.isNaN: It will directly check if the value is NaN without coercion
Eg:
Number.isNaN(NaN) // true
Number.isNaN(“Hey!!”) // false
What does typeof NaN return and why?
It returns “number” because IEEE 754 spec tells that NaN is an invalid “number”.
Quote
typeof NaN return “number”. So, it is better mental model to think of NaN as “invalid number” instead of “Not a Number”.
What is the purpose of -0 in javascript? Give an actual scenario where this could be useful.
-0 can be used in use-cases where direction matters even when the value is 0.
For eg, if a car is moving from certain location, the distance it moved from that point can be represented as positive or negative number based on direction. But in the beginning, how do you determine if the car is facing positive direction or negative direction? It can be done through -0.
What does 0 === -0 return?
true
How to determine if the value is -0?
We can use Object.is()
Object.is(0, -0) // false
Object.is(-0, -0) // true
Quote
Object.is() is like ====. It executes without any weirdness like NaN === NaN (false) or 0 === -0 (true).
So, if a value is NaN can also be checked with Object.is(NaN, NaN).
Write a polyfill for Object.is()
Object.is = (val1, val2) => {
/*
Check for -0: We are basically checking by dividing 1 with values (which gives Infinity or - Infinity based on 0 or -0)
and checking for strict equality.
*/
if (val1 === 0 && val2 === 0) {
return 1 / val1 === 1 / val2;
}
/*
Check for NaN: Checking for NaN by converting values into string and strict equality checking.
*/
if (String(val1) === “NaN” && String(val2) === “NaN”) {
return true;
}
return val1 === val2;
};
Difference between undefined, undeclared & uninitialized?
Will write answer later
