All programming languages have null values.
But in JavaScript, it has both undefined and null.
Undefined:
In other languages, the data type is assigned during the declaration of a variable. In JavaScript, at the time of declaration of a variable, no data type is given. The data type is given after the value is assigned to the variable. So, until the variable is assigned a value, it has a datatype of undefined.
Let's take a look at an example:
let x;
console.log(x); //output -> undefined
console.log(typeof x); //output -> undefined
Here, both the value of x is undefined, and the data type of x is undefined. Undefined is given as datatype to x because it has no value.
Null:
Null is nothing. Null is assigned to a variable or object when you want it to be empty or make it empty.
Let's take a look at an example:
let x = null;
console.log(x); //output -> null
Comparing undefined and null using equality(==):
Equality only compares the value of variables and returns a Boolean value.
Let's take a look at an example:
let x; // here x will have datatype undefined as default
let y = null;
console.log(x == y); //output -> true
This happens because both undefined and null have "nothing" as a value.
Comparing undefined and null using strict equality(===):
Strict equality compares both value and data type of variables and returns a Boolean value.
Let's take a look at an example:
let x; // x has a value of nothing and data type of undefined
let y = null; // y has a value of nothing and data type of null
console.log(x === y); //output -> false
This happens because though the value of both variables is the same, their data types are different.
Interview Questions:
Q1. What is undefined in JavaScript?
Answer: When a variable is declared but not assigned a value, it contains the value undefined, and its data type is also undefined.
Q2. Output for equality between undefined and null?
Answer: The output for equality between undefined and null is true.
Q3. Output for strict equality between undefined and null?
Answer: The output for strict equality between undefined and null is false.
Note: that if you use the typeof operator with null, it displays an object, but that is incorrect from the beginning. In JavaScript, null is treated as a primitive data type.
Q4. Can you explicitly assign undefined to a variable?
Answer: Yes, you can explicitly assign undefined to a variable because undefined itself is a kind of keyword which you can use.
The content of this blog post was inspired by the JavaScript - Marathon Interview Questions Series 2023 course on Udemy (udemy.com/course/javascript-marathon-interv..)