null vs undefined

Author: rohan-paul

Javascript/Tricky-JS-Problems/null-vs-undefined.md

@@ -4,19 +4,22 @@
 
 **Answer**: JavaScript has two distinct values for nothing, null and undefined.
 
-undefined
-undefined means, value of the variable is not defined. JavaScript has a global variable undefined whose value is "undefined" and typeof undefined is also "undefined". Remember, undefined is not a constant or a keyword. undefined is a type with exactly one value: undefined. Assigning a new value to it does not change the value of the type undefined.
+### undefined
+
+In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as: JavaScript has a global variable undefined whose value is "undefined" and typeof undefined is also "undefined".
+
+Remember, undefined is not a constant or a keyword. undefined is a type with exactly one value: undefined. Assigning a new value to it does not change the value of the type undefined.
 
 ### 8 Ways to get Undefined:
 
-A declared variable without assigning any value to it.
-Implicit returns of functions due to missing return statements.
-return statements that do not explicitly return anything.
-Lookups of non-existent properties in an object.
-Function parameters that have not passed.
-Anything that has been set to the value of undefined.
-Any expression in the form of void(expression)
-The value of the global variable undefined
+- A declared variable without assigning any value to it.
+- Implicit returns of functions due to missing return statements.
+- return statements that do not explicitly return anything.
+- Lookups of non-existent properties in an object.
+- Function parameters that have not passed.
+- Anything that has been set to the value of undefined.
+- Any expression in the form of void(expression)
+- The value of the global variable undefined
 
 ### null
 
@@ -25,3 +28,32 @@ null means empty or non-existent value which is used by programmers to indicate
 Btw, null == undefined
 
 ref: [history of typeof null](http://www.2ality.com/2013/10/typeof-null.html)
+
+From the preceding examples, it is clear that `undefined` and `null` are two distinct types: `undefined` is a type itself (undefined) while `null` is an object.
+
+    null === undefined // false
+    null == undefined // true
+    null === null // true
+
+and
+
+    null = 'value' // ReferenceError
+    undefined = 'value' // 'value'
+
+---
+
+### Practical Differences
+
+All of this is great, but what about a practical difference between null and undefined?
+
+Consider the following code snippet:
+
+```js
+let logHi = (str = "hi") => {
+  console.log(str)
+}
+```
+
+The code above creates a function named logHi. This function requires one parameter and sets the default of that parameter to hi if it isn’t supplied. Here’s what that looks like:
+logHi();
+// hi

README.md

@@ -184,6 +184,7 @@ A collection of super-popular Interview questions, along with explanations and i
 - [why-does-adding-two-decimals-in-javascript-produce-a-wrong-result](Javascript/Tricky-JS-Problems/why-does-adding-two-decimals-in-javascript-produce-a-wrong-result.md)
 - [typeof-NaN](Javascript/Tricky-JS-Problems/typeof-NaN.md)
 - [If null is a primitive, why does typeof(null) return "object"?](Javascript/Tricky-JS-Problems/typeof-null-why-its-object.md)
+- [null-vs-undefined](Javascript/Tricky-JS-Problems/null-vs-undefined.md)
 
 ## Most common Async/Await and Promise related Interview Topics & Questions