Wait... Undefined or Not defined or Null?
Some of the simplest concepts in JavaScript can be a little tricky to make sense of. One of these is the difference between undefined, not defined and null
Let's start with the easy one
Undefined:
There are certain cases when undefined value is returned in javascript such as:
1) Whenever we declare a variable without assigning any value to it, javascript implicitly assigns its value as undefined.
let name;console.log(name); //undefined
2) When a value is not assigned in an array or object.
let numArray = [1,2,,4];console.log(numArray); //[1, 2, , 4]typeof(numArray[2])//"undefined"
3) When functions dont have a return statement but are called for assigning a value to a variable.
let add = (a,b) => { let c = a+b;// no return statement}let sum = add(2,3);console.log(sum); //Output: undefined
In the code block above, since we commented the return statement, the value of variable sum is given as undefined in the output.
Not defined:
A not defined variable is one which has not been declared at a given point of time with a keyword like var
, let
or const
.
console.log(a);var a = 5;//Output:- undefined
While if we dont use the var
declaration, the above the output will look like this:
console.log(b);b = 5;//Output:- "ReferenceError: b is not defined
Null:
null
is a reserved keyword in javascript. We can assign a null value to a variable explicitly using this keyword. null
essentially represents a non-existent or an empty value i.e. we explicitly tell the JavaScript interpreter that the variable has no value.
let life = null;console.log(life); //null
Original Link: https://dev.to/zakariaelk/undefined-vs-not-defined-vs-null-185g
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