What is the difference between null and undefined in JavaScript?

Question

Ravi Vishwakarma · Answer

In JavaScript, null and undefined are both values that represent the absence of a value, but they have distinct meanings and uses.and 
Here's a detailed comparison between the two:
Feature
‘undefined’
‘null’
Definitions
It is the default value of variables that have been declared but not initialized.
It is used explicitly to signify that a variable should have no value.
Type
It's type is ‘undefined’.
typeof undefined; // “undefined”
It's type is ‘object’.
typeof null; // ‘object’
Usage
Automatically assigned to variables that are declared but not initialized.
let a;
console.log(a); // undefined
function foo() {}
console.log(foo()); // undefined
let obj = {};
console.log(obj.prop); // undefined
Explicitly assigned to variables to represent "no value".
Commonly used to reset or clear variables.
let b = null;
console.log(b); // null
let obj = { prop: null };
console.log(obj.prop); // null
Comparisons
undefined and null are not strictly equal because they are different types.
console.log(undefined === null); // false
undefined and null are loosely equal because they both represent an absence of value.
console.log(undefined == null); // true
Common Issues
Using undefined can sometimes be a result of not properly initializing a variable.
null is used intentionally and explicitly.
Note:
undefined generally indicates that a variable has been declared but not yet assigned a value, while
null is an assignment value that can be used to represent no value intentionally.
and

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What is the difference between null and undefined in JavaScript?

Sandra Emily

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Feature	‘undefined’	‘null’
Definitions	It is the default value of variables that have been declared but not initialized.	It is used explicitly to signify that a variable should have no value.
Type	It's type is ‘undefined’. `typeof undefined; // “undefined”`	It's type is ‘object’. `typeof null; // ‘object’`
Usage	Automatically assigned to variables that are declared but not initialized. `let a; console.log(a); // undefined function foo() {} console.log(foo()); // undefined let obj = {}; console.log(obj.prop); // undefined`	Explicitly assigned to variables to represent "no value". Commonly used to reset or clear variables. `let b = null; console.log(b); // null let obj = { prop: null }; console.log(obj.prop); // null`
Comparisons	`undefined` and `null` are not strictly equal because they are different types. `console.log(undefined === null); // false`	`undefined` and `null` are loosely equal because they both represent an absence of value. `console.log(undefined == null); // true`
Common Issues	Using `undefined` can sometimes be a result of not properly initializing a variable.	`null` is used intentionally and explicitly.