In JavaScript, null
and undefined
are both primitive values that represent the absence of a value or object. However, they have different meanings and uses.
null
is a value that represents the intentional absence of an object value. It is an explicit representation of no value. You can assign the value null
to a variable as a way of indicating that the variable does not currently have a valid value.
let x = null;
undefined
is a value that is assigned to a variable when the variable is declared, but no value is assigned to it. It represents the absence of a value, but it is not the same as null
. undefined
indicates that the variable has been declared, but it has not been assigned a value.
let y;
console.log(y); // Output: undefined
You can also use the typeof
operator to check the type of a value. The typeof
operator will return "undefined"
for variables that have been declared, but not assigned a value, and "null"
for variables that have been assigned the value null
.
console.log(typeof x); // Output: "object"
console.log(typeof y); // Output: "undefined"
It’s important to note that, in JavaScript, null
is considered an object, while undefined
is considered a primitive value. This can be confusing, as typeof null
returns "object"
, while typeof undefined
returns "undefined"
. This is an error in the design of the language and cannot be fixed without breaking backward compatibility.