undefined

What is undefined?

In JavaScript, the value undefined is a primitive value that represents the absence of a value. It is one of the six primitive values in JavaScript, along with null, boolean, number, string, and symbol.

The undefined value is returned when a variable is declared but not assigned a value, or when a function is called without any arguments.

When is undefined used?

The undefined value is often used to check if a variable has been assigned a value. For example, the following code checks if the name variable has been assigned a value:

“`javascript
if (name === undefined) {
// The name variable has not been assigned a value
}
“`

The undefined value can also be used to check if a function has been called without any arguments. For example, the following code checks if the greet function has been called without any arguments:

“`javascript
function greet(name) {
if (name === undefined) {
// The greet function has been called without any arguments
}
}
“`

What is the difference between undefined and null?

The undefined and null values are both used to represent the absence of a value, but they have different meanings.

The undefined value is used to represent a variable that has not been assigned a value. The null value is used to represent a variable that has been explicitly assigned the value null.

For example, the following code declares a variable called name and assigns it the value undefined:

“`javascript
let name = undefined;
“`

The following code declares a variable called age and assigns it the value null:

“`javascript
let age = null;
“`

Conclusion

The undefined value is a primitive value in JavaScript that represents the absence of a value. It is often used to check if a variable has been assigned a value or if a function has been called without any arguments. The undefined value is different from the null value, which is used to represent a variable that has been explicitly assigned the value null.