What is undefined?

In JavaScript, undefined is a primitive value that represents the absence of a value. It is one of the two special values in JavaScript, the other being null.

Undefined is assigned to a variable when that variable has not been assigned a value. For example:

“`js
let myVariable;
console.log(myVariable); // undefined
“`

Undefined can also be returned from a function if the function does not explicitly return a value. For example:

“`js
function myFunction() {
// Do something
}

console.log(myFunction()); // undefined
“`

Difference between undefined and null

Undefined and null are both values that represent the absence of a value, but they are not the same thing.

Undefined is assigned to a variable when that variable has not been assigned a value. Null, on the other hand, is a value that is explicitly assigned to a variable to indicate that the variable does not have a value.

For example:

“`js
let myVariable; // undefined
myVariable = null; // null
“`

Another difference between undefined and null is that undefined is a primitive value, while null is an object.

When to use undefined

Undefined should be used when a variable has not been assigned a value. It should not be used to represent the absence of a value in a function return value. For that, null should be used.

Conclusion

Undefined is a primitive value in JavaScript that represents the absence of a value. It is one of the two special values in JavaScript, the other being null. Undefined is assigned to a variable when that variable has not been assigned a value. Null, on the other hand, is a value that is explicitly assigned to a variable to indicate that the variable does not have a value.