Understanding ‘undefined’

What is ‘undefined’?

In JavaScript, ‘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.

A variable that is declared but not assigned a value is assigned the value ‘undefined’. For example:

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

You can also explicitly assign the value ‘undefined’ to a variable:

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

When is ‘undefined’ used?

‘undefined’ is typically used to indicate that a variable has not been assigned a value. However, it can also be used in other situations, such as:

• As a return value from a function that does not return a value
• As a property of an object that does not exist
• As a parameter to a function that is not expected to be used

How to check for ‘undefined’

You can check for ‘undefined’ using the typeof operator:

“`javascript
if (typeof myVariable === ‘undefined’) {
// Do something
}
“`

Conclusion

‘undefined’ is a useful value in JavaScript that can be used to indicate the absence of a value. It is important to understand how ‘undefined’ is used in order to write correct and efficient JavaScript code.