What is undefined in JavaScript?

In JavaScript, undefined is a primitive value that represents the lack of a value. It is one of the two falsy values in JavaScript, along with null.

When is undefined used?

There are a few different scenarios in which undefined can be used:

• When a variable has not been assigned a value yet.
• When a function is called without any arguments.
• When an object property does not exist.

How to check for undefined

There are a few different ways to check if a value is undefined:

• The typeof operator
• The == operator
• The === operator

The typeof operator

The typeof operator returns the type of a value. If the value is undefined, the typeof operator will return “undefined”.

    console.log(typeof undefined); // "undefined"
  

The == operator

The == operator compares two values for equality. If the two values are of the same type and have the same value, the == operator will return true. Otherwise, the == operator will return false.

When comparing undefined to another value, the == operator will return true if the other value is also undefined. Otherwise, the == operator will return false.

    console.log(undefined == undefined); // true
    console.log(undefined == null); // true
    console.log(undefined == 0); // false
  

The === operator

The === operator compares two values for strict equality. If the two values are of the same type and have the same value, the === operator will return true. Otherwise, the === operator will return false.

When comparing undefined to another value, the === operator will only return true if the other value is also undefined. Otherwise, the === operator will return false.

    console.log(undefined === undefined); // true
    console.log(undefined === null); // false
    console.log(undefined === 0); // false
  

Conclusion

undefined is a primitive value in JavaScript that represents the lack of a value. It is one of the two falsy values in JavaScript, along with null. There are a few different ways to check if a value is undefined, including the typeof operator, the == operator, and the === operator.