What is 'undefined' in Javascript

• In JavaScript, undefined is a primitive data type that represents a value that is not assigned or defined. When a variable or property is declared but not assigned a value, it is assigned undefined by default.

• Here's an example of a variable that is assigned undefined:

    let foo;

    console.log(foo); // Output: undefined

• In this example, the foo variable is declared but not assigned a value, so it is assigned undefined.

• undefined is also returned by functions that don't explicitly return a value. For example:

    function bar() {

        // No return statement, so it returns undefined by default

    }

    console.log(bar()); // Output: undefined

• It's important to note that undefined is not the same as null. null is a value that represents the intentional absence of any object value, while undefined represents the absence of a value or the lack of definition.

• One common use of undefined is to check if a variable has been assigned a value. For example:

    let baz;

    if (baz === undefined) {

        console.log('baz is undefined');

    } else {

        console.log('baz is defined');

    }

• In this example, we're using the === operator to check if baz is equal to undefined. If it is, we log a message saying that baz is undefined. If it's not, we log a message saying that it's defined.

• Overall, undefined is a primitive data type in JavaScript that represents the absence of a value or the lack of definition. It's commonly used to check if a variable has been assigned a value.