In what situations should the value 'undefined' be used in JavaScript?

In JavaScript, the value undefined is used to indicate that a variable has been declared but has not been assigned any value. It should be used in the following situations:

• When a function does not explicitly return a value, it returns undefined by default.
• To check whether a function parameter or an object property has been provided.
• As an indicator of the absence of a value when a variable or property is uninitialized.

Example:

let a;
console.log(a); // undefined

function foo(x) {
  if (x === undefined) {
    console.log('Parameter not passed');
  }
}
foo();

It is important not to assign undefined manually; it is better to use null to explicitly indicate the absence of a value.