Understanding “undefined”

In JavaScript, the value 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.

The undefined value is often used to indicate that a variable has not been assigned a value yet. For example:

    let myVariable;

    console.log(myVariable); // undefined
  

In the example above, the myVariable variable has not been assigned a value yet, so it has the value undefined. The console.log() function is used to print the value of a variable to the console. In this case, it prints the value undefined to the console.

The undefined value can also be used to indicate that a function does not return a value. For example:

    function myFunction() {
      // ...
    }

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

In the example above, the myFunction() function does not return a value. Therefore, the console.log() function prints the value undefined to the console.

The undefined value can also be used to compare two values. For example:

    if (myVariable === undefined) {
      // ...
    }
  

In the example above, the if statement checks if the myVariable variable is equal to the undefined value. If it is, then the code inside the if statement will execute.

Differences between undefined and null

The undefined and null values are both used to represent the absence of a value. However, there are some important differences between the two values:

• undefined is a primitive value, while null is an object.
• undefined is used to indicate that a variable has not been assigned a value yet, while null is used to indicate that a variable has been assigned the value of null.
• undefined is a global variable, while null is not.

Conclusion

The undefined value is a useful tool for representing the absence of a value in JavaScript. It is important to understand the difference between undefined and null in order to use them correctly in your code.