Understanding Undefined in JavaScript

Introduction

In JavaScript, undefined is a primitive value that represents the absence of a value or property.
It’s one of the six primitive data types in JavaScript, along with null, boolean, number, string, and symbol.

Usage

undefined is automatically assigned to variables that have not been declared or initialized. It can also be
explicitly assigned to a variable using the = undefined syntax.

    let myVariable; // implicitly assigned to undefined
    myVariable = undefined; // explicitly assigned to undefined
  

Checking for undefined is important in JavaScript, as it allows you to distinguish between variables that
haven’t been initialized and those that have been explicitly set to undefined.

    if (myVariable === undefined) {
      // do something
    }
  

Differences from null

undefined and null are often confused, but they are not the same.
undefined represents the absence of a value, while null represents a deliberately assigned
“no value” state.

• undefined is automatically assigned to variables without an initial value.
• null is explicitly assigned to variables to indicate the intentional absence of a value.

Strict Equality (===)

It’s important to note that using strict equality (===) to compare undefined and null
returns false. This is because strict equality checks for both value and type equality.

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

Conclusion

undefined is an important concept in JavaScript, representing the absence of a value.
Understanding the difference between undefined and null is crucial for writing robust
and maintainable code.