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Understanding ‘undefined’ in JavaScript

What is ‘undefined’ in JavaScript?

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

When is a variable undefined?

A variable is undefined when it has not been assigned a value. For example, the following code defines a variable called myVariable, but does not assign it a value:


let myVariable;

When we try to access the value of myVariable, we will get undefined:


console.log(myVariable); // undefined

A variable can also be undefined if it is assigned the value undefined:


let myVariable = undefined;

How to check if a variable is undefined

We can use the typeof operator to check if a variable is undefined. The typeof operator returns a string indicating the type of the variable. For example, the following code checks if myVariable is undefined:


if (typeof myVariable === "undefined") {
// myVariable is undefined
}

When to use undefined

We can use undefined to represent the absence of a value in a variety of situations. For example, we can use undefined to:

  • Initialize a variable that will be assigned a value later
  • Represent the absence of a value in a function argument
  • Indicate that a property does not exist on an object

Conclusion

undefined is a useful value in JavaScript that can be used to represent the absence of a value. It is important to understand when and how to use undefined to avoid errors in your code.


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