Undefined Value in JavaScript

Question
Is there ever a good reason to use undefined?

Answer
No, not really. It’s often confused with null and the string value “undefined” returned by the typeof operator.

Comments
Essentially, variables that are not initialized have a value of undefined. When you set a variable to null, you indicate that it will eventually contain an object.

Examples

// Bad. The account variable is not initialized.
let account;
console.log(account === undefined); // Evaluates to true.

// Bad. The accountTwo variable is not declared.
let accountOne;
console.log(typeof accountOne); // Evaluates to undefined.
console.log(typeof accountTwo); // Evaluates to undefined.

// Good. The account variable is initialized to null.
let account = null;
console.log(account === null); // Evaluates to true.

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