In JavaScript programming, what does the global isNaN() function do when you pass a value to it?

Introduction / Context:
The isNaN() function in JavaScript is a built in global utility that many developers encounter early when learning numeric operations and data validation. Interviewers often ask about isNaN() because it reveals how well you understand JavaScript type conversion, special values like NaN (Not a Number), and the difference between numeric validation and simple type checks.

Given Data / Assumptions:

• We are using standard JavaScript as implemented in modern browsers and Node.js.
• The function in question is the global isNaN() function.
• We pass a single value to isNaN(value) and observe the boolean result.
• NaN stands for Not a Number and is a special numeric value in JavaScript.

Concept / Approach:
The global isNaN() function does not simply test whether the argument is of type number. Instead, it first tries to convert the argument to a numeric value using JavaScript type coercion. After that conversion, it checks whether the resulting numeric value is NaN. If the coerced value is NaN, isNaN() returns true, otherwise it returns false. This means that isNaN() can return true for some non numeric strings that cannot be converted to a meaningful number, while it returns false for numeric strings such as "42".

Step-by-Step Solution:
Step 1: Remember that NaN is a special numeric value that represents an invalid or undefined numeric result, such as 0 / 0. Step 2: The global isNaN() function takes an argument and attempts to convert it to a number using the Number() conversion rules. Step 3: If the conversion results in a valid numeric value, such as 10, 3.14, or even 0, then isNaN() returns false. Step 4: If the conversion fails and results in NaN, for example when the argument is "hello" or an object that cannot be meaningfully converted, then isNaN() returns true. Step 5: Therefore, isNaN() specifically answers the question: is the value, after numeric coercion, considered NaN by the JavaScript engine.

Verification / Alternative check:
You can verify the behaviour with simple tests in a console. For example, isNaN("123") returns false because "123" is converted to the number 123. isNaN("abc") returns true because "abc" cannot be converted and becomes NaN. isNaN(NaN) returns true because the value is already NaN. In contrast, isNaN(undefined) returns true, since converting undefined to a number yields NaN. These tests confirm that isNaN() is about numeric conversion, not just checking the original type.

Why Other Options Are Wrong:
Option B is wrong because isNaN() is not limited to checking for Infinity or Number.POSITIVE_INFINITY. It specifically checks for NaN after conversion. Option C is incorrect because isNaN() does not convert non numeric values to zero, and it does not automatically return false for them; it returns true when the conversion leads to NaN. Option D is incorrect because null converts to 0 (so isNaN(null) is false), and while undefined converts to NaN, isNaN() is not designed purely as a null or undefined checker.

Common Pitfalls:
A common pitfall is assuming that isNaN() checks whether a value is of type number. Another trap is forgetting that isNaN() performs coercion, which is why developers often prefer Number.isNaN() in modern code. Number.isNaN() checks whether a value is the NaN value without coercion, leading to more predictable results. Understanding the difference between these functions helps avoid subtle bugs when validating numeric input in JavaScript applications.

Final Answer:
The global isNaN() function returns true if the given value cannot be converted to a valid numeric value and is treated as NaN after coercion in JavaScript.