What is the difference between isNaN and Number.isNaN?

Peer-reviewed by HireXTech Technical Panel • Updated for 2025/2026 hiring • Editorial standards

Interviewer Expectations for this Question

Core Competency

Assesses fundamental understanding of JavaScript conventions, runtime behavior, and memory/performance considerations.

Evaluation Criteria

Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.

Comprehensive Model Answer Verified Solution

isNaN: The global function isNaN converts the argument to a Number and returns true if the resulting value is NaN.
Number.isNaN: This method does not convert the argument. But it returns true when the type is a Number and value is NaN.

Let's see the difference with an example,

     isNaN(‘hello’);   // true
     Number.isNaN('hello'); // false

Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.

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