Why Use Lodash _.isNaN Instead of Global isNaN

JavaScript developers often prefer Lodash’s _.isNaN over the native global isNaN because the global function performs aggressive type coercion, leading to common and critical false positives. While global isNaN checks whether a value becomes NaN after converting it to a number, _.isNaN strictly determines if the provided value is inherently NaN without modifying its type. This article examines the behavior of both methods, highlights the pitfalls of implicit type coercion, and explains why Lodash provides a more reliable approach to data validation.

The Pitfall of Global isNaN

The primary issue with JavaScript's global isNaN function is that it coerces its argument into a number before performing the check. If the argument cannot be parsed as a valid number, the coercion results in NaN, causing the function to return true.

This leads to counterintuitive behavior where values that are not numbers at all are flagged as NaN:

isNaN("hello");     // true (string coerced to NaN)
isNaN(undefined);   // true (undefined coerced to NaN)
isNaN({});          // true (object coerced to NaN)

Because of this coercion, global isNaN answers the question, "Can this value not be converted to a number?" rather than "Is this value NaN?" In data validation, this frequently introduces bugs by falsely identifying valid strings, objects, or empty states as the numeric NaN type.

How Lodash's _.isNaN Solves the Problem

Lodash's _.isNaN avoids implicit conversion by strictly verifying that the input is both a number primitive (or a Number object) and identical to NaN. Because NaN is the only value in JavaScript that is not equal to itself (NaN !== NaN), _.isNaN uses this property to ensure precision:

_.isNaN(NaN);              // true
_.isNaN(Number(NaN));      // true
_.isNaN("hello");          // false
_.isNaN(undefined);        // false
_.isNaN({});               // false

By ensuring that non-number types return false, _.isNaN prevents unintended side effects and guarantees that your code only reacts to genuine NaN values.

Lodash _.isNaN vs. ES6 Number.isNaN

Modern JavaScript introduced Number.isNaN in the ECMAScript 2015 (ES6) specification, which behaves identically to Lodash’s _.isNaN by skipping type coercion. Despite this native addition, _.isNaN remains widely used for several reasons:

• Legacy Environment Support: _.isNaN works consistently across older browsers and JavaScript runtimes without requiring polyfills.
• Support for Number Objects: Unlike Number.isNaN, which strictly requires a primitive number, _.isNaN correctly detects both primitive NaN and boxed objects like new Number(NaN).
• Consistency in Lodash Pipelines: In codebases that already utilize Lodash for utility chains and functional programming, using _.isNaN maintains stylistic consistency and compatibility with Lodash chaining sequences.

Using _.isNaN eliminates the unpredictable type casting inherent to the global isNaN, providing safe, robust, and accurate identity checks for values in any JavaScript environment.