Finding NaN in Float Arrays with Lodash findIndex

When searching for NaN inside an array of floating-point numbers using Lodash's _.findIndex, the return value depends entirely on how the search criteria is passed. If NaN is passed directly as the second argument (_.findIndex(array, NaN)), the method returns -1 because NaN is not treated as an equality target. However, if a valid predicate function such as _.isNaN or Number.isNaN is provided, _.findIndex returns the zero-based index of the first NaN float in the array, or -1 if no NaN is present.

Passing NaN Directly as a Predicate

If you attempt to find NaN by passing the literal value directly:

const floats = [1.25, 4.5, NaN, 8.75];
const result = _.findIndex(floats, NaN); // Returns: -1

The method returns -1 even though NaN exists at index 2.

This occurs because _.findIndex expects a predicate function. When passed a primitive value that is neither a function, an object, nor an array shorthand, Lodash invokes _.property(NaN). This evaluates each element by accessing the property key NaN on the number primitive (effectively checking floatElement[NaN]). Because numbers in JavaScript do not possess a NaN property, this check evaluates to undefined (falsy) for every element in the array, causing the search to fail and return -1.

Using a Predicate Function to Find NaN

To accurately locate NaN using _.findIndex, you must provide a predicate function capable of evaluating NaN values:

const floats = [1.25, 4.5, NaN, 8.75];
const result = _.findIndex(floats, _.isNaN); // Returns: 2

You can use Lodash's built-in _.isNaN or standard JavaScript's Number.isNaN:

const resultNative = _.findIndex(floats, Number.isNaN); // Returns: 2

In both cases, _.findIndex iterates through the float array, invokes the predicate for each element, and immediately returns the index of the first float that evaluates to true. If the array contains only valid floating-point numbers and no NaN values, it returns -1.

Alternative: Using _.indexOf for Direct Value Search

If the goal is to search for the literal value NaN without writing a callback predicate, use _.indexOf instead of _.findIndex:

const floats = [1.25, 4.5, NaN, 8.75];
const result = _.indexOf(floats, NaN); // Returns: 2

Unlike native Array.prototype.indexOf, Lodash's _.indexOf utilizes the SameValueZero equality algorithm, allowing it to correctly identify NaN directly by value.