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is_nan

Note

This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information.

Returns whether a num is NAN

namespace HH\Lib\Math;

function is_nan(
  num $num,
): bool;

NAN is "the not-a-number special float value"

When comparing NAN to any value (including NAN) using operators false will be returned. NAN === NAN is false.

One must always check for NAN using is_nan and not $x === NAN.

Parameters​

  • num $num

Returns​

  • bool