diff --git a/lib/pure/math.nim b/lib/pure/math.nim
index 1190871073..248653a761 100644
--- a/lib/pure/math.nim
+++ b/lib/pure/math.nim
@@ -581,13 +581,19 @@ when not defined(JS): # C
     proc erf*(x: float32): float32 {.importc: "erff", header: "<math.h>".}
     proc erf*(x: float64): float64 {.importc: "erf", header: "<math.h>".}
       ## Computes the `error function <https://en.wikipedia.org/wiki/Error_function>`_ for ``x``.
+      ##
+      ## Note: Not available for JS backend.
     proc erfc*(x: float32): float32 {.importc: "erfcf", header: "<math.h>".}
     proc erfc*(x: float64): float64 {.importc: "erfc", header: "<math.h>".}
       ## Computes the `complementary error function <https://en.wikipedia.org/wiki/Error_function#Complementary_error_function>`_ for ``x``.
+      ##
+      ## Note: Not available for JS backend.
     proc gamma*(x: float32): float32 {.importc: "tgammaf", header: "<math.h>".}
     proc gamma*(x: float64): float64 {.importc: "tgamma", header: "<math.h>".}
       ## Computes the the `gamma function <https://en.wikipedia.org/wiki/Gamma_function>`_ for ``x``.
       ##
+      ## Note: Not available for JS backend.
+      ##
       ## See also:
       ## * `lgamma proc <#lgamma,float64>`_ for a natural log of gamma function
       ##
@@ -605,6 +611,8 @@ when not defined(JS): # C
     proc lgamma*(x: float64): float64 {.importc: "lgamma", header: "<math.h>".}
       ## Computes the natural log of the gamma function for ``x``.
       ##
+      ## Note: Not available for JS backend.
+      ##
       ## See also:
       ## * `gamma proc <#gamma,float64>`_ for gamma function
       ##