GopherJS v1.21.0 - Go 1.21.13 Runtime Support

GopherJS v1.21.0 was published on June 26, 2026, and moves the compiler support line to Go 1.21.13. For teams that compile Go into JavaScript for browser or Node based data processing, the important change is the Go 1.21 runtime surface: unsafe.Slice, updated maps and slices, new builtins, and better behavior around method resolution and nil checks.