unreachable & nop

• unreachable — traps when executed. Useful for asserting impossible states.
• nop — does nothing. Occasionally useful as a placeholder.

(module
  (func (param $x i32)
    (if (i32.lt_s (local.get $x) (i32.const 0))
      (then (unreachable))
      (else (nop))))
)

Instruction Reference

• Control Flow Instructions — unreachable, nop