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if...else: Wasm text instruction - WebAssembly | MDN

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(module
  ;; import the browser console object, you'll need to pass this in from JavaScript
  (import "console" "log" (func $log (param i32)))

  (func
    i32.const 0 ;; change to positive number (true) if you want to run the if block
    (if
      (then
        i32.const 1
        call $log ;; should log '1'
      )
      (else
        i32.const 0
        call $log ;; should log '0'
      )
    )
  )

  (start 1) ;; run the first function automatically
)

const url = "{%wasm-url%}";
await WebAssembly.instantiateStreaming(fetch(url), { console });

Syntax

wat
i32.const 0
(if
  (then
    ;; do something
  )
  (else
    ;; do something else
  )
)

To leave return values on the stack, add the result statement.

wat
i32.const 0
(if (result i32)
  (then
    ;; do something
    (i32.const 1)
  )
  (else
    ;; do something else
    (i32.const 2)
  )
)
(drop)

Instruction Binary opcode

if	0x04
else	0x05

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This page was last modified on Nov 23, 2025 by MDN contributors.

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if...else: Wasm text instruction

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Syntax

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