block

(module
 ;; import the browser console object, you'll need to pass this in from JavaScript
 (import "console" "log" (func $log (param i32)))
 ;; create a function that takes in a number as a param,
 ;; and logs that number if it's not equal to 100.
 (func (export "log_if_not_100") (param $num i32)
 (block $my_block
 ;; $num is equal to 100
 local.get $num
 i32.const 100
 i32.eq
 (if
 (then
 ;; branch to the end of the block
 br $my_block
 )
 )
 ;; not reachable when $num is 100
 local.get $num
 call $log
 )
 )
)

const url = "{%wasm-url%}";
await WebAssembly.instantiateStreaming(fetch(url), { console }).then(
 (result) => {
 const log_if_not_100 = result.instance.exports.log_if_not_100;
 log_if_not_100(99);
 // Expected output: 99
 log_if_not_100(100);
 // Should not log anything
 log_if_not_100(101);
 // Expected output: 101
 },
);

;; label the block so that it can be branched to.
(block $my_block
 ;; branch to the block.
 ;; most of the time you'll want to put this in an if statement and only branch on condition,
 ;; otherwise the following control flow are unreachable.
 br $my_block
 ;; this will never be reached, since the br jumped out of the block already.
 unreachable
)

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This page was last modified on ⁨2025年3月5日⁩ by MDN contributors.