Control Flow — Overview

WebAssembly has no goto, no raw jump addresses, and no unstructured branches. Every transfer of control happens inside a labeled construct, and br targets that label — not a memory address or a line number. This design makes Wasm programs safe to validate in a single linear pass and easy for browsers to compile efficiently.

Structured constructs

Wasm provides exactly three control-flow constructs:

block — a forward-only group. br $label exits the block, jumping to the instruction immediately after the closing end. Think of it as a labeled break.
loop — a backward-branch target. br $label jumps to the beginning of the loop, repeating it. Exiting a loop requires branching to an outer label or falling off the end.
if — a conditional branch. Pops an i32 from the stack; if non-zero it executes the then arm, otherwise the optional else arm.

All three constructs can carry a result type, making them expressions that leave a value on the stack.

Branch targets and direction

The key insight is that br in Wasm is not a jump to an address. It is a jump relative to the nesting structure. A br $b inside a block $b exits that block; the same br $l inside a loop $l repeats that loop.

flowchart LR
  A["block $b\n  ...body...\n  br $b"] --> B["jumps to END\nof block"]
  C["loop $l\n  ...body...\n  br_if $l"] --> D["jumps to START\nof loop"]

Branch targets in block vs loop

br_if is the conditional variant — it pops an i32 condition and only branches if it is non-zero.

What this module covers

This module walks through each construct in depth:

block and br — using labeled breaks to exit early from a sequence of instructions.
loop and br_if — writing counted and conditional loops.
if / else — conditional execution with optional result values.

Early exit with block and br

The simplest use of block is an early-exit pattern. The function below pushes 42, immediately branches out of the block, and the i32.const 99 after the branch is never reached.

(module
  (func (export "earlyExit") (result i32)
    (block $b (result i32)
      i32.const 42
      br $b
      i32.const 99)  ;; never reached
  ))

The block has a result type of i32. When br $b fires, the value 42 is already on the stack and it becomes the result of the block expression — and therefore the return value of the function.

Run it

WebAssembly

const wat = `(module
  (func (export "earlyExit") (result i32)
    (block $b (result i32)
      i32.const 42
      br $b
      i32.const 99)))`;
const bytes = await compileWat(wat);
const { instance } = await WebAssembly.instantiate(bytes, {});
console.log('earlyExit() =', instance.exports.earlyExit(), '(should be 42, not 99)');