Globals in WebAssembly

Module-level state; only (mut ...) globals accept writes.

(module
  ;; Globals live outside any function; only (mut ...) ones may be
  ;; written after instantiation.
  (global $visits (mut i32) (i32.const 0))
  (global $capacity i32 (i32.const 18))

  (func (export "check_in") (result i32)
    (global.set $visits
      (i32.add (global.get $visits) (i32.const 1)))
    (global.get $visits))

  (func (export "beds_left") (result i32)
    (i32.sub (global.get $capacity) (global.get $visits))))

How it works

1. (global $visits (mut i32) ...) is writable; $capacity is not.
2. global.set after a global.get and add makes an increment.
3. Immutable globals are named constants the engine can fold.

Keywords and builtins used here

• export
• func
• global
• i32
• module
• mut
• result

The run, in numbers

Lines

13

Characters to type

408

Tokens

82

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 76 seconds.

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Step 1 of 3 in Globals & startup, step 20 of 28 in Language basics.

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Globals in other languages

• Igor Pro