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# WebAssembly System Interface (WASI)
<!--introduced_in=v13.3.0-->
> Stability: 1 - Experimental
The WASI API provides an implementation of the [WebAssembly System Interface][]
specification. WASI gives sandboxed WebAssembly applications access to the
underlying operating system via a collection of POSIX-like functions.
```js
'use strict';
const fs = require('fs');
const { WASI } = require('wasi');
const wasi = new WASI({
args: process.argv,
env: process.env,
preopens: {
'/sandbox': '/some/real/path/that/wasm/can/access'
}
});
const importObject = { wasi_unstable: wasi.wasiImport };
(async () => {
const wasm = await WebAssembly.compile(fs.readFileSync('./binary.wasm'));
const instance = await WebAssembly.instantiate(wasm, importObject);
wasi.start(instance);
})();
```
The `--experimental-wasi-unstable-preview0` and `--experimental-wasm-bigint`
CLI arguments are needed for the previous example to run.
## Class: WASI
<!-- YAML
added: v13.3.0
-->
The `WASI` class provides the WASI system call API and additional convenience
methods for working with WASI-based applications. Each `WASI` instance
represents a distinct sandbox environment. For security purposes, each `WASI`
instance must have its command line arguments, environment variables, and
sandbox directory structure configured explicitly.
### new WASI(\[options\])
<!-- YAML
added: v13.3.0
-->
* `options` {Object}
* `args` {Array} An array of strings that the WebAssembly application will
see as command line arguments. The first argument is the virtual path to the
WASI command itself. **Default:** `[]`.
* `env` {Object} An object similar to `process.env` that the WebAssembly
application will see as its environment. **Default:** `{}`.
* `preopens` {Object} This object represents the WebAssembly application's
sandbox directory structure. The string keys of `preopens` are treated as
directories within the sandbox. The corresponding values in `preopens` are
the real paths to those directories on the host machine.
### wasi.start(instance)
<!-- YAML
added: v13.3.0
-->
* `instance` {WebAssembly.Instance}
Attempt to begin execution of `instance` by invoking its `_start()` export.
If `instance` does not contain a `_start()` export, then `start()` attempts to
invoke the `__wasi_unstable_reactor_start()` export. If neither of those exports
is present on `instance`, then `start()` does nothing.
`start()` requires that `instance` exports a [`WebAssembly.Memory`][] named
`memory`. If `instance` does not have a `memory` export an exception is thrown.
### wasi.wasiImport
<!-- YAML
added: v13.3.0
-->
* {Object}
`wasiImport` is an object that implements the WASI system call API. This object
should be passed as the `wasi_unstable` import during the instantiation of a
[`WebAssembly.Instance`][].
[`WebAssembly.Instance`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Instance
[`WebAssembly.Memory`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Memory
[WebAssembly System Interface]: https://wasi.dev/
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