Calculate the SHA1 hash of input

Type some text into the field and the SHA1 hash will be calculated.


Digest will be shown here

The Code

The Rust code for this example is short and exposes a C-like API on top of the sha1 crate.

Download sha1-digest.wat (WebAssembly text format)

extern crate sha1;

use std::mem;
use std::ffi::{CString, CStr};
use std::os::raw::{c_char, c_void};

use sha1::Sha1;

// In order to work with the memory we expose (de)allocation methods
pub extern "C" fn alloc(size: usize) -> *mut c_void {
    let mut buf = Vec::with_capacity(size);
    let ptr = buf.as_mut_ptr();
    return ptr as *mut c_void;

pub extern "C" fn dealloc(ptr: *mut c_void, cap: usize) {
    unsafe  {
        let _buf = Vec::from_raw_parts(ptr, 0, cap);

// The JavaScript side passes a pointer to a C-like string that's already placed into memory.
// On the Rust side we turn this into a CStr, extract the bytes, pass it through the crate
// and then turn it back into an memory-allocated C-like string.
// A pointer to this data is returned.
pub extern "C" fn digest(data: *mut c_char) -> *mut c_char {
    unsafe {
        let data = CStr::from_ptr(data);

        let mut m = Sha1::new();
        let dgst = m.digest().to_string();
        let s = CString::new(dgst).unwrap();

This can be built using cargo with the command cargo +nightly build --target wasm32-unknown-unknown --release.

The JavaScript code loads the WebAssembly module and has access to the exported functions and fields, including the allocation and memory. To pass a JavaScript string, we first allocate some space in the exported memory buffer and copy over the string. On return of the digest function, we copy back the new string from the memory buffer.

var Module = {}
var Sha1 = {
  digest: function(str) {
    let buf = newString(Module, str);
    let outptr = Module.digest(buf);
    let result = copyCStr(Module, outptr);
    return result;

fetchAndInstantiate("./sha1-digest.wasm", {})
.then(mod => {
  Module.alloc   = mod.exports.alloc;
  Module.dealloc = mod.exports.dealloc;
  Module.digest  = mod.exports.digest;
  Module.memory  = new Uint8Array(mod.exports.memory.buffer)

  var input = document.getElementById("input");
  var output = document.getElementById("output");
  input.addEventListener("keyup", function(e) {
    output.innerText = Sha1.digest(input.value);