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new file mode 100644
index 000000000000..144085074ea1
--- /dev/null
+++ b/crates/wasi-common/js-polyfill/Cargo.lock
@@ -0,0 +1,408 @@
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+ "filetime",
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+ "lazy_static",
+ "libc",
+ "log",
+ "num",
+ "thiserror",
+ "wasi-common-cbindgen",
+ "wig",
+ "winapi",
+ "winx",
+ "yanix",
+]
+
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+ "proc-macro2",
+ "quote",
+ "witx",
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+ "winapi-x86_64-pc-windows-gnu",
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+
+[[package]]
+name = "winx"
+version = "0.7.0"
+dependencies = [
+ "bitflags",
+ "cvt",
+ "winapi",
+]
+
+[[package]]
+name = "witx"
+version = "0.6.0"
+dependencies = [
+ "clap",
+ "thiserror",
+ "wast",
+]
+
+[[package]]
+name = "yanix"
+version = "0.1.0"
+dependencies = [
+ "bitflags",
+ "cfg-if",
+ "libc",
+ "log",
+ "thiserror",
+]
diff --git a/crates/wasi-common/js-polyfill/Cargo.toml b/crates/wasi-common/js-polyfill/Cargo.toml
new file mode 100644
index 000000000000..f07f78a33740
--- /dev/null
+++ b/crates/wasi-common/js-polyfill/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+name = "polyfill"
+version = "0.1.0"
+authors = ["The Wasmtime Project Developers"]
+edition = "2018"
+
+[dependencies]
+wasi-common = { path = ".." }
+
+# This crate is built with the wasm32-unknown-emscripten target, so it's separate
+# from the main Wasmtime build, so use this directive to exclude it
+# from the parent directory's workspace.
+[workspace]
diff --git a/crates/wasi-common/js-polyfill/assets/WASI-small.png b/crates/wasi-common/js-polyfill/assets/WASI-small.png
new file mode 100644
index 000000000000..ef55a0bf6df4
Binary files /dev/null and b/crates/wasi-common/js-polyfill/assets/WASI-small.png differ
diff --git a/crates/wasi-common/js-polyfill/assets/load-files.js b/crates/wasi-common/js-polyfill/assets/load-files.js
new file mode 100644
index 000000000000..80c4ee0fc46a
--- /dev/null
+++ b/crates/wasi-common/js-polyfill/assets/load-files.js
@@ -0,0 +1,23 @@
+mergeInto(LibraryManager.library, {
+  loadFiles: function() {
+    const imports = { wasi_snapshot_preview1: WASIPolyfill };
+    let file = document.getElementById("input").files[0];
+    let file_with_mime_type = file.slice(0, file.size, "application/wasm");
+    let response = new Response(file_with_mime_type);
+    wasi_instantiateStreaming(response, imports)
+      .then(obj => {
+        setInstance(obj.instance);
+        try {
+          obj.instance.exports._start();
+        } catch (e) {
+          if (e instanceof WASIExit) {
+            handleWASIExit(e);
+          } else {
+          }
+        }
+      })
+      .catch(error => {
+        console.log("error! " + error);
+      });
+  }
+});
diff --git a/crates/wasi-common/js-polyfill/assets/shell.html b/crates/wasi-common/js-polyfill/assets/shell.html
new file mode 100644
index 000000000000..f44eda52ca03
--- /dev/null
+++ b/crates/wasi-common/js-polyfill/assets/shell.html
@@ -0,0 +1,88 @@
+<!doctype html>
+<!-- This file is derived from src/shell_minimal.html in Emscripten. -->
+<html lang="en-us">
+  <head>
+    <meta charset="utf-8">
+    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+    <title>WASI Web Polyfill</title>
+    <style>
+      .wasi { padding-right: 0; margin-left: auto; margin-right: auto; display: block; }
+      textarea.wasi { font-family: monospace; width: 80%; }
+      div.wasi { text-align: center; }
+      div.wasi_border { border: 1px solid black; }
+    </style>
+  </head>
+  <body>
+    <figure style="overflow:visible;" id="spinner"><div class="spinner"></div><center style="margin-top:0.5em"><strong>WASI</strong></center></figure>
+    <div class="wasi" id="status">Downloading...</div>
+    <div class="wasi">
+      <progress value="0" max="100" id="progress" hidden=1></progress>  
+    </div>
+    <img class="wasi" src="WASI-small.png" width="200" height="200" border="0" alt="WASI logo">
+    <input class="wasi" type="file" id="input" onchange="_handleFiles(this.files)">
+    <hr>
+    <textarea class="wasi" id="output" rows="8"></textarea>
+    <script type='text/javascript'>
+      var statusElement = document.getElementById('status');
+      var progressElement = document.getElementById('progress');
+      var spinnerElement = document.getElementById('spinner');
+
+      var Module = {
+        preRun: [],
+        postRun: [],
+        print: (function() {
+          var element = document.getElementById('output');
+          if (element) element.value = ''; // clear browser cache
+          return function(text) {
+            if (arguments.length > 1) text = Array.prototype.slice.call(arguments).join(' ');
+            console.log(text);
+            if (element) {
+              element.value += text + "\n";
+              element.scrollTop = element.scrollHeight; // focus on bottom
+            }
+          };
+        })(),
+        printErr: function(text) {
+          if (arguments.length > 1) text = Array.prototype.slice.call(arguments).join(' ');
+          console.error(text);
+        },
+        setStatus: function(text) {
+          if (!Module.setStatus.last) Module.setStatus.last = { time: Date.now(), text: '' };
+          if (text === Module.setStatus.last.text) return;
+          var m = text.match(/([^(]+)\((\d+(\.\d+)?)\/(\d+)\)/);
+          var now = Date.now();
+          if (m && now - Module.setStatus.last.time < 30) return; // if this is a progress update, skip it if too soon
+          Module.setStatus.last.time = now;
+          Module.setStatus.last.text = text;
+          if (m) {
+            text = m[1];
+            progressElement.value = parseInt(m[2])*100;
+            progressElement.max = parseInt(m[4])*100;
+            progressElement.hidden = false;
+            spinnerElement.hidden = false;
+          } else {
+            progressElement.value = null;
+            progressElement.max = null;
+            progressElement.hidden = true;
+            if (!text) spinnerElement.hidden = true;
+          }
+          statusElement.innerHTML = text;
+        },
+        totalDependencies: 0,
+        monitorRunDependencies: function(left) {
+          this.totalDependencies = Math.max(this.totalDependencies, left);
+          Module.setStatus(left ? 'Preparing... (' + (this.totalDependencies-left) + '/' + this.totalDependencies + ')' : 'All downloads complete.');
+        }
+      };
+      Module.setStatus('Downloading...');
+      window.onerror = function() {
+        Module.setStatus('Exception thrown, see JavaScript console');
+        spinnerElement.style.display = 'none';
+        Module.setStatus = function(text) {
+          if (text) Module.printErr('[post-exception status] ' + text);
+        };
+      };
+    </script>
+    {{{ SCRIPT }}}
+  </body>
+</html>
diff --git a/crates/wasi-common/js-polyfill/assets/wasi.js b/crates/wasi-common/js-polyfill/assets/wasi.js
new file mode 100644
index 000000000000..237cb5bad40c
--- /dev/null
+++ b/crates/wasi-common/js-polyfill/assets/wasi.js
@@ -0,0 +1,758 @@
+// To implement `proc_exit`, we define a custom exception object
+// that we can throw to unwind the stack and carry the exit value.
+function WASIExit(return_value, message, fileName, lineNumber) {
+  let instance = new Error(message, fileName, lineNumber);
+  instance.return_value = return_value;
+  Object.setPrototypeOf(instance, Object.getPrototypeOf(this));
+  if (Error.captureStackTrace) {
+    Error.captureStackTrace(instance, WASIExit);
+  }
+  return instance;
+}
+
+WASIExit.prototype = Object.create(Error.prototype, {
+  constructor: {
+    value: Error,
+    enumerable: false,
+    writable: true,
+    configurable: true
+  }
+});
+
+if (Object.setPrototypeOf) {
+  Object.setPrototypeOf(WASIExit, Error);
+} else {
+  WASIExit.__proto__ = Error;
+}
+
+function handleWASIExit(e) {
+  if (e.return_value != 0) {
+    console.log("program exited with non-zero exit status " + e.return_value);
+  }
+}
+
+// Safari doesn't have instantiateStreaming
+function wasi_instantiateStreaming(response, imports) {
+  if (WebAssembly && WebAssembly.instantiateStreaming) {
+    return WebAssembly.instantiateStreaming(response, imports);
+  }
+  return response.arrayBuffer().then(function(buffer) {
+    return WebAssembly.instantiate(buffer, imports);
+  });
+}
+
+// The current guest wasm instance.
+var currentInstance;
+
+// There are two heaps in play, the guest heap, which belongs to the WASI-using
+// program, and the host heap, which belongs to the Emscripten-compiled polyfill
+// library. The following declare support for the guest heap in a similar manner
+// to Emscripten's heap.
+
+var GUEST_HEAP,
+  /** @type {ArrayBuffer} */
+  GUEST_buffer,
+  /** @type {Int8Array} */
+  GUEST_HEAP8,
+  /** @type {Uint8Array} */
+  GUEST_HEAPU8,
+  /** @type {Int16Array} */
+  GUEST_HEAP16,
+  /** @type {Uint16Array} */
+  GUEST_HEAPU16,
+  /** @type {Int32Array} */
+  GUEST_HEAP32,
+  /** @type {Uint32Array} */
+  GUEST_HEAPU32,
+  /** @type {Float32Array} */
+  GUEST_HEAPF32,
+  /** @type {Float64Array} */
+  GUEST_HEAPF64;
+
+function setInstance(instance) {
+  currentInstance = instance;
+  updateGuestBuffer();
+}
+
+/// We call updateGuestBuffer any time the guest's memory may have changed,
+/// such as when creating a new instance, or after calling _malloc.
+function updateGuestBuffer() {
+  var buf = currentInstance.exports.memory.buffer;
+  Module["GUEST_buffer"] = GUEST_buffer = buf;
+  Module["GUEST_HEAP8"] = GUEST_HEAP8 = new Int8Array(GUEST_buffer);
+  Module["GUEST_HEAP16"] = GUEST_HEAP16 = new Int16Array(GUEST_buffer);
+  Module["GUEST_HEAP32"] = GUEST_HEAP32 = new Int32Array(GUEST_buffer);
+  Module["GUEST_HEAPU8"] = GUEST_HEAPU8 = new Uint8Array(GUEST_buffer);
+  Module["GUEST_HEAPU16"] = GUEST_HEAPU16 = new Uint16Array(GUEST_buffer);
+  Module["GUEST_HEAPU32"] = GUEST_HEAPU32 = new Uint32Array(GUEST_buffer);
+  Module["GUEST_HEAPF32"] = GUEST_HEAPF32 = new Float32Array(GUEST_buffer);
+  Module["GUEST_HEAPF64"] = GUEST_HEAPF64 = new Float64Array(GUEST_buffer);
+}
+
+function copyin_bytes(src, len) {
+  let dst = _malloc(len);
+  updateGuestBuffer();
+
+  for (let i = 0; i < len; ++i) {
+    HEAP8[dst + i] = GUEST_HEAP8[src + i];
+  }
+  return dst;
+}
+
+function copyout_bytes(dst, src, len) {
+  updateGuestBuffer();
+
+  for (let i = 0; i < len; ++i) {
+    GUEST_HEAP8[dst + i] = HEAP8[src + i];
+  }
+  _free(src);
+}
+
+function copyout_i32(dst, src) {
+  updateGuestBuffer();
+
+  GUEST_HEAP32[dst >> 2] = HEAP32[src >> 2];
+  _free(src);
+}
+
+function copyout_i64(dst, src) {
+  updateGuestBuffer();
+
+  GUEST_HEAP32[dst >> 2] = HEAP32[src >> 2];
+  GUEST_HEAP32[(dst + 4) >> 2] = HEAP32[(src + 4) >> 2];
+  _free(src);
+}
+
+function translate_ciovs(iovs, iovs_len) {
+  host_iovs = _malloc(8 * iovs_len);
+  updateGuestBuffer();
+
+  for (let i = 0; i < iovs_len; ++i) {
+    let ptr = GUEST_HEAP32[(iovs + i * 8 + 0) >> 2];
+    let len = GUEST_HEAP32[(iovs + i * 8 + 4) >> 2];
+    let buf = copyin_bytes(ptr, len);
+    HEAP32[(host_iovs + i * 8 + 0) >> 2] = buf;
+    HEAP32[(host_iovs + i * 8 + 4) >> 2] = len;
+  }
+  return host_iovs;
+}
+
+function free_ciovs(host_iovs, iovs_len) {
+  for (let i = 0; i < iovs_len; ++i) {
+    let buf = HEAP32[(host_iovs + i * 8 + 0) >> 2];
+    _free(buf);
+  }
+  _free(host_iovs);
+}
+
+function translate_iovs(iovs, iovs_len) {
+  host_iovs = _malloc(8 * iovs_len);
+  updateGuestBuffer();
+
+  for (let i = 0; i < iovs_len; ++i) {
+    let len = GUEST_HEAP32[(iovs + i * 8 + 4) >> 2];
+    let buf = _malloc(len);
+    updateGuestBuffer();
+    HEAP32[(host_iovs + i * 8 + 0) >> 2] = buf;
+    HEAP32[(host_iovs + i * 8 + 4) >> 2] = len;
+  }
+  return host_iovs;
+}
+
+function free_iovs(host_iovs, iovs_len, iovs) {
+  updateGuestBuffer();
+  for (let i = 0; i < iovs_len; ++i) {
+    let buf = HEAP32[(host_iovs + i * 8 + 0) >> 2];
+    let len = HEAP32[(host_iovs + i * 8 + 4) >> 2];
+    let ptr = GUEST_HEAP32[(iovs + i * 8 + 0) >> 2];
+    copyout_bytes(ptr, buf, len);
+  }
+  _free(host_iovs);
+}
+
+var WASIPolyfill = {
+  args_get: function(argv, argv_buf) {
+    return 0;
+  },
+
+  args_sizes_get: function(argc, argv_buf_size) {
+    updateGuestBuffer();
+
+    // TODO: Implement command-line arguments.
+    GUEST_HEAP32[argc >> 2] = 0;
+    GUEST_HEAP32[argv_buf_size >> 2] = 0;
+    return 0;
+  },
+
+  clock_res_get: function(clock_id, resolution) {
+    let host_resolution = _malloc(8);
+    let ret = _wasi_common_clock_res_get(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      clock_id,
+      host_resolution
+    );
+    copyout_i64(resolution, host_resolution);
+    return ret;
+  },
+
+  clock_time_get: function(clock_id, precision, time) {
+    let host_time = _malloc(8);
+    let ret = _wasi_common_clock_time_get(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      clock_id,
+      precision,
+      host_time
+    );
+    copyout_i64(time, host_time);
+    return ret;
+  },
+
+  environ_get: function(environ, environ_buf) {
+    return 0;
+  },
+
+  environ_sizes_get: function(environ_size, environ_buf_size) {
+    updateGuestBuffer();
+
+    // TODO: Implement environment variables.
+    GUEST_HEAP32[environ_size >> 2] = 0;
+    GUEST_HEAP32[environ_buf_size >> 2] = 0;
+    return 0;
+  },
+
+  fd_prestat_get: function(fd, buf) {
+    let host_buf = _malloc(8); // sizeof __wasi_prestat_t
+    let ret = _wasi_common_fd_prestat_get(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_buf
+    );
+    copyout_bytes(buf, host_buf, 8);
+    return ret;
+  },
+
+  fd_prestat_dir_name: function(fd, path, path_len) {
+    let host_buf = _malloc(path_len);
+    let ret = _wasi_common_fd_prestat_get(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_buf,
+      path_len
+    );
+    copyout_bytes(path, host_buf, path_len);
+    return ret;
+  },
+
+  fd_close: function(fd) {
+    return _wasi_common_fd_close(_get_wasi_context(), fd);
+  },
+
+  fd_datasync: function(fd) {
+    return _wasi_common_fd_datasync(_get_wasi_context(), fd);
+  },
+
+  fd_pread: function(fd, iovs, iovs_len, offset, nread) {
+    let host_iovs = translate_iovs(iovs, iovs_len);
+    let host_nread = _malloc(4);
+    let ret = _wasi_common_fd_pread(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_iovs,
+      iovs_len,
+      offset,
+      host_nread
+    );
+    copyout_i32(nread, host_nread);
+    free_iovs(host_iovs, iovs_len, iovs);
+    return ret;
+  },
+
+  fd_pwrite: function(fd, iovs, iovs_len, offset, nwritten) {
+    let host_iovs = translate_ciovs(iovs, iovs_len);
+    let host_nwritten = _malloc(4);
+    let ret = _wasi_common_fd_pwrite(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_iovs,
+      iovs_len,
+      offset,
+      host_nwritten
+    );
+    copyout_i32(nwritten, host_nwritten);
+    free_ciovs(host_iovs, iovs_len);
+    return ret;
+  },
+
+  fd_read: function(fd, iovs, iovs_len, nread) {
+    let host_iovs = translate_iovs(iovs, iovs_len);
+    let host_nread = _malloc(4);
+    let ret = _wasi_common_fd_read(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_iovs,
+      iovs_len,
+      host_nread
+    );
+    copyout_i32(nread, host_nread);
+    free_iovs(host_iovs, iovs_len, iovs);
+    return ret;
+  },
+
+  fd_renumber: function(from, to) {
+    return _wasi_common_fd_renumber(_get_wasi_context(), from, to);
+  },
+
+  fd_seek: function(fd, offset, whence, newoffset) {
+    let host_newoffset = _malloc(8);
+    let ret = _wasi_common_fd_seek(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      offset,
+      whence,
+      host_newoffset
+    );
+    copyout_i64(newoffset, host_newoffset);
+    return ret;
+  },
+
+  fd_tell: function(fd, newoffset) {
+    let host_newoffset = _malloc(8);
+    let ret = _wasi_common_fd_seek(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_newoffset
+    );
+    copyout_i64(newoffset, host_newoffset);
+    return ret;
+  },
+
+  fd_fdstat_get: function(fd, buf) {
+    let host_buf = _malloc(24); // sizeof __wasi_fdstat_t
+    let ret = _wasi_common_fd_fdstat_get(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_buf
+    );
+    copyout_bytes(buf, host_buf, 24);
+    return ret;
+  },
+
+  fd_fdstat_set_flags: function(fd, flags) {
+    return _wasi_common_fd_fdstat_set_flags(_get_wasi_context(), fd, flags);
+  },
+
+  fd_fdstat_set_rights: function(fd, fs_rights_base, fs_rights_inheriting) {
+    return _wasi_common_fd_fdstat_set_rights(
+      _get_wasi_context(),
+      fd,
+      fs_rights_base,
+      fs_rights_inheriting
+    );
+  },
+
+  fd_sync: function(fd) {
+    return _wasi_common_fd_sync(_get_wasi_context(), fd);
+  },
+
+  fd_write: function(fd, iovs, iovs_len, nwritten) {
+    let host_iovs = translate_ciovs(iovs, iovs_len);
+    let host_nwritten = _malloc(4);
+    let ret = _wasi_common_fd_write(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_iovs,
+      iovs_len,
+      host_nwritten
+    );
+    copyout_i32(nwritten, host_nwritten);
+    free_ciovs(host_iovs, iovs_len);
+    return ret;
+  },
+
+  fd_advise: function(fd, offset, len, advice) {
+    return _wasi_common_fd_advise(_get_wasi_context(), fd, offset, len, advice);
+  },
+
+  fd_allocate: function(fd, offset, len) {
+    return _wasi_common_fd_allocate(_get_wasi_context(), fd, offset, len);
+  },
+
+  path_create_directory: function(fd, path, path_len) {
+    let host_path = copyin_bytes(path, path_len);
+    let ret = _wasi_common_path_create_directory(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_path,
+      path_len
+    );
+    _free(host_path);
+    return ret;
+  },
+
+  path_link: function(fd0, path0, path_len0, fd1, path1, path_len1) {
+    let host_path0 = copyin_bytes(path0, path_len0);
+    let host_path1 = copyin_bytes(path1, path_len1);
+    let ret = _wasi_common_path_link(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd0,
+      host_path0,
+      path_len0,
+      fd1,
+      host_path1,
+      path1_len
+    );
+    _free(host_path1);
+    _free(host_path0);
+    return ret;
+  },
+
+  path_open: function(
+    dirfd,
+    dirflags,
+    path,
+    path_len,
+    oflags,
+    fs_rights_base,
+    fs_rights_inheriting,
+    fs_flags,
+    fd
+  ) {
+    let host_path = copyin_bytes(path, path_len);
+    let host_fd = _malloc(4);
+    let ret = _wasi_common_path_open(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      dirfd,
+      dirflags,
+      host_path,
+      path_len,
+      oflags,
+      fs_rights_base,
+      fs_rights_inheriting,
+      fs_flags,
+      host_fd
+    );
+    copyout_i32(fd, host_fd);
+    _free(host_path);
+    return ret;
+  },
+
+  fd_readdir: function(fd, buf, buf_len, cookie, buf_used) {
+    let host_buf = _malloc(buf_len);
+    let host_buf_used = _malloc(4);
+    let ret = _wasi_common_fd_readdir(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      buf,
+      buf_len,
+      cookie,
+      host_buf_used
+    );
+    copyout_i32(buf_used, host_buf_used);
+    copyout_bytes(buf, host_buf, buf_len);
+    return ret;
+  },
+
+  path_readlink: function(fd, path, path_len, buf, buf_len, buf_used) {
+    let host_path = copyin_bytes(path, path_len);
+    let host_buf = _malloc(buf_len);
+    let host_buf_used = _malloc(4);
+    let ret = _wasi_common_path_readlink(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      path,
+      path_len,
+      buf,
+      buf_len,
+      host_buf_used
+    );
+    copyout_i32(buf_used, host_buf_used);
+    copyout_bytes(buf, host_buf, buf_len);
+    _free(host_path);
+    return ret;
+  },
+
+  path_rename: function(fd0, path0, path_len0, fd1, path1, path_len1) {
+    let host_path0 = copyin_bytes(path0, path_len0);
+    let host_path1 = copyin_bytes(path1, path_len1);
+    let ret = _wasi_common_path_rename(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd0,
+      host_path0,
+      path_len0,
+      fd1,
+      host_path1,
+      path1_len
+    );
+    _free(host_path1);
+    _free(host_path0);
+    return ret;
+  },
+
+  fd_filestat_get: function(fd, buf) {
+    let host_buf = _malloc(56); // sizeof __wasi_filestat_t
+    let ret = _wasi_common_fd_filestat_get(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_buf
+    );
+    copyout_bytes(buf, host_buf, 56);
+    return ret;
+  },
+
+  fd_filestat_set_size: function(fd, size) {
+    return _wasi_common_fd_filestat_set_size(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      size
+    );
+  },
+
+  fd_filestat_set_times: function(fd, st_atim, st_mtim, fstflags) {
+    return _wasi_common_fd_filestat_set_times(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      st_atim,
+      st_mtim,
+      fstflags
+    );
+  },
+
+  path_filestat_get: function(fd, path, path_len, buf) {
+    let host_path = copyin_bytes(path, path_len);
+    let host_buf = _malloc(56); // sizeof __wasi_filestat_t
+    let ret = _wasi_common_path_filestat_get(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_path,
+      path_len,
+      host_buf
+    );
+    copyout_bytes(buf, host_buf, 56);
+    _free(host_path);
+    return ret;
+  },
+
+  path_filestat_set_times: function(
+    fd,
+    path,
+    path_len,
+    st_atim,
+    st_mtim,
+    flags
+  ) {
+    let host_path = copyin_bytes(path, path_len);
+    let ret = _wasi_common_path_filestat_set_times(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_path,
+      st_atim,
+      st_mtim,
+      fstflags
+    );
+    _free(host_path);
+    return ret;
+  },
+
+  path_symlink: function(path0, path_len0, fd, path1, path_len1) {
+    let host_path0 = copyin_bytes(path0, path0_len);
+    let host_path1 = copyin_bytes(path1, path1_len);
+    let ret = _wasi_common_path_symlink(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      host_path0,
+      path_len0,
+      fd,
+      host_path1,
+      path_len1
+    );
+    _free(host_path1);
+    _free(host_path0);
+    return ret;
+  },
+
+  path_unlink_file: function(fd, path, path_len, flags) {
+    let host_path = copyin_bytes(path, path_len);
+    let ret = _wasi_common_path_unlink_file(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_path,
+      path_len,
+      flags
+    );
+    _free(host_path);
+    return ret;
+  },
+
+  path_remove_directory: function(fd, path, path_len, flags) {
+    let host_path = copyin_bytes(path, path_len);
+    let ret = _wasi_common_path_remove_directory(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      fd,
+      host_path,
+      path_len,
+      flags
+    );
+    _free(host_path);
+    return ret;
+  },
+
+  poll_oneoff: function(in_, out, nsubscriptions, nevents) {
+    let host_in = copyin_bytes(in_, nsubscriptions * 56); // sizeof __wasi_subscription_t
+    let host_out = _malloc(nsubscriptions * 32); // sizeof __wasi_event_t
+    let host_nevents = _malloc(4);
+    let ret = _wasi_common_poll_oneoff(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      host_in,
+      host_out,
+      host_nevents
+    );
+    copyout_bytes(out, host_out, nsubscriptions * 32);
+    copyout_i32(nevents, host_nevents);
+    _free(host_in);
+    return ret;
+  },
+
+  proc_exit: function(rval) {
+    let message;
+    if (rval == 0) {
+      message = "success";
+    } else {
+      message = "error code " + rval;
+    }
+    throw new WASIExit(rval, message);
+  },
+
+  proc_raise: function(sig) {
+    if (
+      sig == 18 || // SIGSTOP
+      sig == 19 || // SIGTSTP
+      sig == 20 || // SIGTTIN
+      sig == 21 || // SIGTTOU
+      sig == 22 || // SIGURG
+      sig == 16 || // SIGCHLD
+      sig == 13
+    ) {
+      // SIGPIPE
+      return 0;
+    }
+
+    let message = "raised signal " + sig;
+    throw new WASIExit(128 + sig, message);
+  },
+
+  random_get: function(buf, buf_len) {
+    let host_buf = _malloc(buf_len);
+    let ret = _wasi_common_random_get(HEAP8, HEAP8.length, host_buf, buf_len);
+    copyout_bytes(buf, host_buf, buf_len);
+    return ret;
+  },
+
+  sched_yield: function() {
+    return _wasi_common_sched_yield();
+  },
+
+  sock_recv: function(
+    sock,
+    ri_data,
+    ri_data_len,
+    ri_flags,
+    ro_datalen,
+    ro_flags
+  ) {
+    let host_ri_data = translate_iovs(ri_data, ri_data_len);
+    let host_ro_datalen = _malloc(4);
+    let ret = _wasi_common_sock_recv(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      sock,
+      host_ri_data,
+      ri_data_len,
+      ri_flags,
+      host_ro_data,
+      ro_flags
+    );
+    copyout_i32(ro_datalen, host_ro_datalen);
+    free_iovs(host_ri_data, ri_data_len, ri_data);
+    return ret;
+  },
+
+  sock_send: function(sock, si_data, si_data_len, si_flags, so_datalen) {
+    let host_si_data = translate_ciovs(si_data, si_data_len);
+    let host_so_datalen = _malloc(4);
+    let ret = _wasi_common_sock_send(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      sock,
+      host_si_data,
+      si_data_len,
+      si_flags,
+      host_so_datalen
+    );
+    copyout_i32(so_datalen, host_so_datalen);
+    free_ciovs(host_si_data, si_data_len);
+    return ret;
+  },
+
+  sock_shutdown: function(sock, how) {
+    return _wasi_common_sock_shutdown(
+      _get_wasi_context(),
+      HEAP8,
+      HEAP8.length,
+      sock,
+      how
+    );
+  }
+};
diff --git a/crates/wasi-common/js-polyfill/build.sh b/crates/wasi-common/js-polyfill/build.sh
new file mode 100755
index 000000000000..f4064918fd58
--- /dev/null
+++ b/crates/wasi-common/js-polyfill/build.sh
@@ -0,0 +1,63 @@
+#!/bin/bash
+set -euo pipefail
+
+WORKDIR=assets                      # our workdir
+JS_LIBRARY=$WORKDIR/load-files.js   # JS helper lib which we use in main.rs to handle loading WASI binaries in Emscripten
+EM_SHELL=$WORKDIR/shell.html        # basic Emscripten shell env
+WASI=$WORKDIR/wasi.js               # WASI polyfill JS glue code
+POLYFILL=$WORKDIR/polyfill.html     # our output
+
+EXPORTED_FUNCTIONS=(
+  _main
+  _get_wasi_context
+  _handleFiles
+  _wasi_common_args_get
+  _wasi_common_args_sizes_get
+  _wasi_common_clock_res_get
+  _wasi_common_clock_time_get
+  _wasi_common_environ_get
+  _wasi_common_environ_sizes_get
+  _wasi_common_fd_advise
+  _wasi_common_fd_allocate
+  _wasi_common_fd_close
+  _wasi_common_fd_datasync
+  _wasi_common_fd_fdstat_get
+  _wasi_common_fd_fdstat_set_flags
+  _wasi_common_fd_fdstat_set_rights
+  _wasi_common_fd_filestat_get
+  _wasi_common_fd_filestat_set_size
+  _wasi_common_fd_filestat_set_times
+  _wasi_common_fd_pread
+  _wasi_common_fd_prestat_dir_name
+  _wasi_common_fd_prestat_get
+  _wasi_common_fd_pwrite
+  _wasi_common_fd_read
+  _wasi_common_fd_readdir
+  _wasi_common_fd_renumber
+  _wasi_common_fd_seek
+  _wasi_common_fd_sync
+  _wasi_common_fd_tell
+  _wasi_common_fd_write
+  _wasi_common_path_create_directory
+  _wasi_common_path_filestat_get
+  _wasi_common_path_filestat_set_times
+  _wasi_common_path_link
+  _wasi_common_path_open
+  _wasi_common_path_readlink
+  _wasi_common_path_remove_directory
+  _wasi_common_path_rename
+  _wasi_common_path_symlink
+  _wasi_common_path_unlink_file
+  _wasi_common_poll_oneoff
+  _wasi_common_proc_exit
+  _wasi_common_proc_raise
+  _wasi_common_random_get
+  _wasi_common_sched_yield
+  _wasi_common_sock_recv
+  _wasi_common_sock_send
+  _wasi_common_sock_shutdown
+)
+EXPORTED_FUNCTIONS_CONCAT=$(printf ",'%s'" "${EXPORTED_FUNCTIONS[@]}")
+EXPORTED_FUNCTIONS_CONCAT=${EXPORTED_FUNCTIONS_CONCAT:1}
+
+cargo +nightly rustc --target wasm32-unknown-emscripten --release -vv -- -C link-args="--js-library ${JS_LIBRARY} --shell-file ${EM_SHELL} --pre-js ${WASI} -s EXPORTED_FUNCTIONS=[${EXPORTED_FUNCTIONS_CONCAT}] -o ${POLYFILL}"
diff --git a/crates/wasi-common/js-polyfill/src/main.rs b/crates/wasi-common/js-polyfill/src/main.rs
new file mode 100644
index 000000000000..366a2201d0d4
--- /dev/null
+++ b/crates/wasi-common/js-polyfill/src/main.rs
@@ -0,0 +1,34 @@
+use std::cell::RefCell;
+use wasi_common::{WasiCtx, WasiCtxBuilder};
+
+extern "C" {
+    fn loadFiles();
+}
+
+thread_local! {
+    static WASI_CTX: RefCell<Option<WasiCtx>> = RefCell::new(None);
+}
+
+fn main() {}
+
+#[no_mangle]
+unsafe fn get_wasi_context() -> *mut WasiCtx {
+    WASI_CTX.with(|ctx| {
+        ctx.borrow_mut()
+            .as_mut()
+            .expect("WasiCtx should be initialized by now") as *mut _
+    })
+}
+
+#[allow(non_snake_case)]
+#[no_mangle]
+fn handleFiles() {
+    WASI_CTX.with(|ctx| {
+        let wasi_ctx = WasiCtxBuilder::new()
+            .inherit_stdio()
+            .build()
+            .expect("could build WasiCtx with stdio inherited");
+        ctx.replace(Some(wasi_ctx));
+    });
+    unsafe { loadFiles() }
+}