diff --git a/napi/Cargo.toml b/napi/Cargo.toml
index 951f9862..c7a10721 100644
--- a/napi/Cargo.toml
+++ b/napi/Cargo.toml
@@ -19,4 +19,13 @@ bindgen = "0.54"
 glob = "0.3"
 napi-build = { version = "0.1", path = "../build" }
 regex = "1.3"
-semver = "0.10"
\ No newline at end of file
+semver = "0.10"
+
+[features]
+default = []
+# See the N-API Version Matrix https://nodejs.org/api/n-api.html#n_api_n_api_version_matrix
+napi2 = []
+napi3 = []
+napi4 = []
+napi5 = []
+napi6 = []
diff --git a/napi/build.rs b/napi/build.rs
index f80d29ad..179683d4 100644
--- a/napi/build.rs
+++ b/napi/build.rs
@@ -57,6 +57,22 @@ fn main() {
     .expect("Unable to generate napi bindings")
     .write_to_file(out_path.join("bindings.rs"))
     .expect("Unable to write napi bindings");
+
+  let napi_version = String::from_utf8(
+    Command::new("node")
+      .args(&[
+        "-e",
+        "console.log(process.versions.napi)",
+      ])
+      .output()
+      .unwrap()
+      .stdout,
+  )
+  .unwrap();
+
+  for version in 2..napi_version.trim().parse::<u32>().unwrap() {
+    println!("cargo:rustc-cfg=napi{}", version);
+  }
 }
 
 #[cfg(target_os = "windows")]
diff --git a/napi/src/lib.rs b/napi/src/lib.rs
index 113a2dfa..1b9ed7c0 100644
--- a/napi/src/lib.rs
+++ b/napi/src/lib.rs
@@ -16,6 +16,8 @@ mod async_work;
 mod call_context;
 pub mod sys;
 mod task;
+#[cfg(napi4)]
+pub mod threadsafe_function;
 mod version;
 
 pub use call_context::CallContext;
diff --git a/napi/src/threadsafe_function.rs b/napi/src/threadsafe_function.rs
new file mode 100644
index 00000000..2d14829b
--- /dev/null
+++ b/napi/src/threadsafe_function.rs
@@ -0,0 +1,233 @@
+use crate::{check_status, ptr, sys, Env, Function, Result, Value};
+use std::os::raw::{c_char, c_void};
+
+use sys::napi_threadsafe_function_call_mode;
+use sys::napi_threadsafe_function_release_mode;
+
+pub trait ToJs: Copy + Clone {
+  type Output;
+  type JsValue;
+
+  fn resolve(
+    &self,
+    env: &mut Env,
+    output: Self::Output,
+  ) -> Result<(u64, Value<Self::JsValue>)>;
+}
+
+/// Communicate with the addon's main thread by invoking a JavaScript function from other threads.
+///
+/// ## Example
+/// An example of using `ThreadsafeFunction`:
+///
+/// ```
+/// #[macro_use]
+/// extern crate napi_rs_derive;
+///
+/// use std::thread;
+/// use napi_rs::{
+///   Number, Result, Value, Env, CallContext, Undefined, Function,
+///   sys::{
+///     napi_threadsafe_function_call_mode::{
+///       napi_tsfn_blocking,
+///     },
+///     napi_threadsafe_function_release_mode::{
+///       napi_tsfn_release,
+///     }
+///   }
+/// };
+/// use napi_rs::threadsafe_function::{
+///   ToJs, ThreadsafeFunction,
+/// };
+///
+/// // Define a struct for handling the data passed from `ThreadsafeFunction::call`
+/// // and return the data to be used for the js callback.
+/// #[derive(Clone, Copy)]
+/// struct HandleNumber;
+///
+/// impl ToJs for HandleNumber {
+///   type Output = u8;
+///   type JsValue = Number;
+///
+///   fn resolve(&self, env: &mut Env, output: Self::Output) -> Result<(u64, Value<Self::JsValue>)> {
+///     let argv: u64 = 1;
+///     let value = env.create_uint32(output as u32)?;
+///     Ok((argv, value))
+///   }
+/// }
+///
+/// #[js_function(1)]
+/// fn test_threadsafe_function(ctx: CallContext) -> Result<Value<Undefined>> {
+///   // The callback function from js which will be called in `ThreadsafeFunction::call`.
+///   let func: Value<Function> = ctx.get::<Function>(0)?;
+///
+///   let to_js = HandleNumber;
+///   let tsfn = ThreadsafeFunction::create(ctx.env, func, to_js, 0)?;
+///
+///   thread::spawn(move || {
+///     let output: u8 = 42;
+///     // It's okay to call a threadsafe function multiple times.
+///     tsfn.call(Ok(output), napi_tsfn_blocking).unwrap();
+///     tsfn.call(Ok(output), napi_tsfn_blocking).unwrap();
+///     // We should call `ThreadsafeFunction::release` manually when we don't
+///     // need the instance anymore, or it will prevent Node.js from exiting
+///     // automatically and possiblely cause memory leaks.
+///     tsfn.release(napi_tsfn_release).unwrap();
+///   });
+///
+///   Ok(Env::get_undefined(ctx.env)?)
+/// }
+/// ```
+#[derive(Debug, Clone, Copy)]
+pub struct ThreadsafeFunction<T: ToJs> {
+  raw_value: sys::napi_threadsafe_function,
+  to_js: T,
+}
+
+unsafe impl<T: ToJs> Send for ThreadsafeFunction<T> {}
+unsafe impl<T: ToJs> Sync for ThreadsafeFunction<T> {}
+
+impl<T: ToJs> ThreadsafeFunction<T> {
+  /// See [napi_create_threadsafe_function](https://nodejs.org/api/n-api.html#n_api_napi_create_threadsafe_function)
+  /// for more information.
+  pub fn create(env: &Env, func: Value<Function>, to_js: T, max_queue_size: u64) -> Result<Self> {
+    let mut async_resource_name = ptr::null_mut();
+    let s = "napi_rs_threadsafe_function";
+    let status = unsafe {
+      sys::napi_create_string_utf8(
+        env.0,
+        s.as_ptr() as *const c_char,
+        s.len() as u64,
+        &mut async_resource_name,
+      )
+    };
+    check_status(status)?;
+
+    let initial_thread_count: u64 = 1;
+    let mut result = ptr::null_mut();
+    let tsfn = ThreadsafeFunction {
+      to_js,
+      raw_value: result,
+    };
+
+    let ptr = Box::into_raw(Box::from(tsfn)) as *mut _ as *mut c_void;
+
+    let status = unsafe {
+      sys::napi_create_threadsafe_function(
+        env.0,
+        func.raw_value,
+        ptr::null_mut(),
+        async_resource_name,
+        max_queue_size,
+        initial_thread_count,
+        ptr,
+        Some(thread_finalize_cb::<T>),
+        ptr,
+        Some(call_js_cb::<T>),
+        &mut result,
+      )
+    };
+    check_status(status)?;
+
+    Ok(ThreadsafeFunction {
+      to_js,
+      raw_value: result,
+    })
+  }
+
+  /// See [napi_call_threadsafe_function](https://nodejs.org/api/n-api.html#n_api_napi_call_threadsafe_function)
+  /// for more information.
+  pub fn call(
+    &self,
+    value: Result<T::Output>,
+    mode: napi_threadsafe_function_call_mode,
+  ) -> Result<()> {
+    check_status(unsafe {
+      sys::napi_call_threadsafe_function(
+        self.raw_value,
+        Box::into_raw(Box::from(value)) as *mut _ as *mut c_void,
+        mode,
+      )
+    })
+  }
+
+  /// See [napi_acquire_threadsafe_function](https://nodejs.org/api/n-api.html#n_api_napi_acquire_threadsafe_function)
+  /// for more information.
+  pub fn acquire(&self) -> Result<()> {
+    check_status(unsafe { sys::napi_acquire_threadsafe_function(self.raw_value) })
+  }
+
+  /// See [napi_release_threadsafe_function](https://nodejs.org/api/n-api.html#n_api_napi_release_threadsafe_function)
+  /// for more information.
+  pub fn release(&self, mode: napi_threadsafe_function_release_mode) -> Result<()> {
+    check_status(unsafe { sys::napi_release_threadsafe_function(self.raw_value, mode) })
+  }
+
+  /// See [napi_ref_threadsafe_function](https://nodejs.org/api/n-api.html#n_api_napi_ref_threadsafe_function)
+  /// for more information.
+  ///
+  /// "ref" is a keyword so that we use "refer" here.
+  pub fn refer(&self, env: &Env) -> Result<()> {
+    check_status(unsafe { sys::napi_ref_threadsafe_function(env.0, self.raw_value) })
+  }
+
+  /// See [napi_unref_threadsafe_function](https://nodejs.org/api/n-api.html#n_api_napi_unref_threadsafe_function)
+  /// for more information.
+  pub fn unref(&self, env: &Env) -> Result<()> {
+    check_status(unsafe { sys::napi_unref_threadsafe_function(env.0, self.raw_value) })
+  }
+}
+
+unsafe extern "C" fn thread_finalize_cb<T: ToJs>(
+  _raw_env: sys::napi_env,
+  finalize_data: *mut c_void,
+  _finalize_hint: *mut c_void,
+) {
+  // cleanup
+  Box::from_raw(finalize_data as *mut ThreadsafeFunction<T>);
+}
+
+unsafe extern "C" fn call_js_cb<T: ToJs>(
+  raw_env: sys::napi_env,
+  js_callback: sys::napi_value,
+  context: *mut c_void,
+  data: *mut c_void,
+) {
+  let mut env = Env::from_raw(raw_env);
+  let mut recv = ptr::null_mut();
+  sys::napi_get_undefined(raw_env, &mut recv);
+
+  let tsfn = Box::leak(Box::from_raw(context as *mut ThreadsafeFunction<T>));
+  let val = Box::from_raw(data as *mut Result<T::Output>);
+
+  let ret = val.and_then(|v| tsfn.to_js.resolve(&mut env, v));
+
+  let status;
+
+  // Follow the convention of Node.js async callback.
+  if ret.is_ok() {
+    let (argv, js_value) = ret.unwrap();
+    let js_null = env.get_null().unwrap();
+    let values = [js_null.raw_value, js_value.raw_value];
+    status = sys::napi_call_function(
+      raw_env,
+      recv,
+      js_callback,
+      argv + 1,
+      values.as_ptr(),
+      ptr::null_mut(),
+    );
+  } else {
+    let mut err = env.create_error(ret.err().unwrap()).unwrap();
+    status = sys::napi_call_function(
+      raw_env,
+      recv,
+      js_callback,
+      1,
+      &mut err.raw_value,
+      ptr::null_mut(),
+    );
+  }
+
+  debug_assert!(status == sys::napi_status::napi_ok, "CallJsCB failed");
+}
diff --git a/test_module/Cargo.toml b/test_module/Cargo.toml
index 81a7e16f..d0dfd8c1 100644
--- a/test_module/Cargo.toml
+++ b/test_module/Cargo.toml
@@ -10,6 +10,7 @@ crate-type = ["cdylib"]
 [dependencies]
 napi-rs = { path = "../napi" }
 napi-rs-derive = { path = "../napi-derive" }
+tokio = { version = "0.2", features = ["default", "fs"]}
 
 [build-dependencies]
 napi-build = { path = "../build" }
diff --git a/test_module/__test__/example.txt b/test_module/__test__/example.txt
new file mode 100644
index 00000000..cd087558
--- /dev/null
+++ b/test_module/__test__/example.txt
@@ -0,0 +1 @@
+Hello world!
diff --git a/test_module/__test__/threadsafe_function.spec.js b/test_module/__test__/threadsafe_function.spec.js
new file mode 100644
index 00000000..627b2952
--- /dev/null
+++ b/test_module/__test__/threadsafe_function.spec.js
@@ -0,0 +1,22 @@
+const test = require('ava')
+const bindings = require('../index.node')
+
+test('should get js function called from a thread', async (t) => {
+  let called = 0
+
+  return new Promise((resolve, reject) => {
+    bindings.testThreadsafeFunction((err, ret) => {
+      called += 1
+      try {
+        t.is(err, null)
+        t.is(ret, 42)
+      } catch (err) {
+        reject(err)
+      }
+
+      if (called === 2) {
+        resolve()
+      }
+    })
+  })
+})
diff --git a/test_module/__test__/tokio_readfile.spec.js b/test_module/__test__/tokio_readfile.spec.js
new file mode 100644
index 00000000..695caa03
--- /dev/null
+++ b/test_module/__test__/tokio_readfile.spec.js
@@ -0,0 +1,21 @@
+const test = require('ava')
+const fs = require('fs')
+const path = require('path')
+const bindings = require('../index.node')
+
+const filepath = path.resolve(__dirname, './example.txt')
+
+test('should read a file and return its a buffer', async (t) => {
+  return new Promise((resolve, reject) => {
+    bindings.testTokioReadfile(filepath, (err, value) => {
+      try {
+        t.is(err, null)
+        t.is(Buffer.isBuffer(value), true)
+        t.is(value.toString(), fs.readFileSync(filepath, 'utf8'))
+        resolve()
+      } catch (err) {
+        reject(err)
+      }
+    })
+  })
+})
diff --git a/test_module/__test__/tsfn_error.spec.js b/test_module/__test__/tsfn_error.spec.js
new file mode 100644
index 00000000..d692a920
--- /dev/null
+++ b/test_module/__test__/tsfn_error.spec.js
@@ -0,0 +1,16 @@
+const test = require('ava')
+const bindings = require('../index.node')
+
+test('should call callback with the first arguments as an Error', async (t) => {
+  return new Promise((resolve, reject) => {
+    bindings.testTsfnError((err) => {
+      try {
+        t.is(err instanceof Error, true)
+        t.is(err.message, 'invalid')
+        resolve()
+      } catch (err) {
+        reject(err)
+      }
+    })
+  })
+})
diff --git a/test_module/src/lib.rs b/test_module/src/lib.rs
index 8b690273..17304501 100644
--- a/test_module/src/lib.rs
+++ b/test_module/src/lib.rs
@@ -5,8 +5,25 @@ extern crate napi_rs_derive;
 
 use napi::{
   Any, Boolean, CallContext, Env, Error, JsString, Number, Object, Result, Status, Task, Value,
+  Undefined, Function, Buffer,
+  threadsafe_function::{
+    ToJs,
+    ThreadsafeFunction,
+  }
+};
+use napi::sys::{
+  napi_threadsafe_function_call_mode:: {
+    napi_tsfn_blocking,
+  },
+  napi_threadsafe_function_release_mode:: {
+    napi_tsfn_release,
+  }
 };
 use std::convert::TryInto;
+use std::thread;
+use std::path::Path;
+use std::ops::Deref;
+use tokio;
 
 register_module!(test_module, init);
 
@@ -24,16 +41,26 @@ fn init(env: &Env, exports: &mut Value<Object>) -> Result<()> {
     "testObjectIsDate",
     env.create_function("testObjectIsDate", test_object_is_date)?,
   )?;
-
   exports.set_named_property(
     "createExternal",
     env.create_function("createExternal", create_external)?,
   )?;
-
   exports.set_named_property(
     "getExternalCount",
     env.create_function("getExternalCount", get_external_count)?,
   )?;
+  exports.set_named_property(
+    "testTsfnError",
+    env.create_function("testTsfnError", test_tsfn_error)?,
+  )?;
+  exports.set_named_property(
+    "testThreadsafeFunction",
+    env.create_function("testThreadsafeFunction", test_threadsafe_function)?
+  )?;
+  exports.set_named_property(
+    "testTokioReadfile",
+    env.create_function("testTokioReadfile", test_tokio_readfile)?
+  )?;
   Ok(())
 }
 
@@ -112,3 +139,94 @@ fn get_external_count(ctx: CallContext) -> Result<Value<Number>> {
   let native_object = ctx.env.get_value_external::<NativeObject>(&attached_obj)?;
   ctx.env.create_int32(native_object.count)
 }
+
+#[derive(Clone, Copy)]
+struct HandleNumber;
+
+impl ToJs for HandleNumber {
+  type Output = u8;
+  type JsValue = Number;
+
+  fn resolve(&self, env: &mut Env, output: Self::Output) -> Result<(u64, Value<Self::JsValue>)> {
+    let argv: u64 = 1;
+
+    let value = env.create_uint32(output as u32)?;
+
+    Ok((argv, value))
+  }
+}
+
+#[js_function(1)]
+fn test_threadsafe_function(ctx: CallContext) -> Result<Value<Undefined>> {
+  let func: Value<Function> = ctx.get::<Function>(0)?;
+
+  let to_js = HandleNumber;
+  let tsfn = ThreadsafeFunction::create(ctx.env, func, to_js, 0)?;
+
+  thread::spawn(move || {
+    let output: u8 = 42;
+    // It's okay to call a threadsafe function multiple times.
+    tsfn.call(Ok(output), napi_tsfn_blocking).unwrap();
+    tsfn.call(Ok(output), napi_tsfn_blocking).unwrap();
+    tsfn.release(napi_tsfn_release).unwrap();
+  });
+
+  Ok(Env::get_undefined(ctx.env)?)
+}
+
+#[js_function(1)]
+fn test_tsfn_error(ctx: CallContext) -> Result<Value<Undefined>> {
+  let func = ctx.get::<Function>(0)?;
+  let to_js = HandleNumber;
+  let tsfn = ThreadsafeFunction::create(ctx.env, func, to_js, 0)?;
+
+  thread::spawn(move || {
+    tsfn.call(Err(Error {
+      status: napi::sys::Status::Unknown,
+      reason: Some(String::from("invalid")),
+    }), napi_tsfn_blocking).unwrap();
+    tsfn.release(napi_tsfn_release).unwrap();
+  });
+
+  Ok(Env::get_undefined(ctx.env)?)
+}
+
+#[derive(Copy, Clone)]
+struct HandleBuffer;
+
+impl ToJs for HandleBuffer {
+  type Output = Vec<u8>;
+  type JsValue = Buffer;
+
+  fn resolve(&self, env: &mut Env, output: Self::Output) -> Result<(u64, Value<Self::JsValue>)> {
+    let value = env.create_buffer_with_data(output.to_vec())?;
+    Ok((1u64, value))
+  }
+}
+
+async fn read_file_content(filepath: &Path) -> Result<Vec<u8>> {
+  tokio::fs::read(filepath).await.map_err(|_| Error {
+    status: Status::Unknown,
+    reason: Some(String::from("failed to read file")),
+  })
+}
+
+#[js_function(2)]
+fn test_tokio_readfile(ctx: CallContext) -> Result<Value<Undefined>> {
+  let js_filepath: Value<JsString> = ctx.get::<JsString>(0)?;
+  let js_func: Value<Function> = ctx.get::<Function>(1)?;
+  let path_str = String::from(js_filepath.as_str()?);
+
+  let to_js = HandleBuffer;
+  let tsfn = ThreadsafeFunction::create(ctx.env, js_func, to_js, 0)?;
+  let mut rt = tokio::runtime::Runtime::new().unwrap();
+
+  rt.block_on(async move {
+    let mut filepath = Path::new(path_str.deref());
+    let ret = read_file_content(&mut filepath).await;
+    let _ = tsfn.call(ret, napi_tsfn_blocking);
+    tsfn.release(napi_tsfn_release).unwrap();
+  });
+
+  Ok(Env::get_undefined(ctx.env)?)
+}