diff --git a/js/mkdir_test.ts b/js/mkdir_test.ts
index 4f82401dcc..27da93c160 100644
--- a/js/mkdir_test.ts
+++ b/js/mkdir_test.ts
@@ -13,7 +13,8 @@ testPerm({ write: true }, function mkdirSyncMode() {
   const path = deno.makeTempDirSync() + "/dir/subdir";
   deno.mkdirSync(path, 0o755); // no perm for x
   const pathInfo = deno.statSync(path);
-  if (pathInfo.mode !== null) { // Skip windows
+  if (pathInfo.mode !== null) {
+    // Skip windows
     assertEqual(pathInfo.mode & 0o777, 0o755);
   }
 });
diff --git a/js/timers_test.ts b/js/timers_test.ts
new file mode 100644
index 0000000000..af172e9765
--- /dev/null
+++ b/js/timers_test.ts
@@ -0,0 +1,97 @@
+import { test, assertEqual } from "./test_util.ts";
+
+function deferred() {
+  let resolve;
+  let reject;
+  const promise = new Promise((res, rej) => {
+    resolve = res;
+    reject = rej;
+  });
+  return {
+    promise,
+    resolve,
+    reject
+  };
+}
+
+function waitForMs(ms) {
+  return new Promise(resolve => setTimeout(resolve, ms));
+}
+
+test(async function timeoutSuccess() {
+  const { promise, resolve } = deferred();
+  let count = 0;
+  setTimeout(() => {
+    count++;
+    resolve();
+  }, 500);
+  await promise;
+  // count should increment
+  assertEqual(count, 1);
+});
+
+test(async function timeoutCancelSuccess() {
+  let count = 0;
+  const id = setTimeout(() => {
+    count++;
+  }, 500);
+  // Cancelled, count should not increment
+  clearTimeout(id);
+  // Wait a bit longer than 500ms
+  await waitForMs(600);
+  assertEqual(count, 0);
+});
+
+test(async function timeoutCancelInvalidSilentFail() {
+  // Expect no panic
+  const { promise, resolve } = deferred();
+  let count = 0;
+  const id = setTimeout(() => {
+    count++;
+    // Should have no effect
+    clearTimeout(id);
+    resolve();
+  }, 500);
+  await promise;
+  assertEqual(count, 1);
+
+  // Should silently fail (no panic)
+  clearTimeout(2147483647);
+});
+
+test(async function intervalSuccess() {
+  const { promise, resolve } = deferred();
+  let count = 0;
+  const id = setInterval(() => {
+    count++;
+    if (count === 2) {
+      // TODO: clearInterval(id) here alone seems not working
+      // causing unit_tests.ts to block forever
+      // Requires further investigation...
+      clearInterval(id);
+      resolve();
+    }
+  }, 200);
+  await promise;
+  // Clear interval
+  clearInterval(id);
+  // count should increment twice
+  assertEqual(count, 2);
+});
+
+test(async function intervalCancelSuccess() {
+  let count = 0;
+  const id = setInterval(() => {
+    count++;
+  }, 500);
+  // Cancelled, count should not increment
+  clearInterval(id);
+  // Wait a bit longer than 500ms
+  await waitForMs(600);
+  assertEqual(count, 0);
+});
+
+test(async function intervalCancelInvalidSilentFail() {
+  // Should silently fail (no panic)
+  clearInterval(2147483647);
+});
diff --git a/js/unit_tests.ts b/js/unit_tests.ts
index e1385e0116..4f7b9681d8 100644
--- a/js/unit_tests.ts
+++ b/js/unit_tests.ts
@@ -12,3 +12,4 @@ import "./make_temp_dir_test.ts";
 import "./stat_test.ts";
 import "./rename_test.ts";
 import "./blob_test.ts";
+import "./timers_test.ts";
diff --git a/src/flags.rs b/src/flags.rs
index 13f0657350..2e48622186 100644
--- a/src/flags.rs
+++ b/src/flags.rs
@@ -154,8 +154,7 @@ fn parse_core_args(args: Vec<String>) -> (Vec<String>, Vec<String>) {
       }
 
       true
-    })
-    .collect();
+    }).collect();
 
   // Replace args being sent to V8
   for idx in 0..args.len() {
@@ -222,7 +221,6 @@ pub fn v8_set_flags(args: Vec<String>) -> Vec<String> {
       let cstr = CStr::from_ptr(*ptr as *const i8);
       let slice = cstr.to_str().unwrap();
       slice.to_string()
-    })
-    .chain(rest.into_iter())
+    }).chain(rest.into_iter())
     .collect()
 }
diff --git a/src/handlers.rs b/src/handlers.rs
index 0917fd22ab..39f2d51aaa 100644
--- a/src/handlers.rs
+++ b/src/handlers.rs
@@ -288,8 +288,7 @@ fn handle_env(d: *const DenoC, base: &msg::Base) -> Box<Op> {
           ..Default::default()
         },
       )
-    })
-    .collect();
+    }).collect();
   let tables = builder.create_vector(&vars);
   let msg = msg::EnvironRes::create(
     builder,
@@ -402,8 +401,7 @@ where
     .and_then(|_| {
       cb();
       Ok(())
-    })
-    .select(cancel_rx)
+    }).select(cancel_rx)
     .map(|_| ())
     .map_err(|_| ());
 
@@ -604,9 +602,7 @@ fn handle_write_file(d: *const DenoC, base: &msg::Base) -> Box<Op> {
 // TODO(ry) Use Deno instead of DenoC as first arg.
 fn remove_timer(d: *const DenoC, timer_id: u32) {
   let deno = from_c(d);
-  assert!(deno.timers.contains_key(&timer_id));
   deno.timers.remove(&timer_id);
-  assert!(!deno.timers.contains_key(&timer_id));
 }
 
 // Prototype: https://github.com/ry/deno/blob/golang/timers.go#L25-L39