WASM support (#491)

Co-authored-by: tmpfs <muji@tmpfs.org>

Cargo.toml

@@ -49,6 +49,9 @@ time-macros = { version = "=0.2.4", path = "time-macros", optional = true }
 libc = "0.2.98"
 num_threads = "0.1.2"
 
+[target.'cfg(all(target_arch = "wasm32", not(any(target_os = "emscripten", target_os = "wasi"))))'.dependencies]
+js-sys = "0.3.58"
+
 [dev-dependencies]
 rand = { version = "0.8.4", default-features = false }
 serde = { version = "1.0.126", default-features = false, features = ["derive"] }

src/offset_date_time.rs

@@ -59,6 +59,18 @@ impl OffsetDateTime {
     #[cfg(feature = "std")]
     #[cfg_attr(__time_03_docs, doc(cfg(feature = "std")))]
     pub fn now_utc() -> Self {
+        #[cfg(all(
+            target_arch = "wasm32",
+            not(any(target_os = "emscripten", target_os = "wasi"))
+        ))]
+        {
+            js_sys::Date::new_0().into()
+        }
+
+        #[cfg(not(all(
+            target_arch = "wasm32",
+            not(any(target_os = "emscripten", target_os = "wasi"))
+        )))]
         SystemTime::now().into()
     }
 
@@ -1320,4 +1332,31 @@ impl From<OffsetDateTime> for SystemTime {
         }
     }
 }
+
+#[allow(clippy::fallible_impl_from)]
+#[cfg(all(
+    target_arch = "wasm32",
+    not(any(target_os = "emscripten", target_os = "wasi"))
+))]
+impl From<js_sys::Date> for OffsetDateTime {
+    fn from(js_date: js_sys::Date) -> Self {
+        // get_time() returns milliseconds
+        let timestamp_nanos = (js_date.get_time() * 1_000_000.0) as i128;
+        Self::from_unix_timestamp_nanos(timestamp_nanos)
+            .expect("invalid timestamp: Timestamp cannot fit in range")
+    }
+}
+
+#[cfg(all(
+    target_arch = "wasm32",
+    not(any(target_os = "emscripten", target_os = "wasi"))
+))]
+impl From<OffsetDateTime> for js_sys::Date {
+    fn from(datetime: OffsetDateTime) -> Self {
+        // new Date() takes milliseconds
+        let timestamp = (datetime.unix_timestamp_nanos() / 1_000_000) as f64;
+        js_sys::Date::new(&timestamp.into())
+    }
+}
+
 // endregion trait impls

src/sys/local_offset_at/mod.rs

@@ -2,6 +2,13 @@
 
 #[cfg_attr(target_family = "windows", path = "windows.rs")]
 #[cfg_attr(target_family = "unix", path = "unix.rs")]
+#[cfg_attr(
+    all(
+        target_arch = "wasm32",
+        not(any(target_os = "emscripten", target_os = "wasi"))
+    ),
+    path = "wasm_js.rs"
+)]
 mod imp;
 
 use crate::{OffsetDateTime, UtcOffset};

src/sys/local_offset_at/wasm_js.rs

@@ -0,0 +1,12 @@
+use crate::{OffsetDateTime, UtcOffset};
+
+/// Obtain the system's UTC offset.
+pub(super) fn local_offset_at(datetime: OffsetDateTime) -> Option<UtcOffset> {
+    let js_date: js_sys::Date = datetime.into();
+    // The number of minutes returned by getTimezoneOffset() is positive if the local time zone
+    // is behind UTC, and negative if the local time zone is ahead of UTC. For example,
+    // for UTC+10, -600 will be returned.
+    let timezone_offset = (js_date.get_timezone_offset() as i32) * -60;
+
+    UtcOffset::from_whole_seconds(timezone_offset).ok()
+}