js-sys: Add js_sys::try_iter for iterating over any JsValue

Fixes #776

crates/js-sys/src/lib.rs

@@ -1255,6 +1255,32 @@ impl IterState {
     }
 }
 
+/// Create an iterator over `val` using the JS iteration protocol and
+/// `Symbol.iterator`.
+pub fn try_iter(val: &JsValue) -> Result<Option<IntoIter>, JsValue> {
+    let iter_sym = Symbol::iterator();
+    let iter_fn = Reflect::get(val, iter_sym.as_ref())?;
+    if !iter_fn.is_function() {
+        return Ok(None);
+    }
+
+    let iter_fn: Function = iter_fn.unchecked_into();
+    let it = iter_fn.call0(val)?;
+    if !it.is_object() {
+        return Ok(None);
+    }
+
+    let next = JsValue::from("next");
+    let next = Reflect::get(&it, &next)?;
+
+    Ok(if next.is_function() {
+        let it: Iterator = it.unchecked_into();
+        Some(it.into_iter())
+    } else {
+        None
+    })
+}
+
 // IteratorNext
 #[wasm_bindgen]
 extern {

crates/js-sys/tests/wasm/Iterator.js

@@ -0,0 +1,19 @@
+exports.get_iterable = () => ["one", "two", "three"];
+
+exports.get_not_iterable = () => new Object;
+
+exports.get_symbol_iterator_throws = () => ({
+  [Symbol.iterator]: () => { throw new Error("nope"); },
+});
+
+exports.get_symbol_iterator_not_function = () => ({
+  [Symbol.iterator]: 5,
+});
+
+exports.get_symbol_iterator_returns_not_object = () => ({
+  [Symbol.iterator]: () => 5,
+});
+
+exports.get_symbol_iterator_returns_object_without_next = () => ({
+  [Symbol.iterator]: () => new Object,
+});

crates/js-sys/tests/wasm/Iterator.rs

@@ -0,0 +1,36 @@
+use js_sys::*;
+use wasm_bindgen::prelude::*;
+use wasm_bindgen_test::*;
+
+#[wasm_bindgen(module = "tests/wasm/Iterator.js")]
+extern "C" {
+    fn get_iterable() -> JsValue;
+
+    fn get_not_iterable() -> JsValue;
+
+    fn get_symbol_iterator_throws() -> JsValue;
+
+    fn get_symbol_iterator_not_function() -> JsValue;
+
+    fn get_symbol_iterator_returns_not_object() -> JsValue;
+
+    fn get_symbol_iterator_returns_object_without_next() -> JsValue;
+}
+
+#[wasm_bindgen_test]
+fn try_iter_handles_iteration_protocol() {
+    assert_eq!(
+        try_iter(&get_iterable())
+            .unwrap()
+            .unwrap()
+            .map(|x| x.unwrap().as_string().unwrap())
+            .collect::<Vec<_>>(),
+        vec!["one", "two", "three"]
+    );
+
+    assert!(try_iter(&get_not_iterable()).unwrap().is_none());
+    assert!(try_iter(&get_symbol_iterator_throws()).is_err());
+    assert!(try_iter(&get_symbol_iterator_not_function()).unwrap().is_none());
+    assert!(try_iter(&get_symbol_iterator_returns_not_object()).unwrap().is_none());
+    assert!(try_iter(&get_symbol_iterator_returns_object_without_next()).unwrap().is_none());
+}

crates/js-sys/tests/wasm/main.rs

@@ -19,6 +19,7 @@ pub mod EvalError;
 pub mod Function;
 pub mod Generator;
 pub mod Intl;
+pub mod Iterator;
 pub mod JsString;
 pub mod JSON;
 pub mod Map;