error-handling.md

src/SUMMARY.md

@@ -301,6 +301,7 @@
 - [Import Method](webassembly/import-method.md)
   - [web-sys](webassembly/web-sys.md)
 - [Expose a method](webassembly/expose-method.md)
+- [Error handling](webassembly/error-handling.md)
 - [Expose user-defined Rust types](webassembly/expose-rust-type.md)
 - [Borrow Checker](webassembly/borrow-checker.md)
 

src/webassembly/error-handling.md

@@ -0,0 +1,54 @@
+# Error handling
+
+`Result<T, E>` is translated to javascript errors. Methods directly exposed and called from Javascript that return a `Result`
+will automatically throw an error if the variant of the return value is `Result::Err`. `E` has to implement `Into<JsValue>`.
+
+```rust
+#[wasm_bindgen]
+pub fn str_to_int(s: &str) -> Option<i32, JsValue> {
+    s.parse::<i32>()
+        .map_err(|_| JsValue::from_str("Failed to parse string"))
+}
+```
+
+`Option` is converted to `undefined` in case of `None`. 
+
+```rust
+#[wasm_bindgen]
+pub fn str_to_int(s: &str) -> Option<i32> {
+    s.parse::<i32>().ok()
+}
+
+```
+
+Javascript (click on the wasm output box) to parse the string:
+
+```javascript
+import init, {set_panic_hook, str_to_int} from '/wasm/project.js';
+
+
+(async () => { 
+    // Run the init method to initiate the WebAssembly module.
+    await init();
+    set_panic_hook();
+    const wasmoutput = document.querySelector('#wasmoutput');
+    const input = document.createElement('input');
+    input.type = 'text';
+    document.body.appendChild(input);
+    wasmoutput.onclick = () => {
+        try {
+            wasmoutput.innerHTML = str_to_int(input.value);
+        } catch (e) {
+            wasmoutput.innerHTML = e;
+        }
+    };
+})();
+```
+
+
+<details>
+
+* Click on the wasm output box to see the output
+* `?` and other error handling toos also work
+
+</details>