Add docs and remove typecheck from variadic attribute

guide/src/SUMMARY.md

@@ -42,6 +42,7 @@
       - [`module = "blah"`](./reference/attributes/on-js-imports/module.md)
       - [`static_method_of = Blah`](./reference/attributes/on-js-imports/static_method_of.md)
       - [`structural`](./reference/attributes/on-js-imports/structural.md)
+      - [variadic](./reference/attributes/on-js-imports/variadic.md)
     - [On Rust Exports](./reference/attributes/on-rust-exports/index.md)
       - [`constructor`](./reference/attributes/on-rust-exports/constructor.md)
       - [`js_name = Blah`](./reference/attributes/on-rust-exports/js_name.md)

guide/src/reference/attributes/on-js-imports/variadic.md

@@ -0,0 +1,38 @@
+# Variadic Parameters
+
+In javascript, both the types of function arguments, and the number of function arguments are
+dynamic. For example
+
+```js
+function sum(...rest) {
+    let i;
+    // the old way
+    let old_way = 0;
+    for (i=0; i<arguments.length; i++) {
+        old_way += arguments[i];
+    }
+    // the new way
+    let new_way = 0;
+    for (i=0; i<rest.length; i++) {
+        new_way += rest[i];
+    }
+    // both give the same answer
+    assert(old_way === new_way);
+    return new_way;
+}
+```
+
+This function doesn't translate directly into rust, since we don't currently support variadic
+arguments on the wasm target. To bind to it, we use a slice as the last argument, and annotate the
+function as variadic:
+
+```rust
+#[wasm_bindgen]
+extern {
+    #[wasm_bindgen(variadic)]
+    fn sum(args: &[i32]) -> i32;
+}
+```
+
+when we call this function, the last argument will be expanded as the javascript expects.
+

guide/src/whirlwind-tour/what-else-can-we-do.md

@@ -64,6 +64,10 @@ extern {
     fn new() -> Awesome;
     #[wasm_bindgen(method)]
     fn get_internal(this: &Awesome) -> u32;
+    // We can call javascript functions that have a dynamic number of arguments,
+    // e.g. rust `sum(&[1, 2, 3])` will be called like `sum(1, 2, 3)`
+    #[wasm_bindgen(variadic)]
+    fn sum(vals: &[u32]) -> u32;
 }
 
 #[wasm_bindgen]
@@ -143,5 +147,13 @@ export class Awesome {
   }
 }
 
+export function sum(...args) {
+    let answer = 0;
+    for(var i=0; i<args.length; i++) {
+        answer += args[i];
+    }
+    return answer;
+}
+
 booted.then(main);
 ```