Add js_sys::Array::set_length (#3246)

Fixes #3244

crates/js-sys/src/lib.rs

@@ -447,6 +447,19 @@ extern "C" {
     #[wasm_bindgen(method, getter, structural)]
     pub fn length(this: &Array) -> u32;
 
+    /// Sets the length of the array.
+    ///
+    /// If it is set to less than the current length of the array, it will
+    /// shrink the array.
+    ///
+    /// If it is set to more than the current length of the array, it will
+    /// increase the length of the array, filling the new space with empty
+    /// slots.
+    ///
+    /// [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/length)
+    #[wasm_bindgen(method, setter)]
+    pub fn set_length(this: &Array, value: u32);
+
     /// `map()` calls a provided callback function once for each element in an array,
     /// in order, and constructs a new array from the results. callback is invoked
     /// only for indexes of the array which have assigned values, including undefined.

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

@@ -534,6 +534,33 @@ fn for_each() {
     assert_eq!(sum_indices_of_evens(&js_array![3, 5, 7, 10]), 3);
 }
 
+#[wasm_bindgen_test]
+fn set_length() {
+    let array = js_array![1, 2, 3, 4, 5];
+    array.set_length(3);
+    assert_eq!(
+        array.iter().collect::<Vec<_>>(),
+        [1.0, 2.0, 3.0].map(|x| JsValue::from_f64(x))
+    );
+
+    array.set_length(7);
+    assert_eq!(
+        array.iter().collect::<Vec<_>>(),
+        [1.0, 2.0, 3.0]
+            .iter()
+            .copied()
+            .map(|x| JsValue::from_f64(x))
+            .chain([JsValue::UNDEFINED; 4])
+            .collect::<Vec<_>>()
+    );
+
+    let mut calls = 0;
+    array.for_each(&mut |_, _, _| calls += 1);
+    // The later elements don't get filled with `undefined`, they get filled with
+    // empty slots, which get skipped by `for_each`.
+    assert_eq!(calls, 3);
+}
+
 #[wasm_bindgen_test]
 fn array_inheritance() {
     let array = Array::new();