feat(js): add typed array toArray (#70)

Author: spiral-ladder

README.md

@@ -90,7 +90,7 @@ c.count; // 1 (getter, not a method call)
 | `Object(T)` | `object` | `get()`, `set(value)` — `T` fields must be DSL types |
 | `Function` | `Function` | `call(args)` |
 | `Value` | `any` | `isNumber()`, `asNumber()`, type checking/narrowing |
-| `Uint8Array` etc. | `TypedArray` | `toSlice()`, `from(slice)` |
+| `Uint8Array` etc. | `TypedArray` | `toSlice()`, `toArray(len)`, `from(slice)` |
 | `Promise(T)` | `Promise` | `resolve(value)`, `reject(err)` |
 
 ---

examples/js_dsl/mod.test.ts

@@ -201,6 +201,15 @@ describe("typed arrays", () => {
 		expect(mod.uint8Sum(data)).toEqual(15);
 	});
 
+	it("toArray copies a TypedArray of the expected length", () => {
+		expect(mod.uint8Array4Sum(new Uint8Array([1, 2, 3, 4]))).toEqual(10);
+	});
+
+	it("toArray rejects TypedArrays of a different length", () => {
+		expect(() => mod.uint8Array4Sum(new Uint8Array([1, 2, 3]))).toThrow();
+		expect(() => mod.uint8Array4Sum(new Uint8Array([1, 2, 3, 4, 5]))).toThrow();
+	});
+
 	it("float64Scale scales values", () => {
 		const data = new Float64Array([1.0, 2.0, 3.0]);
 		const result = mod.float64Scale(data, 2.5);

examples/js_dsl/mod.zig

@@ -190,6 +190,16 @@ pub fn uint8Sum(data: Uint8Array) !Number {
     return Number.from(sum);
 }
 
+/// Sum exactly four bytes copied from a Uint8Array into a Zig array.
+pub fn uint8Array4Sum(data: Uint8Array) !Number {
+    const array = try data.toArray(4);
+    var sum: i32 = 0;
+    for (array) |byte| {
+        sum += @intCast(byte);
+    }
+    return Number.from(sum);
+}
+
 /// Scale all values in a Float64Array by a factor. Returns a new array.
 pub fn float64Scale(data: Float64Array, factor: Number) !Float64Array {
     const slice = try data.toSlice();

src/js/typed_arrays.zig

@@ -50,6 +50,18 @@ pub fn TypedArray(comptime Element: type, comptime array_type: TypedarrayType) t
             return typed_ptr[0..info.length];
         }
 
+        /// Copies the TypedArray into a zig array of exactly `len` elements.
+        ///
+        /// Returns:
+        /// 1) `error.LengthMismatch` when the TypedArray length differs from
+        /// `len`, or
+        /// 2) `error.TypeMismatch` when its element type is wrong.
+        pub fn toArray(self: Self, comptime len: usize) ![len]Element {
+            const slice = try self.toSlice();
+            if (slice.len != len) return error.LengthMismatch;
+            return slice[0..len].*;
+        }
+
         /// Creates a new JavaScript TypedArray backed by an *external* (native-heap)
         /// ArrayBuffer.
         ///