LibJS: Add Array.prototype.filter()

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/f03d005bc4f
Pull-request: https://github.com/SerenityOS/serenity/pull/1793

Libraries/LibJS/Runtime/ArrayPrototype.cpp

@@ -39,6 +39,7 @@ namespace JS {
 
 ArrayPrototype::ArrayPrototype()
 {
+    put_native_function("filter", filter, 1);
     put_native_function("forEach", for_each, 1);
     put_native_function("pop", pop, 0);
     put_native_function("push", push, 1);
@@ -78,6 +79,32 @@ static Function* callback_from_args(Interpreter& interpreter, const String& name
     return static_cast<Function*>(&callback.as_object());
 }
 
+Value ArrayPrototype::filter(Interpreter& interpreter)
+{
+    auto* array = array_from(interpreter);
+    if (!array)
+        return {};
+    auto* callback = callback_from_args(interpreter, "filter");
+    if (!callback)
+        return {};
+    auto this_value = interpreter.argument(1);
+    auto initial_array_size = array->elements().size();
+    auto* new_array = interpreter.heap().allocate<Array>();
+    for (size_t i = 0; i < initial_array_size; ++i) {
+        if (i >= array->elements().size())
+            break;
+        auto value = array->elements()[i];
+        if (value.is_empty())
+            continue;
+        auto result = interpreter.call(callback, this_value, { value, Value((i32)i), array });
+        if (interpreter.exception())
+            return {};
+        if (result.to_boolean())
+            new_array->elements().append(value);
+    }
+    return Value(new_array);
+}
+
 Value ArrayPrototype::for_each(Interpreter& interpreter)
 {
     auto* array = array_from(interpreter);

Libraries/LibJS/Runtime/ArrayPrototype.h

@@ -39,6 +39,7 @@ public:
 private:
     virtual const char* class_name() const override { return "ArrayPrototype"; }
 
+    static Value filter(Interpreter&);
     static Value for_each(Interpreter&);
     static Value pop(Interpreter&);
     static Value push(Interpreter&);

Libraries/LibJS/Tests/Array.prototype.filter.js

@@ -0,0 +1,61 @@
+load("test-common.js");
+
+try {
+    assert(Array.prototype.filter.length === 1);
+
+    try {
+        [].filter();
+        assertNotReached();
+    } catch (e) {
+        assert(e.name === "TypeError");
+        assert(e.message === "Array.prototype.filter() requires at least one argument");
+    }
+
+    try {
+        [].filter(undefined);
+        assertNotReached();
+    } catch (e) {
+        assert(e.name === "TypeError");
+        assert(e.message === "undefined is not a function");
+    }
+
+    var callbackCalled = 0;
+    var callback = () => { callbackCalled++; };
+
+    assert([].filter(callback).length === 0);
+    assert(callbackCalled === 0);
+
+    assert([1, 2, 3].filter(callback).length === 0);
+    assert(callbackCalled === 3);
+
+    var evenNumbers = [0, 1, 2, 3, 4, 5, 6, 7].filter(x => x % 2 === 0);
+    assert(evenNumbers.length === 4);
+    assert(evenNumbers[0] === 0);
+    assert(evenNumbers[1] === 2);
+    assert(evenNumbers[2] === 4);
+    assert(evenNumbers[3] === 6);
+
+    var fruits = ["Apple", "Banana", "Blueberry", "Grape", "Mango", "Orange", "Peach", "Pineapple", "Raspberry", "Watermelon"];
+    const filterItems = (arr, query) => {
+        return arr.filter(el => el.toLowerCase().indexOf(query.toLowerCase()) !== -1)
+    };
+
+    var results;
+
+    results = filterItems(fruits, "Berry");
+    assert(results.length === 2);
+    assert(results[0] === "Blueberry");
+    assert(results[1] === "Raspberry");
+
+    results = filterItems(fruits, "P");
+    assert(results.length === 5);
+    assert(results[0] === "Apple");
+    assert(results[1] === "Grape");
+    assert(results[2] === "Peach");
+    assert(results[3] === "Pineapple");
+    assert(results[4] === "Raspberry");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}