LibJS: Add Array.prototype.concat

Author: https://github.com/kessejones
Commit: https://github.com/SerenityOS/serenity/commit/4931c0feda4
Pull-request: https://github.com/SerenityOS/serenity/pull/1825
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/Runtime/ArrayPrototype.cpp

@@ -48,6 +48,7 @@ ArrayPrototype::ArrayPrototype()
     put_native_function("toString", to_string, 0);
     put_native_function("unshift", unshift, 1);
     put_native_function("join", join, 1);
+    put_native_function("concat", concat, 1);
     put("length", Value(0));
 }
 
@@ -229,4 +230,26 @@ Value ArrayPrototype::join(Interpreter& interpreter)
     return join_array_with_separator(interpreter, *array, separator);
 }
 
+Value ArrayPrototype::concat(Interpreter& interpreter)
+{
+    auto* array = array_from(interpreter);
+    if (!array)
+        return {};
+
+    auto* new_array = interpreter.heap().allocate<Array>();
+    new_array->elements().append(array->elements());
+
+    for (size_t i = 0; i < interpreter.argument_count(); ++i) {
+        auto argument = interpreter.argument(i);
+        if (argument.is_array()) {
+            auto& argument_object = argument.as_object();
+            new_array->elements().append(argument_object.elements());
+        } else {
+            new_array->elements().append(argument);
+        }
+    }
+
+    return Value(new_array);
+}
+
 }

Libraries/LibJS/Runtime/ArrayPrototype.h

@@ -48,6 +48,7 @@ private:
     static Value to_string(Interpreter&);
     static Value unshift(Interpreter&);
     static Value join(Interpreter&);
+    static Value concat(Interpreter&);
 };
 
 }

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

@@ -0,0 +1,37 @@
+load("test-common.js");
+
+try {
+    assert(Array.prototype.concat.length === 1);
+
+    var array = ["hello", "friends"];
+
+    var array_concat = array.concat();
+    assert(array_concat.length === array.length);
+
+    array_concat = array.concat(1)
+    assert(array_concat.length === 3);
+    assert(array_concat[2] === 1);
+
+    array_concat = array.concat([1, 2, 3])
+    assert(array_concat.length === 5);
+    assert(array_concat[2] === 1);
+    assert(array_concat[3] === 2);
+    assert(array_concat[4] === 3);
+
+    array_concat = array.concat(false, "serenity");
+    assert(array_concat.length === 4);
+    assert(array_concat[2] === false);
+    assert(array_concat[3] === "serenity");
+
+    array_concat = array.concat({ name: "libjs" }, [1, [2, 3]]);
+    assert(array_concat.length === 5);
+    assert(array_concat[2].name === "libjs");
+    assert(array_concat[3] === 1);
+    assert(array_concat[4][0] === 2);
+    assert(array_concat[4][1] === 3);
+
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}