LibJS: Add Array.prototype.slice

Author: https://github.com/kessejones
Commit: https://github.com/SerenityOS/serenity/commit/856ab9c600d
Pull-request: https://github.com/SerenityOS/serenity/pull/1846
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/linusg

Libraries/LibJS/Runtime/ArrayPrototype.cpp

@@ -51,6 +51,7 @@ ArrayPrototype::ArrayPrototype()
     put_native_function("unshift", unshift, 1);
     put_native_function("join", join, 1);
     put_native_function("concat", concat, 1);
+    put_native_function("slice", slice, 2);
     put("length", Value(0));
 }
 
@@ -254,4 +255,44 @@ Value ArrayPrototype::concat(Interpreter& interpreter)
     return Value(new_array);
 }
 
+Value ArrayPrototype::slice(Interpreter& interpreter)
+{
+    auto* array = array_from(interpreter);
+    if (!array)
+        return {};
+
+    auto* new_array = Array::create(interpreter.global_object());
+    if (interpreter.argument_count() == 0) {
+        new_array->elements().append(array->elements());
+        return new_array;
+    }
+
+    ssize_t array_size = static_cast<ssize_t>(array->elements().size());
+    auto start_slice = interpreter.argument(0).to_i32();
+    auto end_slice = array_size;
+
+    if (start_slice > array_size)
+        return new_array;
+
+    if (start_slice < 0)
+        start_slice = end_slice + start_slice;
+
+    if (interpreter.argument_count() >= 2) {
+        end_slice = interpreter.argument(1).to_i32();
+
+        if (end_slice < 0)
+            end_slice = array_size + end_slice;
+        else if (end_slice > array_size)
+            end_slice = array_size;
+    }
+
+    size_t array_capacity = start_slice + array_size - end_slice;
+    new_array->elements().ensure_capacity(array_capacity);
+    for (ssize_t i = start_slice; i < end_slice; ++i) {
+        new_array->elements().append(array->elements().at(i));
+    }
+
+    return new_array;
+}
+
 }

Libraries/LibJS/Runtime/ArrayPrototype.h

@@ -49,6 +49,7 @@ private:
     static Value unshift(Interpreter&);
     static Value join(Interpreter&);
     static Value concat(Interpreter&);
+    static Value slice(Interpreter&);
 };
 
 }

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

@@ -0,0 +1,53 @@
+load("test-common.js");
+
+try {
+    assert(Array.prototype.slice.length === 2);
+
+    var array = ["hello", "friends", "serenity", 1];
+
+    var array_slice = array.slice();
+    assert(array_slice.length === array.length);
+    assert(array_slice.length === 4);
+    assert(array_slice[0] === "hello");
+    assert(array_slice[1] === "friends");
+    assert(array_slice[2] === "serenity");
+    assert(array_slice[3] === 1);
+
+    array_slice = array.slice(1)
+    assert(array_slice.length === 3);
+    assert(array_slice[0] === "friends");
+    assert(array_slice[1] === "serenity");
+    assert(array_slice[2] === 1);
+
+    array_slice = array.slice(0, 2);
+    assert(array_slice.length === 2);
+    assert(array_slice[0] === "hello");
+    assert(array_slice[1] === "friends");
+
+    array_slice = array.slice(-1);
+    assert(array_slice.length === 1);
+    assert(array_slice[0] === 1);
+
+    array_slice = array.slice(1, 1);
+    assert(array_slice.length === 0);
+    
+    array_slice = array.slice(1, -1);
+    assert(array_slice.length === 2);
+    assert(array_slice[0] === "friends");
+    assert(array_slice[1] === "serenity");
+    
+    array_slice = array.slice(2, -1);
+    assert(array_slice.length === 1);
+    assert(array_slice[0] === "serenity");
+
+    array_slice = array.slice(0, 100);
+    assert(array_slice.length === 4);
+    assert(array_slice[0] === "hello");
+    assert(array_slice[1] === "friends");
+    assert(array_slice[2] === "serenity");
+    assert(array_slice[3] === 1);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}