LibJS: Add String.prototype.slice

Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -60,6 +60,7 @@ StringPrototype::StringPrototype()
     put_native_function("concat", concat, 1, attr);
     put_native_function("substring", substring, 2, attr);
     put_native_function("includes", includes, 1, attr);
+    put_native_function("slice", slice, 2, attr);
 }
 
 StringPrototype::~StringPrototype()
@@ -399,4 +400,45 @@ Value StringPrototype::includes(Interpreter& interpreter)
     return Value(substring_search.contains(search_string));
 }
 
+Value StringPrototype::slice(Interpreter& interpreter)
+{
+    auto* string_object = string_object_from(interpreter);
+    if (!string_object)
+        return {};
+
+    auto& string = string_object->primitive_string().string();
+
+    if (interpreter.argument_count() == 0)
+        return js_string(interpreter, string);
+
+    i32 string_length = static_cast<i32>(string.length());
+    i32 index_start = interpreter.argument(0).to_i32();
+    i32 index_end = string_length;
+
+    auto negative_min_index = -(string_length - 1);
+    if (index_start < negative_min_index)
+        index_start = 0;
+    else if (index_start < 0)
+        index_start = string_length + index_start;
+
+    if (interpreter.argument_count() >= 2) {
+        index_end = interpreter.argument(1).to_i32();
+
+        if (index_end < negative_min_index)
+            return js_string(interpreter, String::empty());
+
+        if (index_end > string_length)
+            index_end = string_length;
+        else if (index_end < 0)
+            index_end = string_length + index_end;
+    }
+
+    if (index_start >= index_end)
+        return js_string(interpreter, String::empty());
+
+    auto part_length = index_end - index_start;
+    auto string_part = string.substring(index_start, part_length);
+    return js_string(interpreter, string_part);
+}
+
 }

Libraries/LibJS/Runtime/StringPrototype.h

@@ -56,6 +56,7 @@ private:
     static Value trim_end(Interpreter&);
     static Value concat(Interpreter&);
     static Value includes(Interpreter&);
+    static Value slice(Interpreter&);
 };
 
 }

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

@@ -0,0 +1,22 @@
+load("test-common.js");
+
+try {
+    assert(String.prototype.slice.length === 2);
+    assert("hello friends".slice() === "hello friends");
+    assert("hello friends".slice(1) === "ello friends");
+    assert("hello friends".slice(0, 5) === "hello");
+    assert("hello friends".slice(13, 6) === "");
+    assert("hello friends".slice('', 5) === "hello");
+    assert("hello friends".slice(3, 3) === "");
+    assert("hello friends".slice(-1, 13) === "s");
+    assert("hello friends".slice(0, 50) === "hello friends");
+    assert("hello friends".slice(0, "5") === "hello");
+    assert("hello friends".slice("6", "13") === "friends");
+    assert("hello friends".slice(-7) === "friends");
+    assert("hello friends".slice(1000) === "");
+    assert("hello friends".slice(-1000) === "hello friends");
+
+    console.log("PASS");
+} catch (err) {
+    console.log("FAIL: " + err);
+}