LibJS: Add a basic implementation of String.prototype.substr()

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/2d9d8832748

Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -188,8 +188,9 @@ namespace JS {
     P(splice)                                \
     P(sqrt)                                  \
     P(startsWith)                            \
-    P(stringify)                             \
     P(sticky)                                \
+    P(stringify)                             \
+    P(substr)                                \
     P(substring)                             \
     P(tan)                                   \
     P(test)                                  \

Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -85,6 +85,7 @@ void StringPrototype::initialize(GlobalObject& global_object)
     define_native_function(vm.names.trimStart, trim_start, 0, attr);
     define_native_function(vm.names.trimEnd, trim_end, 0, attr);
     define_native_function(vm.names.concat, concat, 1, attr);
+    define_native_function(vm.names.substr, substr, 2, attr);
     define_native_function(vm.names.substring, substring, 2, attr);
     define_native_function(vm.names.includes, includes, 1, attr);
     define_native_function(vm.names.slice, slice, 2, attr);
@@ -355,6 +356,40 @@ JS_DEFINE_NATIVE_FUNCTION(StringPrototype::substring)
     return js_string(vm, string_part);
 }
 
+JS_DEFINE_NATIVE_FUNCTION(StringPrototype::substr)
+{
+    auto string = ak_string_from(vm, global_object);
+    if (string.is_null())
+        return {};
+    if (vm.argument_count() == 0)
+        return js_string(vm, string);
+
+    // FIXME: this should index a UTF-16 code_point view of the string.
+    auto string_length = (i32)string.length();
+
+    auto start_argument = vm.argument(0).to_i32(global_object);
+    if (vm.exception())
+        return {};
+
+    auto start = start_argument < 0 ? (string_length - -start_argument) : start_argument;
+
+    auto length = string_length - start;
+    if (vm.argument_count() >= 2) {
+        auto length_argument = vm.argument(1).to_i32(global_object);
+        if (vm.exception())
+            return {};
+        length = max(0, min(length_argument, length));
+        if (vm.exception())
+            return {};
+    }
+
+    if (length == 0)
+        return js_string(vm, String(""));
+
+    auto string_part = string.substring(start, length);
+    return js_string(vm, string_part);
+}
+
 JS_DEFINE_NATIVE_FUNCTION(StringPrototype::includes)
 {
     auto string = ak_string_from(vm, global_object);

Libraries/LibJS/Runtime/StringPrototype.h

@@ -50,6 +50,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(pad_start);
     JS_DECLARE_NATIVE_FUNCTION(pad_end);
     JS_DECLARE_NATIVE_FUNCTION(substring);
+    JS_DECLARE_NATIVE_FUNCTION(substr);
 
     JS_DECLARE_NATIVE_GETTER(length_getter);
 

Libraries/LibJS/Tests/builtins/String/String.prototype.substr.js

@@ -0,0 +1,16 @@
+test("basic functionality", () => {
+    expect(String.prototype.substr).toHaveLength(2);
+
+    expect("hello friends".substr()).toBe("hello friends");
+    expect("hello friends".substr(1)).toBe("ello friends");
+    expect("hello friends".substr(0, 5)).toBe("hello");
+    expect("hello friends".substr(5, 6)).toBe(" frien");
+    expect("hello friends".substr("", 5)).toBe("hello");
+    expect("hello friends".substr(3, 3)).toBe("lo ");
+    expect("hello friends".substr(-1, 13)).toBe("s");
+    expect("hello friends".substr(0, 50)).toBe("hello friends");
+    expect("hello friends".substr(0, "5")).toBe("hello");
+    expect("hello friends".substr("2", "2")).toBe("ll");
+    expect("hello friends".substr(-7)).toBe("friends");
+    expect("hello friends".substr(-3, -5)).toBe("");
+});