LibJS: Implement String.prototype.startsWith()

Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -41,6 +41,7 @@ StringPrototype::StringPrototype()
     put_native_property("length", length_getter, nullptr);
     put_native_function("charAt", char_at);
     put_native_function("repeat", repeat);
+    put_native_function("startsWith", starts_with);
 }
 
 StringPrototype::~StringPrototype()
@@ -83,6 +84,32 @@ Value StringPrototype::repeat(Interpreter& interpreter)
     return js_string(interpreter.heap(), builder.to_string());
 }
 
+Value StringPrototype::starts_with(Interpreter& interpreter)
+{
+    auto* this_object = interpreter.this_value().to_object(interpreter.heap());
+    if (!this_object)
+        return {};
+    if (interpreter.call_frame().arguments.is_empty())
+        return Value(false);
+    auto search_string = interpreter.call_frame().arguments[0].to_string();
+    auto search_string_length = static_cast<i32>(search_string.length());
+    i32 position = 0;
+    if (interpreter.call_frame().arguments.size() > 1) {
+        auto number = interpreter.call_frame().arguments[1].to_number();
+        if (!number.is_nan())
+            position = number.to_i32();
+    }
+    ASSERT(this_object->is_string_object());
+    auto underlying_string = static_cast<const StringObject*>(this_object)->primitive_string()->string();
+    auto underlying_string_length = static_cast<i32>(underlying_string.length());
+    auto start = min(max(position, 0), underlying_string_length);
+    if (start + search_string_length > underlying_string_length)
+        return Value(false);
+    if (search_string_length == 0)
+        return Value(true);
+    return Value(underlying_string.substring(start, search_string_length) == search_string);
+}
+
 Value StringPrototype::length_getter(Interpreter& interpreter)
 {
     auto* this_object = interpreter.this_value().to_object(interpreter.heap());

Libraries/LibJS/Runtime/StringPrototype.h

@@ -40,6 +40,7 @@ private:
 
     static Value char_at(Interpreter&);
     static Value repeat(Interpreter&);
+    static Value starts_with(Interpreter&);
 
     static Value length_getter(Interpreter&);
 };

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

@@ -0,0 +1,40 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    var s = "foobar";
+    assert(s.startsWith("f") === true);
+    assert(s.startsWith("fo") === true);
+    assert(s.startsWith("foo") === true);
+    assert(s.startsWith("foob") === true);
+    assert(s.startsWith("fooba") === true);
+    assert(s.startsWith("foobar") === true);
+    assert(s.startsWith("foobar1") === false);
+    assert(s.startsWith("f", 0) === true);
+    assert(s.startsWith("fo", 0) === true);
+    assert(s.startsWith("foo", 0) === true);
+    assert(s.startsWith("foob", 0) === true);
+    assert(s.startsWith("fooba", 0) === true);
+    assert(s.startsWith("foobar", 0) === true);
+    assert(s.startsWith("foobar1", 0) === false);
+    assert(s.startsWith("foo", []) === true);
+    assert(s.startsWith("foo", null) === true);
+    assert(s.startsWith("foo", undefined) === true);
+    assert(s.startsWith("foo", false) === true);
+    assert(s.startsWith("foo", true) === false);
+    assert(s.startsWith("foo", "foo") === true);
+    assert(s.startsWith("foo", 0 - 1) === true);
+    assert(s.startsWith("foo", 42) === false);
+    assert(s.startsWith("bar", 3) === true);
+    assert(s.startsWith("bar", "3") === true);
+    assert(s.startsWith("bar1", 3) === false);
+    assert(s.startsWith() === false);
+    assert(s.startsWith("") === true);
+    assert(s.startsWith("", 0) === true);
+    assert(s.startsWith("", 1) === true);
+    assert(s.startsWith("", 0 - 1) === true);
+    assert(s.startsWith("", 42) === true);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}