LibJS: Add String.prototype.concat

Author: https://github.com/kessejones
Commit: https://github.com/SerenityOS/serenity/commit/7725b1970e5
Pull-request: https://github.com/SerenityOS/serenity/pull/1817
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -55,6 +55,7 @@ StringPrototype::StringPrototype()
     put_native_function("trim", trim, 0);
     put_native_function("trimStart", trim_start, 0);
     put_native_function("trimEnd", trim_end, 0);
+    put_native_function("concat", concat, 1);
 }
 
 StringPrototype::~StringPrototype()
@@ -307,4 +308,22 @@ Value StringPrototype::trim_end(Interpreter& interpreter)
     return trim_string(interpreter, *this_object, TrimMode::Right);
 }
 
+Value StringPrototype::concat(Interpreter& interpreter)
+{
+    auto* this_object = interpreter.this_value().to_object(interpreter.heap());
+    if (!this_object)
+        return {};
+    auto& string = this_object->to_string().as_string()->string();
+
+    StringBuilder builder;
+    builder.append(string);
+
+    for (size_t i = 0; i < interpreter.argument_count(); ++i) {
+        auto string_argument = interpreter.argument(i).to_string();
+        builder.append(string_argument);
+    }
+
+    return js_string(interpreter, builder.to_string());
+}
+
 }

Libraries/LibJS/Runtime/StringPrototype.h

@@ -53,6 +53,7 @@ private:
     static Value trim(Interpreter&);
     static Value trim_start(Interpreter&);
     static Value trim_end(Interpreter&);
+    static Value concat(Interpreter&);
 };
 
 }

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

@@ -0,0 +1,22 @@
+load("test-common.js");
+
+try {
+    assert(String.prototype.concat.length === 1);
+    assert("".concat(1) === "1");
+    assert("".concat(3,2,1) === "321");
+    assert("hello".concat(" ", "friends") === "hello friends");
+    assert("".concat(null) === "null");
+    assert("".concat(false) === "false");
+    assert("".concat(true) === "true");
+    assert("".concat([]) === "");
+    assert("".concat([1, 2, 3, 'hello']) === "1,2,3,hello");
+    assert("".concat(true, []) === "true");
+    assert("".concat(true, false) === "truefalse");
+    assert("".concat({}) === "[object Object]");
+    assert("".concat(1, {}) === "1[object Object]");
+    assert("".concat(1, {}, false) === "1[object Object]false");
+
+    console.log("PASS");
+} catch (err) {
+    console.log("FAIL: " + err);
+}