LibJS: Add the Number.prototype.toFixed method

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -297,6 +297,7 @@ namespace JS {
     P(test)                                  \
     P(then)                                  \
     P(toDateString)                          \
+    P(toFixed)                               \
     P(toGMTString)                           \
     P(toISOString)                           \
     P(toJSON)                                \

Userland/Libraries/LibJS/Runtime/ErrorTypes.h

@@ -32,6 +32,7 @@
     M(InstanceOfOperatorBadPrototype, "'prototype' property of {} is not an object")                                                    \
     M(InvalidAssignToConst, "Invalid assignment to const variable")                                                                     \
     M(InvalidCodePoint, "Invalid code point {}, must be an integer no less than 0 and no greater than 0x10FFFF")                        \
+    M(InvalidFractionDigits, "Fraction Digits must be an integer no less than 0, and no greater than 100")                              \
     M(InvalidHint, "Invalid hint: \"{}\"")                                                                                              \
     M(InvalidIndex, "Index must be a positive integer")                                                                                 \
     M(InvalidLeftHandAssignment, "Invalid left-hand side in assignment")                                                                \

Userland/Libraries/LibJS/Runtime/NumberPrototype.cpp

@@ -34,6 +34,7 @@ void NumberPrototype::initialize(GlobalObject& object)
     auto& vm = this->vm();
     Object::initialize(object);
     u8 attr = Attribute::Configurable | Attribute::Writable;
+    define_native_function(vm.names.toFixed, to_fixed, 1, attr);
     define_native_function(vm.names.toString, to_string, 1, attr);
     define_native_function(vm.names.valueOf, value_of, 0, attr);
 }
@@ -54,6 +55,37 @@ static Value this_number_value(GlobalObject& global_object, Value value)
     return {};
 }
 
+// 21.1.3.3 Number.prototype.toFixed ( fractionDigits ), https://tc39.es/ecma262/#sec-number.prototype.tofixed
+JS_DEFINE_NATIVE_FUNCTION(NumberPrototype::to_fixed)
+{
+    auto number_value = this_number_value(global_object, vm.this_value(global_object));
+    if (vm.exception())
+        return {};
+
+    auto fraction_digits = vm.argument(0).to_integer_or_infinity(global_object);
+    if (vm.exception())
+        return {};
+
+    if (!vm.argument(0).is_finite_number()) {
+        vm.throw_exception<RangeError>(global_object, ErrorType::InvalidFractionDigits);
+        return {};
+    }
+
+    if (fraction_digits < 0 || fraction_digits > 100) {
+        vm.throw_exception<RangeError>(global_object, ErrorType::InvalidFractionDigits);
+        return {};
+    }
+
+    if (!number_value.is_finite_number())
+        return js_string(vm, number_value.to_string(global_object));
+
+    auto number = number_value.as_double();
+    if (fabs(number) >= 1e+21)
+        return js_string(vm, number_value.to_string(global_object));
+
+    return js_string(vm, String::formatted("{:0.{1}}", number, static_cast<size_t>(fraction_digits)));
+}
+
 // 21.1.3.6 Number.prototype.toString ( [ radix ] ), https://tc39.es/ecma262/#sec-number.prototype.tostring
 JS_DEFINE_NATIVE_FUNCTION(NumberPrototype::to_string)
 {

Userland/Libraries/LibJS/Runtime/NumberPrototype.h

@@ -18,6 +18,7 @@ public:
     virtual void initialize(GlobalObject&) override;
     virtual ~NumberPrototype() override;
 
+    JS_DECLARE_NATIVE_FUNCTION(to_fixed);
     JS_DECLARE_NATIVE_FUNCTION(to_string);
     JS_DECLARE_NATIVE_FUNCTION(value_of);
 };