LibJS: Implement stringification Temporal.Duration prototypes

Author: https://github.com/trflynn89
Commit: https://github.com/LadybirdBrowser/ladybird/commit/4742775262d
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2431
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/shannonbooth ✅

Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp

@@ -8,10 +8,12 @@
  */
 
 #include <LibCrypto/BigFraction/BigFraction.h>
+#include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/PropertyKey.h>
 #include <LibJS/Runtime/Temporal/AbstractOperations.h>
 #include <LibJS/Runtime/Temporal/Duration.h>
 #include <LibJS/Runtime/Temporal/ISO8601.h>
+#include <LibJS/Runtime/Temporal/Instant.h>
 
 namespace JS::Temporal {
 
@@ -41,6 +43,227 @@ StringView temporal_unit_to_string(Unit unit)
     return temporal_units[to_underlying(unit)].singular_property_name;
 }
 
+// 13.15 GetTemporalFractionalSecondDigitsOption ( options ), https://tc39.es/proposal-temporal/#sec-temporal-gettemporalfractionalseconddigitsoption
+ThrowCompletionOr<Precision> get_temporal_fractional_second_digits_option(VM& vm, Object const& options)
+{
+    // 1. Let digitsValue be ? Get(options, "fractionalSecondDigits").
+    auto digits_value = TRY(options.get(vm.names.fractionalSecondDigits));
+
+    // 2. If digitsValue is undefined, return AUTO.
+    if (digits_value.is_undefined())
+        return Precision { Auto {} };
+
+    // 3. If digitsValue is not a Number, then
+    if (!digits_value.is_number()) {
+        // a. If ? ToString(digitsValue) is not "auto", throw a RangeError exception.
+        auto digits_value_string = TRY(digits_value.to_string(vm));
+
+        if (digits_value_string != "auto"sv)
+            return vm.throw_completion<RangeError>(ErrorType::OptionIsNotValidValue, digits_value, vm.names.fractionalSecondDigits);
+
+        // b. Return AUTO.
+        return Precision { Auto {} };
+    }
+
+    // 4. If digitsValue is NaN, +∞𝔽, or -∞𝔽, throw a RangeError exception.
+    if (digits_value.is_nan() || digits_value.is_infinity())
+        return vm.throw_completion<RangeError>(ErrorType::OptionIsNotValidValue, digits_value, vm.names.fractionalSecondDigits);
+
+    // 5. Let digitCount be floor(ℝ(digitsValue)).
+    auto digit_count = floor(digits_value.as_double());
+
+    // 6. If digitCount < 0 or digitCount > 9, throw a RangeError exception.
+    if (digit_count < 0 || digit_count > 9)
+        return vm.throw_completion<RangeError>(ErrorType::OptionIsNotValidValue, digits_value, vm.names.fractionalSecondDigits);
+
+    // 7. Return digitCount.
+    return Precision { static_cast<u8>(digit_count) };
+}
+
+// 13.16 ToSecondsStringPrecisionRecord ( smallestUnit, fractionalDigitCount ), https://tc39.es/proposal-temporal/#sec-temporal-tosecondsstringprecisionrecord
+SecondsStringPrecision to_seconds_string_precision_record(UnitValue smallest_unit, Precision fractional_digit_count)
+{
+    if (auto const* unit = smallest_unit.get_pointer<Unit>()) {
+        // 1. If smallestUnit is MINUTE, then
+        if (*unit == Unit::Minute) {
+            // a. Return the Record { [[Precision]]: MINUTE, [[Unit]]: MINUTE, [[Increment]]: 1  }.
+            return { .precision = SecondsStringPrecision::Minute {}, .unit = Unit::Minute, .increment = 1 };
+        }
+
+        // 2. If smallestUnit is SECOND, then
+        if (*unit == Unit::Second) {
+            // a. Return the Record { [[Precision]]: 0, [[Unit]]: SECOND, [[Increment]]: 1  }.
+            return { .precision = 0, .unit = Unit::Second, .increment = 1 };
+        }
+
+        // 3. If smallestUnit is MILLISECOND, then
+        if (*unit == Unit::Millisecond) {
+            // a. Return the Record { [[Precision]]: 3, [[Unit]]: MILLISECOND, [[Increment]]: 1  }.
+            return { .precision = 3, .unit = Unit::Millisecond, .increment = 1 };
+        }
+
+        // 4. If smallestUnit is MICROSECOND, then
+        if (*unit == Unit::Microsecond) {
+            // a. Return the Record { [[Precision]]: 6, [[Unit]]: MICROSECOND, [[Increment]]: 1  }.
+            return { .precision = 6, .unit = Unit::Microsecond, .increment = 1 };
+        }
+
+        // 5. If smallestUnit is NANOSECOND, then
+        if (*unit == Unit::Nanosecond) {
+            // a. Return the Record { [[Precision]]: 9, [[Unit]]: NANOSECOND, [[Increment]]: 1  }.
+            return { .precision = 9, .unit = Unit::Nanosecond, .increment = 1 };
+        }
+    }
+
+    // 6. Assert: smallestUnit is UNSET.
+    VERIFY(smallest_unit.has<Unset>());
+
+    // 7. If fractionalDigitCount is auto, then
+    if (fractional_digit_count.has<Auto>()) {
+        // a. Return the Record { [[Precision]]: AUTO, [[Unit]]: NANOSECOND, [[Increment]]: 1  }.
+        return { .precision = Auto {}, .unit = Unit::Nanosecond, .increment = 1 };
+    }
+
+    auto fractional_digits = fractional_digit_count.get<u8>();
+
+    // 8. If fractionalDigitCount = 0, then
+    if (fractional_digits == 0) {
+        // a. Return the Record { [[Precision]]: 0, [[Unit]]: SECOND, [[Increment]]: 1  }.
+        return { .precision = 0, .unit = Unit::Second, .increment = 1 };
+    }
+
+    // 9. If fractionalDigitCount is in the inclusive interval from 1 to 3, then
+    if (fractional_digits >= 1 && fractional_digits <= 3) {
+        // a. Return the Record { [[Precision]]: fractionalDigitCount, [[Unit]]: MILLISECOND, [[Increment]]: 10**(3 - fractionalDigitCount)  }.
+        return { .precision = fractional_digits, .unit = Unit::Millisecond, .increment = static_cast<u8>(pow(10, 3 - fractional_digits)) };
+    }
+
+    // 10. If fractionalDigitCount is in the inclusive interval from 4 to 6, then
+    if (fractional_digits >= 4 && fractional_digits <= 6) {
+        // a. Return the Record { [[Precision]]: fractionalDigitCount, [[Unit]]: MICROSECOND, [[Increment]]: 10**(6 - fractionalDigitCount)  }.
+        return { .precision = fractional_digits, .unit = Unit::Microsecond, .increment = static_cast<u8>(pow(10, 6 - fractional_digits)) };
+    }
+
+    // 11. Assert: fractionalDigitCount is in the inclusive interval from 7 to 9.
+    VERIFY(fractional_digits >= 7 && fractional_digits <= 9);
+
+    // 12. Return the Record { [[Precision]]: fractionalDigitCount, [[Unit]]: NANOSECOND, [[Increment]]: 10**(9 - fractionalDigitCount)  }.
+    return { .precision = fractional_digits, .unit = Unit::Nanosecond, .increment = static_cast<u8>(pow(10, 9 - fractional_digits)) };
+}
+
+// 13.17 GetTemporalUnitValuedOption ( options, key, unitGroup, default [ , extraValues ] ), https://tc39.es/proposal-temporal/#sec-temporal-gettemporalunitvaluedoption
+ThrowCompletionOr<UnitValue> get_temporal_unit_valued_option(VM& vm, Object const& options, PropertyKey const& key, UnitGroup unit_group, UnitDefault const& default_, ReadonlySpan<UnitValue> extra_values)
+{
+    // 1. Let allowedValues be a new empty List.
+    Vector<UnitValue> allowed_values;
+
+    // 2. For each row of Table 21, except the header row, in table order, do
+    for (auto const& row : temporal_units) {
+        // a. Let unit be the value in the "Value" column of the row.
+        auto unit = row.value;
+
+        // b. If the "Category" column of the row is DATE and unitGroup is DATE or DATETIME, append unit to allowedValues.
+        if (row.category == UnitCategory::Date && (unit_group == UnitGroup::Date || unit_group == UnitGroup::DateTime))
+            allowed_values.append(unit);
+
+        // c. Else if the "Category" column of the row is TIME and unitGroup is TIME or DATETIME, append unit to allowedValues.
+        if (row.category == UnitCategory::Time && (unit_group == UnitGroup::Time || unit_group == UnitGroup::DateTime))
+            allowed_values.append(unit);
+    }
+
+    // 3. If extraValues is present, then
+    if (!extra_values.is_empty()) {
+        // a. Set allowedValues to the list-concatenation of allowedValues and extraValues.
+        for (auto value : extra_values)
+            allowed_values.append(value);
+    }
+
+    OptionDefault default_value;
+
+    // 4. If default is UNSET, then
+    if (default_.has<Unset>()) {
+        // a. Let defaultValue be undefined.
+        default_value = {};
+    }
+    // 5. Else if default is REQUIRED, then
+    else if (default_.has<Required>()) {
+        // a. Let defaultValue be REQUIRED.
+        default_value = Required {};
+    }
+    // 6. Else if default is AUTO, then
+    else if (default_.has<Auto>()) {
+        // a. Append default to allowedValues.
+        allowed_values.append(Auto {});
+
+        // b. Let defaultValue be "auto".
+        default_value = "auto"sv;
+    }
+    // 7. Else,
+    else {
+        auto unit = default_.get<Unit>();
+
+        // a. Assert: allowedValues contains default.
+
+        // b. Let defaultValue be the value in the "Singular property name" column of Table 21 corresponding to the row
+        //    with default in the "Value" column.
+        default_value = temporal_units[to_underlying(unit)].singular_property_name;
+    }
+
+    // 8. Let allowedStrings be a new empty List.
+    Vector<StringView> allowed_strings;
+
+    // 9. For each element value of allowedValues, do
+    for (auto value : allowed_values) {
+        // a. If value is auto, then
+        if (value.has<Auto>()) {
+            // i. Append "auto" to allowedStrings.
+            allowed_strings.append("auto"sv);
+        }
+        // b. Else,
+        else {
+            auto unit = value.get<Unit>();
+
+            // i. Let singularName be the value in the "Singular property name" column of Table 21 corresponding to the
+            //    row with value in the "Value" column.
+            auto singular_name = temporal_units[to_underlying(unit)].singular_property_name;
+
+            // ii. Append singularName to allowedStrings.
+            allowed_strings.append(singular_name);
+
+            // iii. Let pluralName be the value in the "Plural property name" column of the corresponding row.
+            auto plural_name = temporal_units[to_underlying(unit)].plural_property_name;
+
+            // iv. Append pluralName to allowedStrings.
+            allowed_strings.append(plural_name);
+        }
+    }
+
+    // 10. NOTE: For each singular Temporal unit name that is contained within allowedStrings, the corresponding plural
+    //     name is also contained within it.
+
+    // 11. Let value be ? GetOption(options, key, STRING, allowedStrings, defaultValue).
+    auto value = TRY(get_option(vm, options, key, OptionType::String, allowed_strings, default_value));
+
+    // 12. If value is undefined, return UNSET.
+    if (value.is_undefined())
+        return UnitValue { Unset {} };
+
+    auto value_string = value.as_string().utf8_string_view();
+
+    // 13. If value is "auto", return AUTO.
+    if (value_string == "auto"sv)
+        return UnitValue { Auto {} };
+
+    // 14. Return the value in the "Value" column of Table 21 corresponding to the row with value in its "Singular
+    //     property name" or "Plural property name" column.
+    for (auto const& row : temporal_units) {
+        if (value_string.is_one_of(row.singular_property_name, row.plural_property_name))
+            return UnitValue { row.value };
+    }
+
+    VERIFY_NOT_REACHED();
+}
+
 // 13.18 GetTemporalRelativeToOption ( options ), https://tc39.es/proposal-temporal/#sec-temporal-gettemporalrelativetooption
 ThrowCompletionOr<RelativeTo> get_temporal_relative_to_option(VM& vm, Object const& options)
 {
@@ -101,6 +324,305 @@ UnitCategory temporal_unit_category(Unit unit)
     return temporal_units[to_underlying(unit)].category;
 }
 
+// AD-HOC
+Crypto::UnsignedBigInteger const& temporal_unit_length_in_nanoseconds(Unit unit)
+{
+    switch (unit) {
+    case Unit::Day:
+        return NANOSECONDS_PER_DAY;
+    case Unit::Hour:
+        return NANOSECONDS_PER_HOUR;
+    case Unit::Minute:
+        return NANOSECONDS_PER_MINUTE;
+    case Unit::Second:
+        return NANOSECONDS_PER_SECOND;
+    case Unit::Millisecond:
+        return NANOSECONDS_PER_MILLISECOND;
+    case Unit::Microsecond:
+        return NANOSECONDS_PER_MICROSECOND;
+    case Unit::Nanosecond:
+        return NANOSECONDS_PER_NANOSECOND;
+    default:
+        VERIFY_NOT_REACHED();
+    }
+}
+
+// 13.24 FormatFractionalSeconds ( subSecondNanoseconds, precision ), https://tc39.es/proposal-temporal/#sec-temporal-formatfractionalseconds
+String format_fractional_seconds(u64 sub_second_nanoseconds, Precision precision)
+{
+    String fraction_string;
+
+    // 1. If precision is auto, then
+    if (precision.has<Auto>()) {
+        // a. If subSecondNanoseconds = 0, return the empty String.
+        if (sub_second_nanoseconds == 0)
+            return String {};
+
+        // b. Let fractionString be ToZeroPaddedDecimalString(subSecondNanoseconds, 9).
+        fraction_string = MUST(String::formatted("{:09}", sub_second_nanoseconds));
+
+        // c. Set fractionString to the longest prefix of fractionString ending with a code unit other than 0x0030 (DIGIT ZERO).
+        fraction_string = MUST(fraction_string.trim("0"sv, TrimMode::Right));
+    }
+    // 2. Else,
+    else {
+        // a. If precision = 0, return the empty String.
+        if (precision.get<u8>() == 0)
+            return String {};
+
+        // b. Let fractionString be ToZeroPaddedDecimalString(subSecondNanoseconds, 9).
+        fraction_string = MUST(String::formatted("{:09}", sub_second_nanoseconds));
+
+        // c. Set fractionString to the substring of fractionString from 0 to precision.
+        fraction_string = MUST(fraction_string.substring_from_byte_offset(0, precision.get<u8>()));
+    }
+
+    // 3. Return the string-concatenation of the code unit 0x002E (FULL STOP) and fractionString.
+    return MUST(String::formatted(".{}", fraction_string));
+}
+
+// 13.26 GetUnsignedRoundingMode ( roundingMode, sign ), https://tc39.es/proposal-temporal/#sec-getunsignedroundingmode
+UnsignedRoundingMode get_unsigned_rounding_mode(RoundingMode rounding_mode, Sign sign)
+{
+    // 1. Return the specification type in the "Unsigned Rounding Mode" column of Table 22 for the row where the value
+    //    in the "Rounding Mode" column is roundingMode and the value in the "Sign" column is sign.
+    switch (rounding_mode) {
+    case RoundingMode::Ceil:
+        return sign == Sign::Positive ? UnsignedRoundingMode::Infinity : UnsignedRoundingMode::Zero;
+    case RoundingMode::Floor:
+        return sign == Sign::Positive ? UnsignedRoundingMode::Zero : UnsignedRoundingMode::Infinity;
+    case RoundingMode::Expand:
+        return UnsignedRoundingMode::Infinity;
+    case RoundingMode::Trunc:
+        return UnsignedRoundingMode::Zero;
+    case RoundingMode::HalfCeil:
+        return sign == Sign::Positive ? UnsignedRoundingMode::HalfInfinity : UnsignedRoundingMode::HalfZero;
+    case RoundingMode::HalfFloor:
+        return sign == Sign::Positive ? UnsignedRoundingMode::HalfZero : UnsignedRoundingMode::HalfInfinity;
+    case RoundingMode::HalfExpand:
+        return UnsignedRoundingMode::HalfInfinity;
+    case RoundingMode::HalfTrunc:
+        return UnsignedRoundingMode::HalfZero;
+    case RoundingMode::HalfEven:
+        return UnsignedRoundingMode::HalfEven;
+    }
+
+    VERIFY_NOT_REACHED();
+}
+
+// 13.27 ApplyUnsignedRoundingMode ( x, r1, r2, unsignedRoundingMode ), https://tc39.es/proposal-temporal/#sec-applyunsignedroundingmode
+double apply_unsigned_rounding_mode(double x, double r1, double r2, UnsignedRoundingMode unsigned_rounding_mode)
+{
+    // 1. If x = r1, return r1.
+    if (x == r1)
+        return r1;
+
+    // 2. Assert: r1 < x < r2.
+    VERIFY(r1 < x && x < r2);
+
+    // 3. Assert: unsignedRoundingMode is not undefined.
+
+    // 4. If unsignedRoundingMode is ZERO, return r1.
+    if (unsigned_rounding_mode == UnsignedRoundingMode::Zero)
+        return r1;
+
+    // 5. If unsignedRoundingMode is INFINITY, return r2.
+    if (unsigned_rounding_mode == UnsignedRoundingMode::Infinity)
+        return r2;
+
+    // 6. Let d1 be x – r1.
+    auto d1 = x - r1;
+
+    // 7. Let d2 be r2 – x.
+    auto d2 = r2 - x;
+
+    // 8. If d1 < d2, return r1.
+    if (d1 < d2)
+        return r1;
+
+    // 9. If d2 < d1, return r2.
+    if (d2 < d1)
+        return r2;
+
+    // 10. Assert: d1 is equal to d2.
+    VERIFY(d1 == d2);
+
+    // 11. If unsignedRoundingMode is HALF-ZERO, return r1.
+    if (unsigned_rounding_mode == UnsignedRoundingMode::HalfZero)
+        return r1;
+
+    // 12. If unsignedRoundingMode is HALF-INFINITY, return r2.
+    if (unsigned_rounding_mode == UnsignedRoundingMode::HalfInfinity)
+        return r2;
+
+    // 13. Assert: unsignedRoundingMode is HALF-EVEN.
+    VERIFY(unsigned_rounding_mode == UnsignedRoundingMode::HalfEven);
+
+    // 14. Let cardinality be (r1 / (r2 – r1)) modulo 2.
+    auto cardinality = modulo((r1 / (r2 - r1)), 2);
+
+    // 15. If cardinality = 0, return r1.
+    if (cardinality == 0)
+        return r1;
+
+    // 16. Return r2.
+    return r2;
+}
+
+// 13.27 ApplyUnsignedRoundingMode ( x, r1, r2, unsignedRoundingMode ), https://tc39.es/proposal-temporal/#sec-applyunsignedroundingmode
+Crypto::SignedBigInteger apply_unsigned_rounding_mode(Crypto::SignedDivisionResult const& x, Crypto::SignedBigInteger const& r1, Crypto::SignedBigInteger const& r2, UnsignedRoundingMode unsigned_rounding_mode, Crypto::UnsignedBigInteger const& increment)
+{
+    // 1. If x = r1, return r1.
+    if (x.quotient == r1 && x.remainder.unsigned_value().is_zero())
+        return r1;
+
+    // 2. Assert: r1 < x < r2.
+    // NOTE: Skipped for the sake of performance.
+
+    // 3. Assert: unsignedRoundingMode is not undefined.
+
+    // 4. If unsignedRoundingMode is ZERO, return r1.
+    if (unsigned_rounding_mode == UnsignedRoundingMode::Zero)
+        return r1;
+
+    // 5. If unsignedRoundingMode is INFINITY, return r2.
+    if (unsigned_rounding_mode == UnsignedRoundingMode::Infinity)
+        return r2;
+
+    // 6. Let d1 be x – r1.
+    auto d1 = x.remainder.unsigned_value();
+
+    // 7. Let d2 be r2 – x.
+    auto d2 = increment.minus(x.remainder.unsigned_value());
+
+    // 8. If d1 < d2, return r1.
+    if (d1 < d2)
+        return r1;
+
+    // 9. If d2 < d1, return r2.
+    if (d2 < d1)
+        return r2;
+
+    // 10. Assert: d1 is equal to d2.
+    // NOTE: Skipped for the sake of performance.
+
+    // 11. If unsignedRoundingMode is HALF-ZERO, return r1.
+    if (unsigned_rounding_mode == UnsignedRoundingMode::HalfZero)
+        return r1;
+
+    // 12. If unsignedRoundingMode is HALF-INFINITY, return r2.
+    if (unsigned_rounding_mode == UnsignedRoundingMode::HalfInfinity)
+        return r2;
+
+    // 13. Assert: unsignedRoundingMode is HALF-EVEN.
+    VERIFY(unsigned_rounding_mode == UnsignedRoundingMode::HalfEven);
+
+    // 14. Let cardinality be (r1 / (r2 – r1)) modulo 2.
+    auto cardinality = modulo(r1.divided_by(r2.minus(r1)).quotient, "2"_bigint);
+
+    // 15. If cardinality = 0, return r1.
+    if (cardinality.unsigned_value().is_zero())
+        return r1;
+
+    // 16. Return r2.
+    return r2;
+}
+
+// 13.28 RoundNumberToIncrement ( x, increment, roundingMode ), https://tc39.es/proposal-temporal/#sec-temporal-roundnumbertoincrement
+double round_number_to_increment(double x, u64 increment, RoundingMode rounding_mode)
+{
+    // 1. Let quotient be x / increment.
+    auto quotient = x / static_cast<double>(increment);
+
+    Sign is_negative;
+
+    // 2. If quotient < 0, then
+    if (quotient < 0) {
+        // a. Let isNegative be NEGATIVE.
+        is_negative = Sign::Negative;
+
+        // b. Set quotient to -quotient.
+        quotient = -quotient;
+    }
+    // 3. Else,
+    else {
+        // a. Let isNegative be POSITIVE.
+        is_negative = Sign::Positive;
+    }
+
+    // 4. Let unsignedRoundingMode be GetUnsignedRoundingMode(roundingMode, isNegative).
+    auto unsigned_rounding_mode = get_unsigned_rounding_mode(rounding_mode, is_negative);
+
+    // 5. Let r1 be the largest integer such that r1 ≤ quotient.
+    auto r1 = floor(quotient);
+
+    // 6. Let r2 be the smallest integer such that r2 > quotient.
+    auto r2 = ceil(quotient);
+    if (quotient == r2)
+        r2++;
+
+    // 7. Let rounded be ApplyUnsignedRoundingMode(quotient, r1, r2, unsignedRoundingMode).
+    auto rounded = apply_unsigned_rounding_mode(quotient, r1, r2, unsigned_rounding_mode);
+
+    // 8. If isNegative is NEGATIVE, set rounded to -rounded.
+    if (is_negative == Sign::Negative)
+        rounded = -rounded;
+
+    // 9. Return rounded × increment.
+    return rounded * static_cast<double>(increment);
+}
+
+// 13.28 RoundNumberToIncrement ( x, increment, roundingMode ), https://tc39.es/proposal-temporal/#sec-temporal-roundnumbertoincrement
+Crypto::SignedBigInteger round_number_to_increment(Crypto::SignedBigInteger const& x, Crypto::UnsignedBigInteger const& increment, RoundingMode rounding_mode)
+{
+    // OPTIMIZATION: If the increment is 1 the number is always rounded.
+    if (increment == 1)
+        return x;
+
+    // 1. Let quotient be x / increment.
+    auto division_result = x.divided_by(increment);
+
+    // OPTIMIZATION: If there's no remainder the number is already rounded.
+    if (division_result.remainder.unsigned_value().is_zero())
+        return x;
+
+    Sign is_negative;
+
+    // 2. If quotient < 0, then
+    if (division_result.quotient.is_negative()) {
+        // a. Let isNegative be NEGATIVE.
+        is_negative = Sign::Negative;
+
+        // b. Set quotient to -quotient.
+        division_result.quotient.negate();
+        division_result.remainder.negate();
+    }
+    // 3. Else,
+    else {
+        // a. Let isNegative be POSITIVE.
+        is_negative = Sign::Positive;
+    }
+
+    // 4. Let unsignedRoundingMode be GetUnsignedRoundingMode(roundingMode, isNegative).
+    auto unsigned_rounding_mode = get_unsigned_rounding_mode(rounding_mode, is_negative);
+
+    // 5. Let r1 be the largest integer such that r1 ≤ quotient.
+    auto r1 = division_result.quotient;
+
+    // 6. Let r2 be the smallest integer such that r2 > quotient.
+    auto r2 = division_result.quotient.plus(1_bigint);
+
+    // 7. Let rounded be ApplyUnsignedRoundingMode(quotient, r1, r2, unsignedRoundingMode).
+    auto rounded = apply_unsigned_rounding_mode(division_result, r1, r2, unsigned_rounding_mode, increment);
+
+    // 8. If isNegative is NEGATIVE, set rounded to -rounded.
+    if (is_negative == Sign::Negative)
+        rounded.negate();
+
+    // 9. Return rounded × increment.
+    return rounded.multiplied_by(increment);
+}
+
 // 13.35 ParseTemporalDurationString ( isoString ), https://tc39.es/proposal-temporal/#sec-temporal-parsetemporaldurationstring
 ThrowCompletionOr<GC::Ref<Duration>> parse_temporal_duration_string(VM& vm, StringView iso_string)
 {
@@ -353,12 +875,12 @@ ThrowCompletionOr<Value> get_option(VM& vm, Object const& options, PropertyKey c
     // 2. If value is undefined, then
     if (value.is_undefined()) {
         // a. If default is REQUIRED, throw a RangeError exception.
-        if (default_.has<DefaultRequired>())
+        if (default_.has<Required>())
             return vm.throw_completion<RangeError>(ErrorType::OptionIsNotValidValue, "undefined"sv, property.as_string());
 
         // b. Return default.
         return default_.visit(
-            [](DefaultRequired) -> Value { VERIFY_NOT_REACHED(); },
+            [](Required) -> Value { VERIFY_NOT_REACHED(); },
             [](Empty) -> Value { return js_undefined(); },
             [](bool default_) -> Value { return Value { default_ }; },
             [](double default_) -> Value { return Value { default_ }; },
@@ -392,4 +914,20 @@ ThrowCompletionOr<Value> get_option(VM& vm, Object const& options, PropertyKey c
     return value;
 }
 
+// 14.4.1.3 GetRoundingModeOption ( options, fallback ), https://tc39.es/proposal-temporal/#sec-temporal-getroundingmodeoption
+ThrowCompletionOr<RoundingMode> get_rounding_mode_option(VM& vm, Object const& options, RoundingMode fallback)
+{
+    // 1. Let allowedStrings be the List of Strings from the "String Identifier" column of Table 26.
+    static constexpr auto allowed_strings = to_array({ "ceil"sv, "floor"sv, "expand"sv, "trunc"sv, "halfCeil"sv, "halfFloor"sv, "halfExpand"sv, "halfTrunc"sv, "halfEven"sv });
+
+    // 2. Let stringFallback be the value from the "String Identifier" column of the row with fallback in its "Rounding Mode" column.
+    auto string_fallback = allowed_strings[to_underlying(fallback)];
+
+    // 3. Let stringValue be ? GetOption(options, "roundingMode", STRING, allowedStrings, stringFallback).
+    auto string_value = TRY(get_option(vm, options, vm.names.roundingMode, OptionType::String, allowed_strings, string_fallback));
+
+    // 4. Return the value from the "Rounding Mode" column of the row with stringValue in its "String Identifier" column.
+    return static_cast<RoundingMode>(allowed_strings.first_index_of(string_value.as_string().utf8_string_view()).value());
+}
+
 }

Libraries/LibJS/Runtime/Temporal/AbstractOperations.h

@@ -9,6 +9,8 @@
 #pragma once
 
 #include <AK/Variant.h>
+#include <LibCrypto/BigInt/SignedBigInteger.h>
+#include <LibCrypto/BigInt/UnsignedBigInteger.h>
 #include <LibGC/Ptr.h>
 #include <LibJS/Forward.h>
 #include <LibJS/Runtime/Completion.h>
@@ -51,8 +53,49 @@ enum class UnitGroup {
     DateTime,
 };
 
+// https://tc39.es/proposal-temporal/#table-unsigned-rounding-modes
+enum class RoundingMode {
+    Ceil,
+    Floor,
+    Expand,
+    Trunc,
+    HalfCeil,
+    HalfFloor,
+    HalfExpand,
+    HalfTrunc,
+    HalfEven,
+};
+
+// https://tc39.es/proposal-temporal/#table-unsigned-rounding-modes
+enum class UnsignedRoundingMode {
+    HalfEven,
+    HalfInfinity,
+    HalfZero,
+    Infinity,
+    Zero,
+};
+
+// https://tc39.es/proposal-temporal/#table-unsigned-rounding-modes
+enum class Sign {
+    Negative,
+    Positive,
+};
+
+struct Auto { };
+struct Required { };
 struct Unset { };
+using Precision = Variant<Auto, u8>;
 using RoundingIncrement = Variant<Unset, u64>;
+using UnitDefault = Variant<Required, Unset, Auto, Unit>;
+using UnitValue = Variant<Unset, Auto, Unit>;
+
+struct SecondsStringPrecision {
+    struct Minute { };
+
+    Variant<Minute, Auto, u8> precision;
+    Unit unit;
+    u8 increment { 0 };
+};
 
 struct RelativeTo {
     // FIXME: Make these objects represent their actual types when we re-implement them.
@@ -60,10 +103,20 @@ struct RelativeTo {
     GC::Ptr<JS::Object> zoned_relative_to; // [[ZonedRelativeTo]]
 };
 
+ThrowCompletionOr<Precision> get_temporal_fractional_second_digits_option(VM&, Object const& options);
+SecondsStringPrecision to_seconds_string_precision_record(UnitValue, Precision);
+ThrowCompletionOr<UnitValue> get_temporal_unit_valued_option(VM&, Object const& options, PropertyKey const&, UnitGroup, UnitDefault const&, ReadonlySpan<UnitValue> extra_values = {});
 ThrowCompletionOr<RelativeTo> get_temporal_relative_to_option(VM&, Object const& options);
 Unit larger_of_two_temporal_units(Unit, Unit);
 bool is_calendar_unit(Unit);
 UnitCategory temporal_unit_category(Unit);
+Crypto::UnsignedBigInteger const& temporal_unit_length_in_nanoseconds(Unit);
+String format_fractional_seconds(u64, Precision);
+UnsignedRoundingMode get_unsigned_rounding_mode(RoundingMode, Sign);
+double apply_unsigned_rounding_mode(double, double r1, double r2, UnsignedRoundingMode);
+Crypto::SignedBigInteger apply_unsigned_rounding_mode(Crypto::SignedDivisionResult const&, Crypto::SignedBigInteger const& r1, Crypto::SignedBigInteger const& r2, UnsignedRoundingMode, Crypto::UnsignedBigInteger const& increment);
+double round_number_to_increment(double, u64 increment, RoundingMode);
+Crypto::SignedBigInteger round_number_to_increment(Crypto::SignedBigInteger const&, Crypto::UnsignedBigInteger const& increment, RoundingMode);
 ThrowCompletionOr<GC::Ref<Duration>> parse_temporal_duration_string(VM&, StringView iso_string);
 
 // 13.38 ToIntegerWithTruncation ( argument ), https://tc39.es/proposal-temporal/#sec-tointegerwithtruncation
@@ -118,8 +171,7 @@ enum class OptionType {
     String,
 };
 
-struct DefaultRequired { };
+using OptionDefault = Variant<Required, Empty, bool, StringView, double>;
-using OptionDefault = Variant<DefaultRequired, Empty, bool, StringView, double>;
 
 ThrowCompletionOr<GC::Ref<Object>> get_options_object(VM&, Value options);
 ThrowCompletionOr<Value> get_option(VM&, Object const& options, PropertyKey const& property, OptionType type, ReadonlySpan<StringView> values, OptionDefault const&);
@@ -130,4 +182,6 @@ ThrowCompletionOr<Value> get_option(VM& vm, Object const& options, PropertyKey c
     return get_option(vm, options, property, type, ReadonlySpan<StringView> { values }, default_);
 }
 
+ThrowCompletionOr<RoundingMode> get_rounding_mode_option(VM&, Object const& options, RoundingMode fallback);
+
 }

Libraries/LibJS/Runtime/Temporal/Duration.cpp

@@ -701,6 +701,20 @@ i8 compare_time_duration(TimeDuration const& one, TimeDuration const& two)
     return 0;
 }
 
+// 7.5.27 RoundTimeDurationToIncrement ( d, increment, roundingMode ), https://tc39.es/proposal-temporal/#sec-temporal-roundtimedurationtoincrement
+ThrowCompletionOr<TimeDuration> round_time_duration_to_increment(VM& vm, TimeDuration const& duration, Crypto::UnsignedBigInteger const& increment, RoundingMode rounding_mode)
+{
+    // 1. Let rounded be RoundNumberToIncrement(d, increment, roundingMode).
+    auto rounded = round_number_to_increment(duration, increment, rounding_mode);
+
+    // 2. If abs(rounded) > maxTimeDuration, throw a RangeError exception.
+    if (rounded.unsigned_value() > MAX_TIME_DURATION.unsigned_value())
+        return vm.throw_completion<RangeError>(ErrorType::TemporalInvalidDuration);
+
+    // 3. Return rounded.
+    return rounded;
+}
+
 // 7.5.28 TimeDurationSign ( d ), https://tc39.es/proposal-temporal/#sec-temporal-timedurationsign
 i8 time_duration_sign(TimeDuration const& time_duration)
 {
@@ -716,6 +730,108 @@ i8 time_duration_sign(TimeDuration const& time_duration)
     return 0;
 }
 
+// 7.5.30 RoundTimeDuration ( timeDuration, increment, unit, roundingMode ), https://tc39.es/proposal-temporal/#sec-temporal-roundtimeduration
+ThrowCompletionOr<TimeDuration> round_time_duration(VM& vm, TimeDuration const& time_duration, Crypto::UnsignedBigInteger const& increment, Unit unit, RoundingMode rounding_mode)
+{
+    // 1. Let divisor be the value in the "Length in Nanoseconds" column of the row of Table 21 whose "Value" column contains unit.
+    auto const& divisor = temporal_unit_length_in_nanoseconds(unit);
+
+    // 2. Return ? RoundTimeDurationToIncrement(timeDuration, divisor × increment, roundingMode).
+    return TRY(round_time_duration_to_increment(vm, time_duration, divisor.multiplied_by(increment), rounding_mode));
+}
+
+// 7.5.39 TemporalDurationToString ( duration, precision ), https://tc39.es/proposal-temporal/#sec-temporal-temporaldurationtostring
+String temporal_duration_to_string(Duration const& duration, Precision precision)
+{
+    // 1. Let sign be DurationSign(duration).
+    auto sign = duration_sign(duration);
+
+    // 2. Let datePart be the empty String.
+    StringBuilder date_part;
+
+    // 3. If duration.[[Years]] ≠ 0, then
+    if (duration.years() != 0) {
+        // a. Set datePart to the string concatenation of abs(duration.[[Years]]) formatted as a decimal number and the
+        //    code unit 0x0059 (LATIN CAPITAL LETTER Y).
+        date_part.appendff("{}Y", AK::fabs(duration.years()));
+    }
+    // 4. If duration.[[Months]] ≠ 0, then
+    if (duration.months() != 0) {
+        // a. Set datePart to the string concatenation of datePart, abs(duration.[[Months]]) formatted as a decimal number,
+        //    and the code unit 0x004D (LATIN CAPITAL LETTER M).
+        date_part.appendff("{}M", AK::fabs(duration.months()));
+    }
+    // 5. If duration.[[Weeks]] ≠ 0, then
+    if (duration.weeks() != 0) {
+        // a. Set datePart to the string concatenation of datePart, abs(duration.[[Weeks]]) formatted as a decimal number,
+        //    and the code unit 0x0057 (LATIN CAPITAL LETTER W).
+        date_part.appendff("{}W", AK::fabs(duration.weeks()));
+    }
+    // 6. If duration.[[Days]] ≠ 0, then
+    if (duration.days() != 0) {
+        // a. Set datePart to the string concatenation of datePart, abs(duration.[[Days]]) formatted as a decimal number,
+        //    and the code unit 0x0044 (LATIN CAPITAL LETTER D).
+        date_part.appendff("{}D", AK::fabs(duration.days()));
+    }
+
+    // 7. Let timePart be the empty String.
+    StringBuilder time_part;
+
+    // 8. If duration.[[Hours]] ≠ 0, then
+    if (duration.hours() != 0) {
+        // a. Set timePart to the string concatenation of abs(duration.[[Hours]]) formatted as a decimal number and the
+        //    code unit 0x0048 (LATIN CAPITAL LETTER H).
+        time_part.appendff("{}H", AK::fabs(duration.hours()));
+    }
+    // 9. If duration.[[Minutes]] ≠ 0, then
+    if (duration.minutes() != 0) {
+        // a. Set timePart to the string concatenation of timePart, abs(duration.[[Minutes]]) formatted as a decimal number,
+        //    and the code unit 0x004D (LATIN CAPITAL LETTER M).
+        time_part.appendff("{}M", AK::fabs(duration.minutes()));
+    }
+
+    // 10. Let zeroMinutesAndHigher be false.
+    auto zero_minutes_and_higher = false;
+
+    // 11. If DefaultTemporalLargestUnit(duration) is SECOND, MILLISECOND, MICROSECOND, or NANOSECOND, set zeroMinutesAndHigher to true.
+    if (auto unit = default_temporal_largest_unit(duration); unit == Unit::Second || unit == Unit::Millisecond || unit == Unit::Microsecond || unit == Unit::Nanosecond)
+        zero_minutes_and_higher = true;
+
+    // 12. Let secondsDuration be TimeDurationFromComponents(0, 0, duration.[[Seconds]], duration.[[Milliseconds]], duration.[[Microseconds]], duration.[[Nanoseconds]]).
+    auto seconds_duration = time_duration_from_components(0, 0, duration.seconds(), duration.milliseconds(), duration.microseconds(), duration.nanoseconds());
+
+    // 13. If secondsDuration ≠ 0, or zeroMinutesAndHigher is true, or precision is not auto, then
+    if (!seconds_duration.is_zero() || zero_minutes_and_higher || !precision.has<Auto>()) {
+        auto division_result = seconds_duration.divided_by(NANOSECONDS_PER_SECOND);
+
+        // a. Let secondsPart be abs(truncate(secondsDuration / 10**9)) formatted as a decimal number.
+        auto seconds_part = MUST(division_result.quotient.unsigned_value().to_base(10));
+
+        // b. Let subSecondsPart be FormatFractionalSeconds(abs(remainder(secondsDuration, 10**9)), precision).
+        auto sub_seconds_part = format_fractional_seconds(division_result.remainder.unsigned_value().to_u64(), precision);
+
+        // c. Set timePart to the string concatenation of timePart, secondsPart, subSecondsPart, and the code unit
+        //    0x0053 (LATIN CAPITAL LETTER S).
+        time_part.appendff("{}{}S", seconds_part, sub_seconds_part);
+    }
+
+    // 14. Let signPart be the code unit 0x002D (HYPHEN-MINUS) if sign < 0, and otherwise the empty String.
+    auto sign_part = sign < 0 ? "-"sv : ""sv;
+
+    // 15. Let result be the string concatenation of signPart, the code unit 0x0050 (LATIN CAPITAL LETTER P) and datePart.
+    StringBuilder result;
+    result.appendff("{}P{}", sign_part, date_part.string_view());
+
+    // 16. If timePart is not the empty String, then
+    if (!time_part.is_empty()) {
+        // a. Set result to the string concatenation of result, the code unit 0x0054 (LATIN CAPITAL LETTER T), and timePart.
+        result.appendff("T{}", time_part.string_view());
+    }
+
+    // 17. Return result.
+    return MUST(result.to_string());
+}
+
 // 7.5.40 AddDurations ( operation, duration, other ), https://tc39.es/proposal-temporal/#sec-temporal-adddurations
 ThrowCompletionOr<GC::Ref<Duration>> add_durations(VM& vm, ArithmeticOperation operation, Duration const& duration, Value other_value)
 {

Libraries/LibJS/Runtime/Temporal/Duration.h

@@ -118,7 +118,10 @@ TimeDuration time_duration_from_components(double hours, double minutes, double
 ThrowCompletionOr<TimeDuration> add_time_duration(VM&, TimeDuration const&, TimeDuration const&);
 ThrowCompletionOr<TimeDuration> add_24_hour_days_to_time_duration(VM&, TimeDuration const&, double days);
 i8 compare_time_duration(TimeDuration const&, TimeDuration const&);
+ThrowCompletionOr<TimeDuration> round_time_duration_to_increment(VM&, TimeDuration const&, Crypto::UnsignedBigInteger const& increment, RoundingMode);
 i8 time_duration_sign(TimeDuration const&);
+ThrowCompletionOr<TimeDuration> round_time_duration(VM&, TimeDuration const&, Crypto::UnsignedBigInteger const& increment, Unit, RoundingMode);
+String temporal_duration_to_string(Duration const&, Precision);
 ThrowCompletionOr<GC::Ref<Duration>> add_durations(VM&, ArithmeticOperation, Duration const&, Value);
 
 }

Libraries/LibJS/Runtime/Temporal/DurationPrototype.cpp

@@ -41,6 +41,9 @@ void DurationPrototype::initialize(Realm& realm)
     define_native_function(realm, vm.names.abs, abs, 0, attr);
     define_native_function(realm, vm.names.add, add, 1, attr);
     define_native_function(realm, vm.names.subtract, subtract, 1, attr);
+    define_native_function(realm, vm.names.toString, to_string, 0, attr);
+    define_native_function(realm, vm.names.toJSON, to_json, 0, attr);
+    define_native_function(realm, vm.names.toLocaleString, to_locale_string, 0, attr);
 }
 
 // 7.3.3 get Temporal.Duration.prototype.years, https://tc39.es/proposal-temporal/#sec-get-temporal.duration.prototype.years
@@ -214,4 +217,85 @@ JS_DEFINE_NATIVE_FUNCTION(DurationPrototype::subtract)
     return TRY(add_durations(vm, ArithmeticOperation::Subtract, duration, other));
 }
 
+// 7.3.22 Temporal.Duration.prototype.toString ( [ options ] ), https://tc39.es/proposal-temporal/#sec-temporal.duration.prototype.tostring
+JS_DEFINE_NATIVE_FUNCTION(DurationPrototype::to_string)
+{
+    // 1. Let duration be the this value.
+    // 2. Perform ? RequireInternalSlot(duration, [[InitializedTemporalDuration]]).
+    auto duration = TRY(typed_this_object(vm));
+
+    // 3. Let resolvedOptions be ? GetOptionsObject(options).
+    auto resolved_options = TRY(get_options_object(vm, vm.argument(0)));
+
+    // 4. NOTE: The following steps read options and perform independent validation in alphabetical order
+    //    (GetTemporalFractionalSecondDigitsOption reads "fractionalSecondDigits" and GetRoundingModeOption reads
+    //    "roundingMode").
+
+    // 5. Let digits be ? GetTemporalFractionalSecondDigitsOption(resolvedOptions).
+    auto digits = TRY(get_temporal_fractional_second_digits_option(vm, resolved_options));
+
+    // 6. Let roundingMode be ? GetRoundingModeOption(resolvedOptions, TRUNC).
+    auto rounding_mode = TRY(get_rounding_mode_option(vm, resolved_options, RoundingMode::Trunc));
+
+    // 7. Let smallestUnit be ? GetTemporalUnitValuedOption(resolvedOptions, "smallestUnit", TIME, UNSET).
+    auto smallest_unit = TRY(get_temporal_unit_valued_option(vm, resolved_options, vm.names.smallestUnit, UnitGroup::Time, Unset {}));
+
+    // 8. If smallestUnit is HOUR or MINUTE, throw a RangeError exception.
+    if (auto const* unit = smallest_unit.get_pointer<Unit>(); unit && (*unit == Unit::Hour || *unit == Unit::Minute))
+        return vm.throw_completion<RangeError>(ErrorType::OptionIsNotValidValue, temporal_unit_to_string(*unit), vm.names.smallestUnit);
+
+    // 9. Let precision be ToSecondsStringPrecisionRecord(smallestUnit, digits).
+    auto precision = to_seconds_string_precision_record(smallest_unit, digits);
+
+    // 10. If precision.[[Unit]] is NANOSECOND and precision.[[Increment]] = 1, then
+    if (precision.unit == Unit::Nanosecond && precision.increment == 1) {
+        // a. Return TemporalDurationToString(duration, precision.[[Precision]]).
+        return PrimitiveString::create(vm, temporal_duration_to_string(duration, precision.precision.downcast<Auto, u8>()));
+    }
+
+    // 11. Let largestUnit be DefaultTemporalLargestUnit(duration).
+    auto largest_unit = default_temporal_largest_unit(duration);
+
+    // 12. Let internalDuration be ToInternalDurationRecord(duration).
+    auto internal_duration = to_internal_duration_record(vm, duration);
+
+    // 13. Let timeDuration be ? RoundTimeDuration(internalDuration.[[Time]], precision.[[Increment]], precision.[[Unit]], roundingMode).
+    auto time_duration = TRY(round_time_duration(vm, internal_duration.time, precision.increment, precision.unit, rounding_mode));
+
+    // 14. Set internalDuration to ! CombineDateAndTimeDuration(internalDuration.[[Date]], timeDuration).
+    internal_duration = MUST(combine_date_and_time_duration(vm, internal_duration.date, move(time_duration)));
+
+    // 15. Let roundedLargestUnit be LargerOfTwoTemporalUnits(largestUnit, SECOND).
+    auto rounded_largest_unit = larger_of_two_temporal_units(largest_unit, Unit::Second);
+
+    // 16. Let roundedDuration be ? TemporalDurationFromInternal(internalDuration, roundedLargestUnit).
+    auto rounded_duration = TRY(temporal_duration_from_internal(vm, internal_duration, rounded_largest_unit));
+
+    // 17. Return TemporalDurationToString(roundedDuration, precision.[[Precision]]).
+    return PrimitiveString::create(vm, temporal_duration_to_string(rounded_duration, precision.precision.downcast<Auto, u8>()));
+}
+
+// 7.3.23 Temporal.Duration.prototype.toJSON ( ), https://tc39.es/proposal-temporal/#sec-temporal.duration.prototype.tojson
+JS_DEFINE_NATIVE_FUNCTION(DurationPrototype::to_json)
+{
+    // 1. Let duration be the this value.
+    // 2. Perform ? RequireInternalSlot(duration, [[InitializedTemporalDuration]]).
+    auto duration = TRY(typed_this_object(vm));
+
+    // 3. Return TemporalDurationToString(duration, AUTO).
+    return PrimitiveString::create(vm, temporal_duration_to_string(duration, Auto {}));
+}
+
+// 7.3.24 Temporal.Duration.prototype.toLocaleString ( [ locales [ , options ] ] ), https://tc39.es/proposal-temporal/#sec-temporal.duration.prototype.tolocalestring
+// NOTE: This is the minimum toLocaleString implementation for engines without ECMA-402.
+JS_DEFINE_NATIVE_FUNCTION(DurationPrototype::to_locale_string)
+{
+    // 1. Let duration be the this value.
+    // 2. Perform ? RequireInternalSlot(duration, [[InitializedTemporalDuration]]).
+    auto duration = TRY(typed_this_object(vm));
+
+    // 3. Return TemporalDurationToString(duration, AUTO).
+    return PrimitiveString::create(vm, temporal_duration_to_string(duration, Auto {}));
+}
+
 }

Libraries/LibJS/Runtime/Temporal/DurationPrototype.h

@@ -35,6 +35,9 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(abs);
     JS_DECLARE_NATIVE_FUNCTION(add);
     JS_DECLARE_NATIVE_FUNCTION(subtract);
+    JS_DECLARE_NATIVE_FUNCTION(to_string);
+    JS_DECLARE_NATIVE_FUNCTION(to_json);
+    JS_DECLARE_NATIVE_FUNCTION(to_locale_string);
 };
 
 }

Libraries/LibJS/Tests/builtins/Temporal/Duration/Duration.prototype.toJSON.js

@@ -0,0 +1,19 @@
+describe("correct behavior", () => {
+    test("length is 0", () => {
+        expect(Temporal.Duration.prototype.toJSON).toHaveLength(0);
+    });
+
+    test("basic functionality", () => {
+        expect(new Temporal.Duration(1, 2, 3, 4, 5, 6, 7, 8, 9, 10).toJSON()).toBe(
+            "P1Y2M3W4DT5H6M7.00800901S"
+        );
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.Duration object", () => {
+        expect(() => {
+            Temporal.Duration.prototype.toJSON.call("foo");
+        }).toThrowWithMessage(TypeError, "Not an object of type Temporal.Duration");
+    });
+});

Libraries/LibJS/Tests/builtins/Temporal/Duration/Duration.prototype.toLocaleString.js

@@ -0,0 +1,19 @@
+describe("correct behavior", () => {
+    test("length is 0", () => {
+        expect(Temporal.Duration.prototype.toLocaleString).toHaveLength(0);
+    });
+
+    test("basic functionality", () => {
+        expect(new Temporal.Duration(1, 2, 3, 4, 5, 6, 7, 8, 9, 10).toLocaleString()).toBe(
+            "P1Y2M3W4DT5H6M7.00800901S"
+        );
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.Duration object", () => {
+        expect(() => {
+            Temporal.Duration.prototype.toLocaleString.call("foo");
+        }).toThrowWithMessage(TypeError, "Not an object of type Temporal.Duration");
+    });
+});

Libraries/LibJS/Tests/builtins/Temporal/Duration/Duration.prototype.toString.js

@@ -0,0 +1,94 @@
+describe("correct behavior", () => {
+    test("length is 0", () => {
+        expect(Temporal.Duration.prototype.toString).toHaveLength(0);
+    });
+
+    test("basic functionality", () => {
+        const values = [
+            [[0], "PT0S"],
+            [[1], "P1Y"],
+            [[0, 1], "P1M"],
+            [[0, 0, 1], "P1W"],
+            [[0, 0, 0, 1], "P1D"],
+            [[0, 0, 0, 0, 1], "PT1H"],
+            [[0, 0, 0, 0, 0, 1], "PT1M"],
+            [[0, 0, 0, 0, 0, 0, 1], "PT1S"],
+            [[0, 0, 0, 0, 0, 0, 0, 1], "PT0.001S"],
+            [[0, 0, 0, 0, 0, 0, 0, 0, 1], "PT0.000001S"],
+            [[0, 0, 0, 0, 0, 0, 0, 0, 0, 1], "PT0.000000001S"],
+            [[1, 2], "P1Y2M"],
+            [[1, 2, 3], "P1Y2M3W"],
+            [[1, 2, 3, 4], "P1Y2M3W4D"],
+            [[1, 2, 3, 4, 5], "P1Y2M3W4DT5H"],
+            [[1, 2, 3, 4, 5, 6], "P1Y2M3W4DT5H6M"],
+            [[1, 2, 3, 4, 5, 6, 7], "P1Y2M3W4DT5H6M7S"],
+            [[1, 2, 3, 4, 5, 6, 7, 8], "P1Y2M3W4DT5H6M7.008S"],
+            [[1, 2, 3, 4, 5, 6, 7, 8, 9], "P1Y2M3W4DT5H6M7.008009S"],
+            [[1, 2, 3, 4, 5, 6, 7, 8, 9, 10], "P1Y2M3W4DT5H6M7.00800901S"],
+            [
+                [100, 200, 300, 400, 500, 600, 700, 800, 900, 1000],
+                "P100Y200M300W400DT500H600M700.800901S",
+            ],
+            [[-1], "-P1Y"],
+        ];
+        for (const [args, expected] of values) {
+            expect(new Temporal.Duration(...args).toString()).toBe(expected);
+        }
+    });
+
+    test("smallestUnit option", () => {
+        const values = [
+            ["second", "P1Y2M3W4DT5H6M7S"],
+            ["millisecond", "P1Y2M3W4DT5H6M7.008S"],
+            ["microsecond", "P1Y2M3W4DT5H6M7.008010S"],
+            ["nanosecond", "P1Y2M3W4DT5H6M7.008010000S"],
+        ];
+        for (const [smallestUnit, expected] of values) {
+            expect(
+                new Temporal.Duration(1, 2, 3, 4, 5, 6, 7, 8, 10).toString({ smallestUnit })
+            ).toBe(expected);
+        }
+    });
+
+    test("fractionalSecondDigits option", () => {
+        const values = [
+            [0, "P1Y2M3W4DT5H6M7S"],
+            [1, "P1Y2M3W4DT5H6M7.0S"],
+            [2, "P1Y2M3W4DT5H6M7.00S"],
+            [3, "P1Y2M3W4DT5H6M7.008S"],
+            [4, "P1Y2M3W4DT5H6M7.0080S"],
+            [5, "P1Y2M3W4DT5H6M7.00801S"],
+            [6, "P1Y2M3W4DT5H6M7.008010S"],
+            [7, "P1Y2M3W4DT5H6M7.0080100S"],
+            [8, "P1Y2M3W4DT5H6M7.00801000S"],
+            [9, "P1Y2M3W4DT5H6M7.008010000S"],
+        ];
+        for (const [fractionalSecondDigits, expected] of values) {
+            expect(
+                new Temporal.Duration(1, 2, 3, 4, 5, 6, 7, 8, 10).toString({
+                    fractionalSecondDigits,
+                })
+            ).toBe(expected);
+        }
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.Duration object", () => {
+        expect(() => {
+            Temporal.Duration.prototype.toString.call("foo");
+        }).toThrowWithMessage(TypeError, "Not an object of type Temporal.Duration");
+    });
+
+    test("disallowed smallestUnit option values", () => {
+        const values = ["year", "month", "week", "day", "hour", "minute"];
+        for (const smallestUnit of values) {
+            expect(() => {
+                new Temporal.Duration(0).toString({ smallestUnit });
+            }).toThrowWithMessage(
+                RangeError,
+                `${smallestUnit} is not a valid value for option smallestUnit`
+            );
+        }
+    });
+});