Build out partial record functionality, property bag construction, and `with` methods (#3955)

* Build out partial functionality and with methods

* Use temporal_rs's TinyAsciiStr

* Apply review feedback for specification comments

* Bump temporal-rs and add DateTime::with

Cargo.lock

@@ -3220,7 +3220,7 @@ checksum = "55937e1799185b12863d447f42597ed69d9928686b8d88a1df17376a097d8369"
 [[package]]
 name = "temporal_rs"
 version = "0.0.3"
-source = "git+https://github.com/boa-dev/temporal.git?rev=af94bbc31d409a2bfdce473e667e08e16c677149#af94bbc31d409a2bfdce473e667e08e16c677149"
+source = "git+https://github.com/boa-dev/temporal.git?rev=1e7901d07a83211e62373ab94284a7d1ada4c913#1e7901d07a83211e62373ab94284a7d1ada4c913"
 dependencies = [
  "bitflags 2.6.0",
  "icu_calendar",

Cargo.toml

@@ -111,7 +111,7 @@ intrusive-collections = "0.9.6"
 cfg-if = "1.0.0"
 either = "1.13.0"
 sys-locale = "0.3.1"
-temporal_rs = { git = "https://github.com/boa-dev/temporal.git", rev = "af94bbc31d409a2bfdce473e667e08e16c677149" }
+temporal_rs = { git = "https://github.com/boa-dev/temporal.git", rev = "1e7901d07a83211e62373ab94284a7d1ada4c913" }
 web-time = "1.1.0"
 criterion = "0.5.1"
 float-cmp = "0.9.0"

core/engine/src/builtins/temporal/mod.rs

@@ -274,6 +274,46 @@ pub(crate) fn to_relative_temporal_object(
 // 13.26 `GetUnsignedRoundingMode ( roundingMode, isNegative )`
 // Implemented on RoundingMode in builtins/options.rs
 
+// 13.26 IsPartialTemporalObject ( object )
+pub(crate) fn is_partial_temporal_object<'value>(
+    value: &'value JsValue,
+    context: &mut Context,
+) -> JsResult<Option<&'value JsObject>> {
+    // 1. If value is not an Object, return false.
+    let Some(obj) = value.as_object() else {
+        return Ok(None);
+    };
+
+    // 2. If value has an [[InitializedTemporalDate]], [[InitializedTemporalDateTime]],
+    // [[InitializedTemporalMonthDay]], [[InitializedTemporalTime]],
+    // [[InitializedTemporalYearMonth]], or
+    // [[InitializedTemporalZonedDateTime]] internal slot, return false.
+    if obj.is::<PlainDate>()
+        || obj.is::<PlainDateTime>()
+        || obj.is::<PlainMonthDay>()
+        || obj.is::<PlainYearMonth>()
+        || obj.is::<PlainTime>()
+        || obj.is::<ZonedDateTime>()
+    {
+        return Ok(None);
+    }
+
+    // 3. Let calendarProperty be ? Get(value, "calendar").
+    let calendar_property = obj.get(js_str!("calendar"), context)?;
+    // 4. If calendarProperty is not undefined, return false.
+    if !calendar_property.is_undefined() {
+        return Ok(None);
+    }
+    // 5. Let timeZoneProperty be ? Get(value, "timeZone").
+    let time_zone_property = obj.get(js_str!("timeZone"), context)?;
+    // 6. If timeZoneProperty is not undefined, return false.
+    if !time_zone_property.is_undefined() {
+        return Ok(None);
+    }
+    // 7. Return true.
+    Ok(Some(obj))
+}
+
 // 13.27 `ApplyUnsignedRoundingMode ( x, r1, r2, unsignedRoundingMode )`
 // Migrated to `temporal_rs`
 

core/engine/src/builtins/temporal/plain_date/mod.rs

@@ -3,6 +3,8 @@
 
 // TODO (nekevss): DOCS DOCS AND MORE DOCS
 
+use std::str::FromStr;
+
 use crate::{
     builtins::{
         options::{get_option, get_options_object},
@@ -14,7 +16,8 @@ use crate::{
     property::Attribute,
     realm::Realm,
     string::StaticJsStrings,
-    Context, JsArgs, JsData, JsNativeError, JsObject, JsResult, JsString, JsSymbol, JsValue,
+    Context, JsArgs, JsData, JsError, JsNativeError, JsObject, JsResult, JsString, JsSymbol,
+    JsValue,
 };
 use boa_gc::{Finalize, Trace};
 use boa_macros::js_str;
@@ -22,16 +25,18 @@ use boa_profiler::Profiler;
 use temporal_rs::{
     components::{
         calendar::{Calendar, GetTemporalCalendar},
-        Date as InnerDate, DateTime,
+        Date as InnerDate, DateTime, MonthCode, PartialDate,
     },
     iso::IsoDateSlots,
     options::ArithmeticOverflow,
+    TemporalFields, TinyAsciiStr,
 };
 
 use super::{
-    calendar::to_temporal_calendar_slot_value, create_temporal_datetime, create_temporal_duration,
+    calendar::{get_temporal_calendar_slot_value_with_default, to_temporal_calendar_slot_value},
-    options::get_difference_settings, to_temporal_duration_record, to_temporal_time, PlainDateTime,
+    create_temporal_datetime, create_temporal_duration,
-    ZonedDateTime,
+    options::get_difference_settings,
+    to_temporal_duration_record, to_temporal_time, PlainDateTime, ZonedDateTime,
 };
 
 /// The `Temporal.PlainDate` object.
@@ -613,10 +618,39 @@ impl PlainDate {
             .map(Into::into)
     }
 
-    fn with(this: &JsValue, _: &[JsValue], _: &mut Context) -> JsResult<JsValue> {
+    // 3.3.24 Temporal.PlainDate.prototype.with ( temporalDateLike [ , options ] )
-        Err(JsNativeError::error()
+    fn with(this: &JsValue, args: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
-            .with_message("not yet implemented.")
+        // 1. Let temporalDate be the this value.
-            .into())
+        // 2. Perform ? RequireInternalSlot(temporalDate, [[InitializedTemporalDate]]).
+        let date = this
+            .as_object()
+            .and_then(JsObject::downcast_ref::<Self>)
+            .ok_or_else(|| {
+                JsNativeError::typ().with_message("the this object must be a PlainDate object.")
+            })?;
+
+        // 3. If ? IsPartialTemporalObject(temporalDateLike) is false, throw a TypeError exception.
+        let Some(partial_object) =
+            super::is_partial_temporal_object(args.get_or_undefined(0), context)?
+        else {
+            return Err(JsNativeError::typ()
+                .with_message("with object was not a PartialTemporalObject.")
+                .into());
+        };
+        let options = get_options_object(args.get_or_undefined(1))?;
+
+        // SKIP: Steps 4-9 are handled by the with method of temporal_rs's Date
+        // 4. Let resolvedOptions be ? SnapshotOwnProperties(? GetOptionsObject(options), null).
+        // 5. Let calendarRec be ? CreateCalendarMethodsRecord(temporalDate.[[Calendar]], « date-from-fields, fields, merge-fields »).
+        // 6. Let fieldsResult be ? PrepareCalendarFieldsAndFieldNames(calendarRec, temporalDate, « "day", "month", "monthCode", "year" »).
+        // 7. Let partialDate be ? PrepareTemporalFields(temporalDateLike, fieldsResult.[[FieldNames]], partial).
+        // 8. Let fields be ? CalendarMergeFields(calendarRec, fieldsResult.[[Fields]], partialDate).
+        // 9. Set fields to ? PrepareTemporalFields(fields, fieldsResult.[[FieldNames]], «»).
+        let overflow = get_option::<ArithmeticOverflow>(&options, js_str!("overflow"), context)?;
+        let partial = to_partial_date_record(partial_object, context)?;
+
+        // 10. Return ? CalendarDateFromFields(calendarRec, fields, resolvedOptions).
+        create_temporal_date(date.inner.with(partial, overflow)?, None, context).map(Into::into)
     }
 
     /// 3.3.26 Temporal.PlainDate.prototype.withCalendar ( calendarLike )
@@ -807,13 +841,30 @@ pub(crate) fn to_temporal_date(
         }
 
         // d. Let calendar be ? GetTemporalCalendarSlotValueWithISODefault(item).
+        let calendar = get_temporal_calendar_slot_value_with_default(object, context)?;
+        let overflow =
+            get_option::<ArithmeticOverflow>(&options_obj, js_str!("overflow"), context)?
+                .unwrap_or(ArithmeticOverflow::Constrain);
+
         // e. Let fieldNames be ? CalendarFields(calendar, « "day", "month", "monthCode", "year" »).
         // f. Let fields be ? PrepareTemporalFields(item, fieldNames, «»).
-        // g. Return ? CalendarDateFromFields(calendar, fields, options).
+        let partial = to_partial_date_record(object, context)?;
-        return Err(JsNativeError::error()
+        // TODO: Move validation to `temporal_rs`.
-            .with_message("CalendarDateFields not yet implemented.")
+        if !(partial.day.is_some()
+            && (partial.month.is_some() || partial.month_code.is_some())
+            && (partial.year.is_some() || (partial.era.is_some() && partial.era_year.is_some())))
+        {
+            return Err(JsNativeError::typ()
+                .with_message("A partial date must have at least one defined field.")
                 .into());
         }
+        let mut fields = TemporalFields::from(partial);
+
+        // g. Return ? CalendarDateFromFields(calendar, fields, options).
+        return calendar
+            .date_from_fields(&mut fields, overflow)
+            .map_err(Into::into);
+    }
 
     // 5. If item is not a String, throw a TypeError exception.
     let JsValue::String(date_like_string) = item else {
@@ -837,3 +888,63 @@ pub(crate) fn to_temporal_date(
 
     Ok(result)
 }
+
+pub(crate) fn to_partial_date_record(
+    partial_object: &JsObject,
+    context: &mut Context,
+) -> JsResult<PartialDate> {
+    let day = partial_object
+        .get(js_str!("day"), context)?
+        .map(|v| super::to_integer_if_integral(v, context))
+        .transpose()?;
+    let month = partial_object
+        .get(js_str!("month"), context)?
+        .map(|v| super::to_integer_if_integral(v, context))
+        .transpose()?;
+    let month_code = partial_object
+        .get(js_str!("monthCode"), context)?
+        .map(|v| {
+            let JsValue::String(month_code) =
+                v.to_primitive(context, crate::value::PreferredType::String)?
+            else {
+                return Err(JsNativeError::typ()
+                    .with_message("The monthCode field value must be a string.")
+                    .into());
+            };
+            MonthCode::from_str(&month_code.to_std_string_escaped()).map_err(Into::<JsError>::into)
+        })
+        .transpose()?;
+    let year = partial_object
+        .get(js_str!("year"), context)?
+        .map(|v| super::to_integer_if_integral(v, context))
+        .transpose()?;
+    let era_year = partial_object
+        .get(js_str!("eraYear"), context)?
+        .map(|v| super::to_integer_if_integral(v, context))
+        .transpose()?;
+    let era = partial_object
+        .get(js_str!("era"), context)?
+        .map(|v| {
+            let JsValue::String(era) =
+                v.to_primitive(context, crate::value::PreferredType::String)?
+            else {
+                return Err(JsError::from(
+                    JsNativeError::typ()
+                        .with_message("The monthCode field value must be a string."),
+                ));
+            };
+            // TODO: double check if an invalid monthCode is a range or type error.
+            TinyAsciiStr::<16>::from_str(&era.to_std_string_escaped())
+                .map_err(|e| JsError::from(JsNativeError::range().with_message(e.to_string())))
+        })
+        .transpose()?;
+
+    Ok(PartialDate {
+        year,
+        month,
+        month_code,
+        day,
+        era,
+        era_year,
+    })
+}

core/engine/src/builtins/temporal/plain_date_time/mod.rs

@@ -4,7 +4,7 @@
 use crate::{
     builtins::{
         options::{get_option, get_options_object},
-        temporal::to_integer_with_truncation,
+        temporal::{to_integer_with_truncation, to_partial_date_record, to_partial_time_record},
         BuiltInBuilder, BuiltInConstructor, BuiltInObject, IntrinsicObject,
     },
     context::intrinsics::{Intrinsics, StandardConstructor, StandardConstructors},
@@ -25,14 +25,15 @@ mod tests;
 use temporal_rs::{
     components::{
         calendar::{Calendar, GetTemporalCalendar},
-        DateTime as InnerDateTime,
+        DateTime as InnerDateTime, PartialDateTime, Time,
     },
     iso::{IsoDate, IsoDateSlots},
     options::{ArithmeticOverflow, RoundingIncrement, RoundingOptions, TemporalRoundingMode},
+    TemporalFields,
 };
 
 use super::{
-    calendar::to_temporal_calendar_slot_value,
+    calendar::{get_temporal_calendar_slot_value_with_default, to_temporal_calendar_slot_value},
     create_temporal_duration,
     options::{get_difference_settings, get_temporal_unit, TemporalUnitGroup},
     to_temporal_duration_record, to_temporal_time, PlainDate, ZonedDateTime,
@@ -278,6 +279,7 @@ impl IntrinsicObject for PlainDateTime {
             )
             .static_method(Self::from, js_string!("from"), 1)
             .static_method(Self::compare, js_string!("compare"), 2)
+            .method(Self::with, js_string!("with"), 1)
             .method(Self::with_plain_time, js_string!("withPlainTime"), 1)
             .method(Self::with_calendar, js_string!("withCalendar"), 1)
             .method(Self::add, js_string!("add"), 1)
@@ -679,6 +681,35 @@ impl PlainDateTime {
 // ==== PlainDateTime.prototype method implementations ====
 
 impl PlainDateTime {
+    ///  5.3.25 Temporal.PlainDateTime.prototype.with ( temporalDateTimeLike [ , options ] )
+    fn with(this: &JsValue, args: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
+        let dt = this
+            .as_object()
+            .and_then(JsObject::downcast_ref::<Self>)
+            .ok_or_else(|| {
+                JsNativeError::typ().with_message("the this object must be a PlainDateTime object.")
+            })?;
+
+        let Some(partial_object) =
+            super::is_partial_temporal_object(args.get_or_undefined(0), context)?
+        else {
+            return Err(JsNativeError::typ()
+                .with_message("with object was not a PartialTemporalObject.")
+                .into());
+        };
+        let options = get_options_object(args.get_or_undefined(1))?;
+
+        let date = to_partial_date_record(partial_object, context)?;
+        let time = to_partial_time_record(partial_object, context)?;
+
+        let partial_dt = PartialDateTime { date, time };
+
+        let overflow = get_option::<ArithmeticOverflow>(&options, js_str!("overflow"), context)?;
+
+        create_temporal_datetime(dt.inner.with(partial_dt, overflow)?, None, context)
+            .map(Into::into)
+    }
+
     /// 5.3.26 Temporal.PlainDateTime.prototype.withPlainTime ( `[ plainTimeLike ]` )
     fn with_plain_time(
         this: &JsValue,
@@ -975,17 +1006,56 @@ pub(crate) fn to_temporal_datetime(
                 date.inner.calendar().clone(),
             )?);
         }
+
         // d. Let calendar be ? GetTemporalCalendarSlotValueWithISODefault(item).
+        let calendar = get_temporal_calendar_slot_value_with_default(object, context)?;
+
         // e. Let calendarRec be ? CreateCalendarMethodsRecord(calendar, « date-from-fields, fields »).
         // f. Let fields be ? PrepareCalendarFields(calendarRec, item, « "day", "month",
         // "monthCode", "year" », « "hour", "microsecond", "millisecond", "minute",
-        // "nanosecond", "second" », «»).
+        // "nanosecond", "second" », «»)
-        // g. Let result be ? InterpretTemporalDateTimeFields(calendarRec, fields, resolvedOptions).
+        let partial_date = to_partial_date_record(object, context)?;
-        // TODO: Implement d-g.
+        let partial_time = to_partial_time_record(object, context)?;
-        return Err(JsNativeError::range()
+        // TODO: Move validation to `temporal_rs`.
-            .with_message("Not yet implemented.")
+        if !(partial_date.day.is_some()
+            && (partial_date.month.is_some() || partial_date.month_code.is_some())
+            && (partial_date.year.is_some()
+                || (partial_date.era.is_some() && partial_date.era_year.is_some())))
+        {
+            return Err(JsNativeError::typ()
+                .with_message("A partial date must have at least one defined field.")
                 .into());
         }
+        // g. Let result be ? InterpretTemporalDateTimeFields(calendarRec, fields, resolvedOptions).
+        let overflow = get_option::<ArithmeticOverflow>(&options, js_str!("overflow"), context)?;
+        let date = calendar.date_from_fields(
+            &mut TemporalFields::from(partial_date),
+            overflow.unwrap_or(ArithmeticOverflow::Constrain),
+        )?;
+        let time = Time::new(
+            partial_time.hour.unwrap_or(0),
+            partial_time.minute.unwrap_or(0),
+            partial_time.second.unwrap_or(0),
+            partial_time.millisecond.unwrap_or(0),
+            partial_time.microsecond.unwrap_or(0),
+            partial_time.nanosecond.unwrap_or(0),
+            ArithmeticOverflow::Constrain,
+        )?;
+
+        return InnerDateTime::new(
+            date.iso_year(),
+            date.iso_month().into(),
+            date.iso_day().into(),
+            time.hour().into(),
+            time.minute().into(),
+            time.second().into(),
+            time.millisecond().into(),
+            time.microsecond().into(),
+            time.nanosecond().into(),
+            calendar,
+        )
+        .map_err(Into::into);
+    }
     // 4. Else,
     //     a. If item is not a String, throw a TypeError exception.
     let Some(string) = value.as_string() else {

core/engine/src/builtins/temporal/plain_time/mod.rs

@@ -17,7 +17,7 @@ use boa_gc::{Finalize, Trace};
 use boa_macros::js_str;
 use boa_profiler::Profiler;
 use temporal_rs::{
-    components::Time,
+    components::{PartialTime, Time},
     options::{ArithmeticOverflow, TemporalRoundingMode},
 };
 
@@ -112,6 +112,7 @@ impl IntrinsicObject for PlainTime {
             .static_method(Self::compare, js_string!("compare"), 2)
             .method(Self::add, js_string!("add"), 1)
             .method(Self::subtract, js_string!("subtract"), 1)
+            .method(Self::with, js_string!("with"), 1)
             .method(Self::until, js_string!("until"), 1)
             .method(Self::since, js_string!("since"), 1)
             .method(Self::round, js_string!("round"), 1)
@@ -374,6 +375,33 @@ impl PlainTime {
         create_temporal_time(time.inner.subtract(&duration)?, None, context).map(Into::into)
     }
 
+    fn with(this: &JsValue, args: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
+        // 1.Let temporalTime be the this value.
+        // 2. Perform ? RequireInternalSlot(temporalTime, [[InitializedTemporalTime]]).
+        let time = this
+            .as_object()
+            .and_then(JsObject::downcast_ref::<Self>)
+            .ok_or_else(|| {
+                JsNativeError::typ().with_message("the this object must be a PlainTime object.")
+            })?;
+
+        // 3. If ? IsPartialTemporalObject(temporalTimeLike) is false, throw a TypeError exception.
+        // 4. Set options to ? GetOptionsObject(options).
+        let Some(partial_object) =
+            super::is_partial_temporal_object(args.get_or_undefined(0), context)?
+        else {
+            return Err(JsNativeError::typ()
+                .with_message("with object was not a PartialTemporalObject.")
+                .into());
+        };
+
+        let options = get_options_object(args.get_or_undefined(1))?;
+        let overflow = get_option::<ArithmeticOverflow>(&options, js_str!("overflow"), context)?;
+        let partial = to_partial_time_record(partial_object, context)?;
+
+        create_temporal_time(time.inner.with(partial, overflow)?, None, context).map(Into::into)
+    }
+
     /// 4.3.12 Temporal.PlainTime.prototype.until ( other [ , options ] )
     fn until(this: &JsValue, args: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
         let time = this
@@ -598,8 +626,8 @@ pub(crate) fn create_temporal_time(
 
 pub(crate) fn to_temporal_time(
     value: &JsValue,
-    _overflow: Option<ArithmeticOverflow>,
+    overflow: Option<ArithmeticOverflow>,
-    _context: &mut Context,
+    context: &mut Context,
 ) -> JsResult<Time> {
     // 1.If overflow is not present, set overflow to "constrain".
     // 2. If item is an Object, then
@@ -625,23 +653,24 @@ pub(crate) fn to_temporal_time(
                 // i. Return ! CreateTemporalTime(item.[[ISOHour]], item.[[ISOMinute]],
                 // item.[[ISOSecond]], item.[[ISOMillisecond]], item.[[ISOMicrosecond]],
                 // item.[[ISONanosecond]]).
-                return Ok(Time::new(
+                return Ok(Time::from(dt.inner.clone()));
-                    dt.inner.hour().into(),
-                    dt.inner.minute().into(),
-                    dt.inner.second().into(),
-                    dt.inner.millisecond().into(),
-                    dt.inner.microsecond().into(),
-                    dt.inner.nanosecond().into(),
-                    ArithmeticOverflow::Reject,
-                )?);
             }
             // d. Let result be ? ToTemporalTimeRecord(item).
             // e. Set result to ? RegulateTime(result.[[Hour]], result.[[Minute]],
             // result.[[Second]], result.[[Millisecond]], result.[[Microsecond]],
             // result.[[Nanosecond]], overflow).
-            Err(JsNativeError::range()
+            let partial = to_partial_time_record(object, context)?;
-                .with_message("Not yet implemented.")
+
-                .into())
+            Time::new(
+                partial.hour.unwrap_or(0),
+                partial.minute.unwrap_or(0),
+                partial.second.unwrap_or(0),
+                partial.millisecond.unwrap_or(0),
+                partial.microsecond.unwrap_or(0),
+                partial.nanosecond.unwrap_or(0),
+                overflow.unwrap_or(ArithmeticOverflow::Constrain),
+            )
+            .map_err(Into::into)
         }
         // 3. Else,
         JsValue::String(str) => {
@@ -659,3 +688,47 @@ pub(crate) fn to_temporal_time(
 
     // 4. Return ! CreateTemporalTime(result.[[Hour]], result.[[Minute]], result.[[Second]], result.[[Millisecond]], result.[[Microsecond]], result.[[Nanosecond]]).
 }
+
+pub(crate) fn to_partial_time_record(
+    partial_object: &JsObject,
+    context: &mut Context,
+) -> JsResult<PartialTime> {
+    let hour = partial_object
+        .get(js_str!("hour"), context)?
+        .map(|v| super::to_integer_if_integral(v, context))
+        .transpose()?;
+
+    let minute = partial_object
+        .get(js_str!("minute"), context)?
+        .map(|v| super::to_integer_if_integral(v, context))
+        .transpose()?;
+
+    let second = partial_object
+        .get(js_str!("second"), context)?
+        .map(|v| super::to_integer_if_integral(v, context))
+        .transpose()?;
+
+    let millisecond = partial_object
+        .get(js_str!("millisecond"), context)?
+        .map(|v| super::to_integer_if_integral(v, context))
+        .transpose()?;
+
+    let microsecond = partial_object
+        .get(js_str!("microsecond"), context)?
+        .map(|v| super::to_integer_if_integral(v, context))
+        .transpose()?;
+
+    let nanosecond = partial_object
+        .get(js_str!("nanosecond"), context)?
+        .map(|v| super::to_integer_if_integral(v, context))
+        .transpose()?;
+
+    Ok(PartialTime {
+        hour,
+        minute,
+        second,
+        millisecond,
+        microsecond,
+        nanosecond,
+    })
+}

core/engine/src/value/mod.rs

@@ -1029,6 +1029,40 @@ impl JsValue {
         .into()
     }
 
+    /// Maps a `JsValue` into a `Option<T>` where T is the result of an
+    /// operation on a defined value. If the value is `JsValue::undefined`,
+    /// then `JsValue::map` will return None.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use boa_engine::{JsValue, Context};
+    ///
+    /// let mut context = Context::default();
+    ///
+    /// let defined_value = JsValue::from(5);
+    /// let undefined = JsValue::undefined();
+    ///
+    /// let defined_result = defined_value.map(|v| v.add(&JsValue::from(5), &mut context)).transpose().unwrap();
+    /// let undefined_result = undefined.map(|v| v.add(&JsValue::from(5), &mut context)).transpose().unwrap();
+    ///
+    /// assert_eq!(defined_result, Some(JsValue::Integer(10)));
+    /// assert_eq!(undefined_result, None);
+    ///
+    /// ```
+    ///
+    #[inline]
+    #[must_use]
+    pub fn map<T, F>(&self, f: F) -> Option<T>
+    where
+        F: FnOnce(&JsValue) -> T,
+    {
+        if self.is_undefined() {
+            return None;
+        }
+        Some(f(self))
+    }
+
     /// Abstract operation `IsArray ( argument )`
     ///
     /// Check if a value is an array.