diff --git a/crates/js/src/lib.rs b/crates/js/src/lib.rs
index 0e7de9891..8f3bb412d 100644
--- a/crates/js/src/lib.rs
+++ b/crates/js/src/lib.rs
@@ -825,6 +825,7 @@ enum DateUnit {
 }
 
 impl DateUnit {
+    /// A year, month or day: a format needs one of these for `util.scand` to read anything.
     fn is_date(self) -> bool {
         matches!(self, Self::Year | Self::ShortYear | Self::Month | Self::MonthName | Self::Day)
     }
@@ -845,6 +846,9 @@ enum ScanPart {
     Word(String),
 }
 
+/// The fields and literal words of a `util.scand` format, in order. A numeric field is marked
+/// `joined` when another numeric field follows it directly; anything but letters and digits
+/// outside a field is left out.
 fn scan_parts(fmt: &str) -> Vec<ScanPart> {
     let chars: Vec<char> = fmt.chars().collect();
     let mut parts: Vec<ScanPart> = Vec::new();
@@ -956,6 +960,8 @@ struct ScanValues {
 }
 
 impl ScanValues {
+    /// Records `v` as the value of `unit`: a `yy` year as 1950–2049, and an `h` or `hh` hour
+    /// marked as 12-hour. Month names, weekdays and am/pm are not read here.
     fn set(&mut self, unit: DateUnit, v: u32) {
         match unit {
             DateUnit::Year => self.y = i32::try_from(v).ok(),
@@ -1202,6 +1208,9 @@ fn color_object(ctx: &mut Context) -> JsObject {
     o.build()
 }
 
+/// Defines the form object model's globals: the document's functions (`getField`, `resetForm`,
+/// `print`, `submitForm` and others) and properties (`numFields`, `pageNum`, `numPages`,
+/// `documentFileName`, `info`), and the `app`, `util`, `console`, `display` and `color` objects.
 fn install(ctx: &mut Context) -> JsResult<()> {
     let globals: [(&str, usize, NativeFunction); 9] = [
         ("getField", 1, NativeFunction::from_fn_ptr(get_field)),
diff --git a/crates/js/src/tests.rs b/crates/js/src/tests.rs
index 791da93a2..c6b6d80d3 100644
--- a/crates/js/src/tests.rs
+++ b/crates/js/src/tests.rs
@@ -1157,6 +1157,8 @@ mod scand {
 
     // The cases below are another viewer's answers, observed black-box with a synthetic file.
 
+    /// Any separator may come between fields, whether the format has that one, another or none,
+    /// but a text with a space or a letter before or after the date reads nothing.
     #[test]
     fn scand_allows_separators_between_fields_but_nothing_before_or_after_them() {
         assert_scand(&[
@@ -1171,6 +1173,8 @@ mod scand {
         ]);
     }
 
+    /// Digits typed together fill the joined fields with exactly as many digits as each field has
+    /// letters, so a run of any other length reads nothing.
     #[test]
     fn scand_splits_digits_typed_together_by_the_formats_letter_counts() {
         assert_scand(&[
@@ -1188,6 +1192,8 @@ mod scand {
         ]);
     }
 
+    /// A month past 12, a day past the month's end and February 29 outside a leap year are
+    /// refused, while a month or day of 0 is read as 1.
     #[test]
     fn scand_refuses_impossible_dates_and_reads_a_zero_month_or_day_as_1() {
         assert_scand(&[
@@ -1201,6 +1207,8 @@ mod scand {
         ]);
     }
 
+    /// A field typed by itself may have one or two digits, but a `yyyy` year needs four; `yy` reads
+    /// as 1950–2049; month names may be full or three letters in any case; weekday names are read.
     #[test]
     fn scand_reads_lone_fields_two_digit_years_and_month_and_weekday_names() {
         assert_scand(&[
@@ -1223,6 +1231,7 @@ mod scand {
         ]);
     }
 
+    /// A time is read along with a date, but a format with only time fields reads nothing.
     #[test]
     fn scand_reads_a_time_only_with_a_date() {
         assert_scand(&[
@@ -1236,6 +1245,8 @@ mod scand {
         ]);
     }
 
+    /// An empty text reads as today at noon, and a text that is not a string (a number, or none
+    /// at all) reads nothing.
     #[test]
     fn scand_reads_only_text_and_an_empty_text_is_today_at_noon() {
         let o = go(
@@ -1250,6 +1261,8 @@ mod scand {
 
     // Not observed: the choices made where the other viewer wasn't asked.
 
+    /// A year left out is this year, `h:MM tt` reads a 12-hour time and refuses hours it cannot
+    /// have, and a letter the format writes must be typed there.
     #[test]
     fn scand_takes_this_year_when_none_is_given_and_reads_12_hour_times_and_written_letters() {
         assert_scand(&[
@@ -1268,6 +1281,8 @@ mod scand {
         assert_eq!(o.value, "true|9|18");
     }
 
+    /// A format of 50,000 literal letters and one of 100,000 fields, with texts to match, are read
+    /// in under two seconds.
     #[test]
     fn scand_reads_long_formats_and_texts_in_linear_time() {
         // Scripts can pass anything; native code isn't bounded by the loop limit.
@@ -1282,6 +1297,8 @@ mod scand {
         assert!(started.elapsed() < std::time::Duration::from_secs(2), "{:?}", started.elapsed());
     }
 
+    /// The check an XFA form runs on leaving a date field tries six formats with `util.scand` and
+    /// writes the first that reads the text back as `yyyymmdd`, or reports an invalid date.
     #[test]
     fn xfa_scripts_check_typed_dates_with_util_scand() {
         // The check a form runs when a date field is left: the first of six formats that reads
diff --git a/crates/js/src/xfa.rs b/crates/js/src/xfa.rs
index 9a48ff16a..ef06e8418 100644
--- a/crates/js/src/xfa.rs
+++ b/crates/js/src/xfa.rs
@@ -1098,6 +1098,9 @@ fn xfa_resolve(ctx: &mut Context, many: bool) -> JsResult<JsValue> {
     resolver(ctx, current, many)
 }
 
+/// Defines the globals for one script on node `current`: `xfa` (`form`, `host`, `layout`, `event`,
+/// `resolveNode` and more), `event` as the same object as `xfa.event`, `app`, `console`, `util`,
+/// and the nodes in scope by name, with `_name` for each repeatable one's instance manager.
 fn install(ctx: &mut Context, event: &XfaEvent, doc: &XfaDoc, current: usize) -> JsResult<()> {
     let rw = Attribute::CONFIGURABLE;
     let form = node_object(ctx, 0)?;