fixes #190: add support for single configuration option by hardwiring some of the configuration missing from CLDR availableFormats collection.

Author: caridy

src/12.datetimeformat.js

@@ -21,6 +21,7 @@ import {
 } from "./11.numberformat.js";
 
 import {
+    generateSyntheticFormat,
     createDateTimeFormats
 } from "./cldr";
 
@@ -607,6 +608,30 @@ function BasicFormatMatcher (options, formats) {
  * not expect to see the returned format containing narrow, short or long part names
  */
 function BestFitFormatMatcher (options, formats) {
+    /** Diverging: this block implements the hack for single property configuration, eg.:
+     *
+     *      `new Intl.DateTimeFormat('en', {day: 'numeric'})`
+     *
+     * should produce a single digit with the day of the month. This is needed because
+     * CLDR `availableFormats` data structure doesn't cover these cases.
+     */
+    {
+        const optionsPropNames = [];
+        for (let property in dateTimeComponents) {
+            if (!hop.call(dateTimeComponents, property))
+                continue;
+
+            if (options['[['+ property +']]'] !== undefined) {
+                optionsPropNames.push(property);
+            }
+        }
+        if (optionsPropNames.length === 1) {
+            const bestFormat = generateSyntheticFormat(optionsPropNames[0], options['[['+ optionsPropNames[0] +']]']);
+            if (bestFormat) {
+                return bestFormat;
+            }
+        }
+    }
 
     // 1. Let removalPenalty be 120.
     let removalPenalty = 120;

src/cldr.js

@@ -9,7 +9,7 @@ let expPatternTrimmer = /^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g;
 // timezone, weekday, amoung others
 let unwantedDTCs = /[rqQASjJgwWIQq]/; // xXVO were removed from this list in favor of computing matches with timeZoneName values but printing as empty string
 
-let dtKeys = ["weekday", "era", "year", "month", "day", "weekday", "quarter"];
+let dtKeys = ["era", "year", "month", "day", "weekday", "quarter"];
 let tmKeys = ["hour", "minute", "second", "hour12", "timeZoneName"];
 
 function isDateFormatOnly(obj) {
@@ -306,3 +306,53 @@ export function createDateTimeFormats(formats) {
 
     return result;
 }
+
+// this represents the exceptions of the rule that are not covered by CLDR availableFormats
+// for single property configurations, they play no role when using multiple properties, and
+// those that are not in this table, are not exceptions or are not covered by the data we
+// provide.
+const validSyntheticProps = {
+    second: {
+        numeric:   's',
+        '2-digit': 'ss'
+    },
+    minute: {
+        numeric:   'm',
+        '2-digit': 'mm'
+    },
+    year: {
+        numeric:   'y',
+        '2-digit': 'yy'
+    },
+    day: {
+        numeric:   'd',
+        '2-digit': 'dd'
+    },
+    month: {
+        numeric:   'L',
+        '2-digit': 'LL',
+        narrow:    'LLLLL',
+        short:     'LLL',
+        long:      'LLLL'
+    },
+    weekday: {
+        narrow:  'ccccc',
+        short:   'ccc',
+        long:    'cccc'
+    }
+};
+
+export function generateSyntheticFormat(propName, propValue) {
+    if (validSyntheticProps[propName] && validSyntheticProps[propName][propValue]) {
+        return {
+            originalPattern: validSyntheticProps[propName][propValue],
+            _: {
+                [propName]: propValue
+            },
+            extendedPattern: `{${propName}}`,
+            [propName]: propValue,
+            pattern12: `{${propName}}`,
+            pattern: `{${propName}}`
+        };
+    }
+}