Add Interval.toLocaleString()

This method is like DateTime.toLocaleString() but for intervals. It
takes the same arguments. It uses the native
DateTimeFormat.formatRange() method, now well supported by browsers.

src/impl/formatter.js

@@ -108,6 +108,11 @@ export default class Formatter {
     return df.formatToParts();
   }
 
+  formatInterval(interval, opts = {}) {
+    const df = this.loc.dtFormatter(interval.start, { ...this.opts, ...opts });
+    return df.dtf.formatRange(interval.start.toJSDate(), interval.end.toJSDate());
+  }
+
   resolvedOptions(dt, opts = {}) {
     const df = this.loc.dtFormatter(dt, { ...this.opts, ...opts });
     return df.resolvedOptions();

src/interval.js

@@ -3,6 +3,8 @@ import Duration from "./duration.js";
 import Settings from "./settings.js";
 import { InvalidArgumentError, InvalidIntervalError } from "./errors.js";
 import Invalid from "./impl/invalid.js";
+import Formatter from "./impl/formatter.js";
+import * as Formats from "./impl/formats.js";
 
 const INVALID = "Invalid Interval";
 
@@ -32,7 +34,7 @@ function validateStartEnd(start, end) {
  * * **Interrogation** To analyze the Interval, use {@link Interval#count}, {@link Interval#length}, {@link Interval#hasSame}, {@link Interval#contains}, {@link Interval#isAfter}, or {@link Interval#isBefore}.
  * * **Transformation** To create other Intervals out of this one, use {@link Interval#set}, {@link Interval#splitAt}, {@link Interval#splitBy}, {@link Interval#divideEqually}, {@link Interval.merge}, {@link Interval.xor}, {@link Interval#union}, {@link Interval#intersection}, or {@link Interval#difference}.
  * * **Comparison** To compare this Interval to another one, use {@link Interval#equals}, {@link Interval#overlaps}, {@link Interval#abutsStart}, {@link Interval#abutsEnd}, {@link Interval#engulfs}
- * * **Output** To convert the Interval into other representations, see {@link Interval#toString}, {@link Interval#toISO}, {@link Interval#toISODate}, {@link Interval#toISOTime}, {@link Interval#toFormat}, and {@link Interval#toDuration}.
+ * * **Output** To convert the Interval into other representations, see {@link Interval#toString}, {@link Interval#toLocaleString}, {@link Interval#toISO}, {@link Interval#toISODate}, {@link Interval#toISOTime}, {@link Interval#toFormat}, and {@link Interval#toDuration}.
  */
 export default class Interval {
   /**
@@ -529,6 +531,30 @@ export default class Interval {
     return `[${this.s.toISO()} – ${this.e.toISO()})`;
   }
 
+  /**
+   * Returns a localized string representing this Interval. Accepts the same options as the
+   * Intl.DateTimeFormat constructor and any presets defined by Luxon, such as
+   * {@link DateTime.DATE_FULL} or {@link DateTime.TIME_SIMPLE}. The exact behavior of this method
+   * is browser-specific, but in general it will return an appropriate representation of the
+   * Interval in the assigned locale. Defaults to the system's locale if no locale has been
+   * specified.
+   * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat
+   * @param {Object} [formatOpts=DateTime.DATE_SHORT] - Either a DateTime preset or
+   * Intl.DateTimeFormat constructor options.
+   * @param {Object} opts - Options to override the configuration of the start DateTime.
+   * @example Interval.fromISO('2022-11-07T09:00Z/2022-11-08T09:00Z').toLocaleString(); //=> 11/7/2022 – 11/8/2022
+   * @example Interval.fromISO('2022-11-07T09:00Z/2022-11-08T09:00Z').toLocaleString(DateTime.DATE_FULL); //=> November 7 – 8, 2022
+   * @example Interval.fromISO('2022-11-07T09:00Z/2022-11-08T09:00Z').toLocaleString(DateTime.DATE_FULL, { locale: 'fr-FR' }); //=> 7–8 novembre 2022
+   * @example Interval.fromISO('2022-11-07T17:00Z/2022-11-07T19:00Z').toLocaleString(DateTime.TIME_SIMPLE); //=> 6:00 – 8:00 PM
+   * @example Interval.fromISO('2022-11-07T17:00Z/2022-11-07T19:00Z').toLocaleString({ weekday: 'short', month: 'short', day: '2-digit', hour: '2-digit', minute: '2-digit' }); //=> Mon, Nov 07, 6:00 – 8:00 p
+   * @return {string}
+   */
+  toLocaleString(formatOpts = Formats.DATE_SHORT, opts = {}) {
+    return this.isValid
+      ? Formatter.create(this.s.loc.clone(opts), formatOpts).formatInterval(this)
+      : INVALID;
+  }
+
   /**
    * Returns an ISO 8601-compliant string representation of this Interval.
    * @see https://en.wikipedia.org/wiki/ISO_8601#Time_intervals
@@ -564,10 +590,14 @@ export default class Interval {
   }
 
   /**
-   * Returns a string representation of this Interval formatted according to the specified format string.
-   * @param {string} dateFormat - the format string. This string formats the start and end time. See {@link DateTime#toFormat} for details.
-   * @param {Object} opts - options
-   * @param {string} [opts.separator =  ' – '] - a separator to place between the start and end representations
+   * Returns a string representation of this Interval formatted according to the specified format
+   * string. **You may not want this.** See {@link Interval#toLocaleString} for a more flexible
+   * formatting tool.
+   * @param {string} dateFormat - The format string. This string formats the start and end time.
+   * See {@link DateTime#toFormat} for details.
+   * @param {Object} opts - Options.
+   * @param {string} [opts.separator =  ' – '] - A separator to place between the start and end
+   * representations.
    * @return {string}
    */
   toFormat(dateFormat, { separator = " – " } = {}) {