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Stable Formatting

A TC39 proposal to bring stable Intl-inspired formatting options to ECMAScript.

Stage: 1

Presentation: Slides for Stage 1 acceptance at September 2023 TC39 meeting

Motivation

The Intl formatters and other interfaces defined by ECMA-402 are highly implementation-dependent, and liable to change over time. This means that the output of these functions should not be relied upon, as they're only intended to provide "best effort" results meant for immediate display to users.

However, this has not prevented users from relying on string formatters having an exact output shape, in particular with date formatting using the en-US locale. Recent examples are available from June 2022 and November 2022. Learn more in this presentation from June 2023 TC39-TG2.

Separately, sometimes it is desirable to format values for an international audience, or for other reasons use formats that are not tied to a specific locale. The Intl formatters do not currently support this well. For example, the top StackOverflow suggestion for how to format a date using ISO-8601 formatting is to use Swedish as the locale.

Possible Solutions

It's entirely possible for a solution to this to be found in ECMA-262 outside Intl. Two possible approaches are presented: one that extends the Intl formatters to support non-internationalization usage for the desired formatting, and another that's a purely ECMA-262 solution.

Add a new "null" locale

Define in ECMA-402 the behaviour of each of the formatters for the zxx null locale. This locale identifier (which stands for for "no linguistic content; not applicable") is a valid BCP 47 primary language tag defined in ISO 639.2 but its behaviour is not otherwise well defined.

For ease of use, Intl formatters would accept null as an alias for the canonical "zxx" identifier.

Wherever possible, the zxx locale would use well-defined standardized behaviour, such as using ISO-8601 for date formatting.

The "localized" output for zxx would avoid including actually localized text in its output, such as fully written-out unit names or the names of months.

Intl.Collator.supportedLocalesOf(null) → ['zxx']

new Intl.DateTimeFormat('zxx').format(new Date()) === '2023-09-01'

(12345.67).toLocaleString(null) === '12345.67'

Add options to ECMA-262 formatters

Change

  • Date.prototype.toString() 21.4.4.41
  • Number.prototype.toString([radix]) 21.1.3.6
  • BigInt.prototype.toString([radix]) 21.2.3.3

to include a new optional options argument:

  • Date.prototype.toString([options])
  • Number.prototype.toString([radix] [, options])
  • BigInt.prototype.toString([radix] [, options])

and specify how these three functions should read the options and create the formatted result string differently

The options read and respected by Date.prototype.toString will be only a subset of what the toLocaleString method accepts. For example, it will NOT read "localeMatcher", "calendar", "numberingSystem", "hour12", "dateStyle", and "timeStyle", but will read "hourCycle", "timeZone". The options listed in Table 7 could be decided by the proposal to include for reading or not.

The options read and respected by Number.prototype.toString and BigInt.prototype.toString will be only a subset of what the toLocaleString methods accept. For example, they will NOT read "localeMatcher", "numberingSystem", "style", "currency", "currencyDisplay", "currencySign", "unit", "unitDisplay", but will read other options listed in Table 12.

The Number and BigInt methods could also allow for the options argument to replace the radix argument, determining behaviour based on that argument's type.

This approach would not include any equivalent of the Intl formatters' formatToParts methods.

Alternatives

Add an "undetermined" locale

A prior version of this proposal used the "undetermined" und locale instead of zxx. This is also a valid ISO 639.2 language identifier, but it is used as the canonical root locale identifier in CLDR, which has well-defined behaviour e.g. in java.util.Locale.

The und locale is also currently supported by Safari as an alias for en-US-u-va-posix, and it's recognised by Chrome and Node.js for Intl.Locale.

Add new ECMA-262 formatting methods

Rather than modifying the existing toString methods of Date, Number and BigInt, new methods toFormattedString could be added to each of these, with an options argument as defined above.

Prior Art