Input Masking

Input Masking

Author: Thomas Steiner (, @tomayac)

Last updated: 2021-06-29

The problem

By "restricting input using preformatted input masks (telephone number, birthday, Social Security number), or even auto-correcting input by appending or removing unnecessary characters" [cf. Klimczak], input masking helps users enter information in forms more correctly, and existing data can be formatted adequately.
Many implementations of input masking exist in user land, proving that there is a true need for this feature.

A beginning of a proposal

This proposal is to gauge interest in making input masking part of the language, maybe as a new interface of Intl. Here're some code snippets how this could look like in practice:

  • Globally agreed-on input mask:

    new Intl.InputMask('credit-card- 
    // "4012 8888 8888 1881"
  • Locale-aware input mask with customization options:

    new Intl.InputMask('phone-number', {
      locale: 'de-DE',
      countryCode: 'leadingPlus',
      areaCode: 'leadingZero',
      groupSize: 2,
    // "+49 (0)40 12 34 56 78"
  • Fully custom input mask with a mask function:

    new Intl.InputMask('custom', {
      maskFunction: (input) => input.toLowerCase().replaceAll(' ', '')
    }).format('No Spaces No Uppercase'));
    // "nospacesnouppercase"

Isomporhic JS™

The idea of making this part of the language would allow people to use this on the client and the server.

On the client

For example, a client-side implementation could use this as follows (note that the type="tel" of the <input> does not mean "the value is actually the input value is not automatically validated to a particular format before the form can be submitted, because formats for telephone numbers vary so much around the world" [cf. MDN]):

<label for="phone">Enter your phone number:</label>
<input type="tel" id="phone" name="phone">
const formatPhoneNumber = (value) => {
  return new Intl.InputMask('phone-number', {
     locale: 'de-DE',
     countryCode: 'leadingPlus',
     areaCode: 'leadingZero',
     groupSize: 2,

const input = document.querySelector('#phone').addEventListener('input', (e) => { = formatPhoneNumber(;

On the server

For a server-side implementation, this could look as follows:

SELECT phone FROM users_legacy;
# Returns a mix of formats from a legacy dataset:
# 00494012345678\n+494012345678\n04012345678
const formatPhoneNumber = (value) => {
  return new Intl.InputMask('phone-number', {
     locale: 'de-DE',
     countryCode: 'leadingPlus',
     areaCode: 'leadingZero',
     groupSize: 2,

// Express.js YOLO example.
app.get('/phones', (req, res) => {
  const rawPhones = getPhoneNumbersFromLegacyDB();
  const formattedPhonesHTML = => {
    return formatPhoneNumber(rawPhone);


Just leaving this to the user land is an obvious alternative. The ecosystem of input masking libraries is alive, and great implementations exist. Pulling any of those in comes at a cost for each locale that's needed, and the weight of the library itself.

Another alternative would be to add new (and smarter) input types. A recent example is the input[type=currency] proposal. The declarative burden for advanced formatting needs is not to be underestimated, though (see the above countryCode and areaCode examples). This also does not solve the issue on the server side.


Feedback on this early-stage idea is welcome. Please open a new Issue on the GitHub repo where this was crossposted.

(I'm a new user and can't add more than five links to a post, which means now that my first post was accepted by an admin that I can't make any modifications to it. A version of this post that fixes the syntax errors is on GitHub.)

I have personally faced challenges, especially with credit-card also formatting with a separator.
I would like to champion this if there are no obvious objections from others on the committee.


It's extremely important to implement such standard. But I think it requires a lot of work from many different parties. It's not a TC39 problem, IMO it's more W3C. Here are the issues with plain text-to-text formatting:

  1. It's incorrect to replace input value with the new one, you need to preserve caret position. So there should be some way to map old caret position to the new one.
  2. It would add formatting to the value itself, so formatting would be copied to buffer with the value, what seems wrong.
  3. Modifications of such text could break the formatting or lead to glitches or awkward behavior. Like adding numbers in the middle of phone number.

Also I'm sure it should support rich input to make the technology suitable for the future. Plain text-to-text conversion is good for 90s, but it wouldn't solve user-land solutions issue.

1 Like

I've developed MightyInput (demo). It's some-kind of rich input with input masking and input suggestion. It allows format values, colorize formatting and to highlight input errors right in the place where they appear. It uses hacks which I strongly don't like and would like to avoid, but it's good as an example.

Thanks, @hemanth! Super appreciate the kind offer! I'm all new to TC39, so double-value you taking the time to champion this. What would be the next step?

Thanks, @rumkin, for taking the time to reply!

  • For 1.):
    Yes, caret position needs to be taken into account. This is a bit orthogonal since it touches upon the UX of form fields. I'll dig deeper.
  • For 2.):
    This is what libraries like cleave.js seem to do, and to me this seems right.
  • For 3.):
    Yes, absolutely. Again, like 1.), I think this is super important, but orthogonal. As I wrote above, I'll dig deeper.
  • For rich text:
    I see this proposal more on the level of Intl.NumberFormat.
  • For MightyInput:
    This is a really impressive component, especially at this file size!

About MightyInput seems like I've described it wrong, and it might look like the component does what it doesn't. MightyInput only allows to replace plain text input with HTML. And it lets developers to implement formatting, colorization, error highlighting, etc.

FYI, I have presented this proposal to ECMA-402 folks, you can see the slides online.