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Formatting user's text input on-the-fly

See Demo

GitHub Ban

On March 9th, 2020, GitHub, Inc. silently banned my account (and all my libraries) without any notice for an unknown reason. I opened a support ticked but they didn't answer. Because of that, I had to move all my libraries to GitLab.


npm install input-format --save


Start with defining parse() and format() functions:

  • parse() function parses a single character from an input string (will be called for each character in the input string). After parsing each subsequent character, it returns the entire parsed value: string so far.
  • format() function formats the entire parsed value back to a stringified representation. Returns an object of shape: { value: string, template: string }.

Because "masked input" is a common use case, this library also provides parse() and format() function creators from a template:

  • templateParser() creates a parse() function from a template.
  • templateFormatter() creates a format() function from a template.

An example for parsing and formatting a US phone number:

import { templateParser, templateFormatter, parseDigit } from 'input-format'

// US phone number template
const TEMPLATE = '(xxx) xxx-xxxx'

// `parse` function parses input text characters one-by-one.
// `function parse(character, value) { return character }`
// Arguments:
//  * `character` is the currently parsed input text character.
//  * `value` is the parsed value so far.
// Returns:
//  * If it returns anything (not `undefined`) then it is appended to the `value`
// `parseDigit` is an exported helper `parse` function
// that returns `character` if it's a digit
// (a common case, e.g. phone numbers input).
// `templateParser` wrapper is a small helper
// which enhances `parse` function to limit `value` max length
// to the number of "x"-es in the template.
const parse = templateParser(TEMPLATE, parseDigit)

// `format` function formats parsed value.
// function format(value) { return { text: '(800) 555-3535', template: '(xxx) xxx-xxxx' } }
// Arguments:
//  * `value` is the parsed value to be formatted.
// Returns `{ text, template }`, where:
//  * `text` is the formatted text
//  * `template` is the template used to format the `text`
//    (can be a partial template or a full template)
// If the `value` couldn't be formatted then
// `format()` should just return `undefined`.
// `templateFormatter` helper creates a formatter based on a template.
const format = templateFormatter(TEMPLATE)

When parse() and format() functions have been defined, they should be used either in a DOM environment or in React.


import ReactInput from 'input-format/react'

const [phone, setPhone] = useState()


Phone: {phone}


import {
} from 'input-format'

const input = document.querySelector('input')

// (optional)
const optionalOnChangeListener = (value) => {
  console.log('Value has changed:', value)

input.addEventListener('change', (event) => {
  onChange(event, input, parse, format, optionalOnChangeListener)

input.addEventListener('keydown', (event) => {
  onKeyDown(event, input, parse, format, optionalOnChangeListener)

Low-level API

This is an example of using the low-level API — the exported parse() and format() functions.

import { parse, format } from 'input-format'

// Input character parser for `parse()`.
function _parse(character, value) {
  if (value.length < 10) {
    if (character >= '0' && character <= '9') {
      return character

// Output text formatter for `format()`.
function _format(value) {
  // Just as an example of a return value
  return {
    text: '(800) 555-3535',
    template: '(xxx) xxx-xxxx'

// Testing.

let value
let text = '(800) 555-3535'
let caret = 4 // before the first zero

// `parse()`.

{ value, caret } = parse(text, caret, _parse)

value === '8005553535'
caret === 2

// `format()`.

{ text, caret } = format(value, caret, _format)

value === '(800) 555-3535'
caret === 4


After cloning this repo, ensure dependencies are installed by running:

npm install

This module is written in ES6 and uses Babel for ES5 transpilation. Widely consumable JavaScript can be produced by running:

npm run build

Once npm run build has run, you may import or require() directly from node.

After developing, the full test suite can be evaluated by running:

npm test

When you're ready to test your new functionality on a real project, you can run

npm pack

It will build, test and then create a .tgz archive which you can then install in your project folder

npm install [module name with version].tar.gz




npm i input-format

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