Ian McNally

Input validity in the HTML spec

June 25, 2015

Last night, while staving off insomnia, I started working on an enhancement to react-currency-masked-input.

I wanted an empty input value (i.e., when a user deletes all text from the input) to be replaced (a.k.a masked) as null, but invalid input (in this case, any non-digits were invalid) to replaced as 0.00. The solution sounds simple: check the input value before it’s replaced. But it’s not that easy! Both empty input and invalid input (input that doesn’t match the pattern attribute’s expression) get passed as an empty string.

To illustrate that, both events look like:

// the event when input cleared OR when bad input
event = {
  // the target is our input
  target: {
    value: ''

But, in swooped the HTML spec that saved me. Enter HTML ValidityState.

The <element>.validity is an object with many different states of validation, including valid, tooLong, tooShort, and the most important for me: badInput.

// an example element's ValidityState object, i.e., 
  badInput: false
  customError: false,
  patternMismatch: false,
  rangeOverflow: false,
  rangeUnderflow: false,
  stepMismatch: false,
  tooLong: false,
  tooShort: false,
  typeMismatch: false,
  valid: false,
  valueMissing: true

As the key suggests, badInput is set to true when the input does not match the pattern expression, so when I enter 'abc' into this input:

<input type="number" pattern="\d*" />

The elements validity state includes:

// the event fired on bad input on the element
event =  {
  target: {
    value: '',
    validity: {
      badInput: true // tada!
      // ...

In contrast, when I delete all my text from the above input, validity.badInput is false (and validity.valueMissing is true).

Putting it all together, if I want to return null for an empty input, but return zero for bad input, my masking function includes:

// var input = document.getElementsByTagName('input')[0]
input.addEventListener('change', function(event) {
  var replacedValue;
  if (!event.target.value) {
    replacedValue =  (event.target.validity.badInput) ? '0' : null;
  // ... do stuff with the replaced value

Thanks HTML spec!

Ian McNally

Hey, I'm Ian. I build websites and write about what I learn as I go. Follow me on Twitter.