Enable pattern attribute for input (+ custom date input attributes)

[colinrotherham]

We're converting a service from GOV.UK Elements to GOV.UK Frontend and noticed we can't provide custom attributes to our day/month/year fields.

Here's a pull request to add the feature.

As there's no "object merge" nunjucks filter the pattern: '[0-9]*' default attribute must be added manually when the entire attributes object is overridden from the defaults.

[36degrees]

This would fix #995, if we're happy with this as a solution.

I am a little concerned about users forgetting (or not realising) that they need to pass in the pattern attribute – would need to be careful about how we documented this.

[colinrotherham]

@36degrees Yeah I thought the same.

I added a new global called merge which combined the custom attribute over the defaults, but then realised most people's nunjucks setups would need to re-add the global.

Would have been like:

{
  autocomplete: item.autocomplete,
  attributes: merge(item.attributes, { pattern: "[0-9]*" })
}

Sadly nunjucks doesn't provide object merging.

[HughePaul]

We want to add maxlength to the fields, as this worked much better in testing. However this requires the type to not be number as you can't set a maxlength for number fields (although you can set a min and max with some success)

[malcolmhire]

This is the same way I have done it in my forked version of govuk-frontend, will be interesting if this gets accepted as I was also concerned that users might forget to add the pattern attribute.

[colinrotherham]

Rebased with latest master

[NickColley]

Hanna and I have reviewed this, we like this approach but want to consider the others too.

If we were to go with this approach we'd need to update the YAML description to be clear that setting this attributes requires you to add pattern back in.

You can see #995 (comment) for more details on what we've talked about.

[colinrotherham]

@NickColley @hannalaakso If you'd like to go with this approach, I've updated both the yaml description and changelog entry to include pattern: '[0-9]*'.

Looking forward to seeing __additionalAttributes and/or a merge global filter in future 👍

[NickColley]

I think this is a good compromise that will also unblock @malcolmhire and we can still consider a more robust developer friendly way in the future as discussed.

The work itself is clearly documented at the point of use, and implemented well with tests, thanks a lot for this Colin.

Will require a second review.

Discussion with team

[NickColley]

I've talked this through with the team and we think there's an alternative to this that avoids some of the negative aspects of this approach without making breaking changes:
#995 (comment)

[NickColley]

I've reviewed the live examples and they are outputting correctly.

And the api changes to the macros look great.

There's a lot going on here though so would appreciate another proper good look from someone on the team.

[36degrees]

Looks good to me, thanks @colinrotherham 👍

I've added one minor comment about one of the tests, but happy to merge as-is if you prefer.

[colinrotherham]

Thanks @36degrees.

I've removed that extra assertion, it was definitely already covered.

All ready to merge if you're happy 😊

[NickColley]

Going to run out of superlatives to describe these pull requests thanks so much for your help Colin.

👏