Support passing custom template functions

[davidjamesstone]

• is this issue currently blocking your project? (yes/no): no
• is this issue affecting a production system? (yes/no): no

Context

• node version: 12-14
• module version: 17
• environment (e.g. node, browser, native): node
• used with (e.g. hapi application, another framework, standalone, ...): hapi app
• any other relevant information:

What problem are you trying to solve?

Support custom functions in joi templates. I can see that the package formula is used and is passed an internal set of functions here: https://github.com/sideway/joi/blob/master/lib/template.js#L384

I think it would be great if we could supply some custom functions to allow string manipulations like lower and slice. This would allow re-use of the label property

joi
  .string()
  .label('First name')
  .max(50)
  .required()
  .options({
    messages: {
      'string.empty': 'Enter your {{lower(#label)}}',
      'string.max': '{{#label}} must be {{#limit}} characters or fewer'
    }
  })

Do you have a new or modified API suggestion to solve the problem?

Not sure. Maybe by creating a new customized instance of the joi module like for extensions?

Or maybe as part of the call to options

joi
  .string()
  .label('First name')
  .max(50)
  .required()
  .options({
    messages: {
      'string.empty': 'Enter your {{lower(#label)}}',
      'string.max': '{{#label}} must be {{#limit}} characters or fewer'
    },
    functions: {
      slice: (str, start, end) => str.slice(start, end),
      lower: (str) => str.toLowerCase(),
      ...
    }
  })

[yeongjet]

I think the solution that passing a function to messages() to custom message is more nimble. like: messages({label, limit} => { 'string.empty': `Enter your ${lower(label)}`} Hope joi to provide this. @Marsup

[Marsup]

Extra functions can be supported with minimal effort, it's already something we're using internally, while what you're suggesting would hardly work (multiple messages) and is way more effort imho.

[hueniverse]

@Marsup got an API in mind you like for this? The API is the hardest part especially if we want it to be namespace-safe and work with extentions.