index.js

// Helpers

// stackoverflow.com/questions/767486/how-do-you-check-if-a-variable-is-an-array-in-javascript
var isArray = Array.isArray ? Array.isArray
  : function(array) { return array.constructor === Array; };

// stackoverflow.com/questions/646628/how-to-check-if-a-string-startswith-another-string
var startsWith = function(str, pattern) {
  return str.indexOf(pattern) === 0;
};

// stackoverflow.com/questions/1144783/replacing-all-occurrences-of-a-string-in-javascript
function replaceAll(str, find, replace) {
  return str.replace(new RegExp(escapeRegExp(find), 'g'), replace);
}

function escapeRegExp(str) {
  return str.replace(/([.*+?^=!:${}()|\[\]\/\\])/g, "\\$1");
}

// Package

function joiErrorsForForms(format, options) {
  var convert = useNothing;
  if (typeof options === 'string') { convert = useFixedMsg; }
  if (typeof options === 'object') {
    convert = isArray(options) ? useJoiMsg : useJoiType;
  }

  return function joiErrorsForFormsInner(joiErrs) {
    if (!joiErrs || !joiErrs.details) {
      return null;
    }

    var newErrs = {};
    for (var i = 0, leni = joiErrs.details.length; i < leni; i++) {
      var detail = joiErrs.details[i];
      var path = detail.path;

      switch (format) {
        case 'mongoose':
          newErrs[path] = {
            message: convert(detail, options),
            name: 'ValidatorError',
            path: path,
            type: detail.type
          };
          break;
        default:
          newErrs[path] = convert(detail, options);
      }
    }

    return newErrs;
  };
}


function useNothing(detail) {
  return detail.message;
}

function useFixedMsg(detail, options) {
  return substituteContext(detail.context, options);
}

function useJoiMsg(detail, options) {
  var detailMessage = detail.message;

  for (var i = 0, leni = options.length; i < leni; i++) {
    var option = options[i];
    var regex = option.regex;

    if (typeof regex === 'string'
        ? detailMessage.indexOf(regex) !== -1
        : detailMessage.search(regex) !== -1) {
      return substituteContext(detail.context, option.message);
    }
  }

  return detailMessage;
}

function useJoiType(detail, options) {
  var context = detail.context;
  var message;

  if (options[detail.type]) {
      message = options[detail.type](context)
  }
  
  if (!message) {
      message = detail.message;
  }

  return substituteContext(context, message);
}

function substituteContext(context, message) {
  for (var prop in context) {
    if (context.hasOwnProperty(prop)) {
      message = replaceAll(message, '${' + prop + '}', '' + context[prop]);
    }
  }

  return message;
}

module.exports = {
  form: function form(options) { return joiErrorsForForms('form', options); },
  mongoose: function mongoose(options) { return joiErrorsForForms('mongoose', options); }
};