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Frequently Asked Questions

Why does Final Form set my '' field value to undefined?

If you change a form value to '', Final Form will set the value in its state to undefined. This can be counterintutive, because '' !== undefined in javascript. The reason Final Form does this is so that pristine will be true if you start with an uninitialized form field (i.e. value === undefined), type into it (pristine is now false), and then empty the form field. In this case, pristine should return to true, but the value that the HTML DOM gives for that input is ''. If Final Form did not treat '' and undefined as the same, any field that was ever typed in would forever be dirty, no matter what the user did.

Your validation functions should also treat undefined and '' as the same. This is not too difficult since both undefined and '' are falsy in javascript. So a "required" validation rule would just be error = value ? undefined : 'Required'. If you are doing a regular expression check, your function should handle undefined as a potential value.

// WRONG ❌ - values.myField might be undefined!
if (!values.myField.match(/myexpression/)) {
  errors.myField = 'Bad user'
}

// RIGHT ✅
if (!values.myField || !values.myField.match(/myexpression/)) {
  errors.myField = 'Bad user'
}

// RIGHT ✅
if (!/myexpression/.test(values.myField)) {
  errors.myField = 'Bad user'
}

In practice, if you need a field value to conform to some specific rule, you also need it to be required, so normally your validation functions will look like:

if (!values.myField) {
  errors.myField = 'Required'
} else if (!someOtherCheck(values.myField)) {
  errors.myField = 'Not acceptable'
}

Why do my numeric keys result in an array structure?

The field name parsing code in Final Form was modeled after Lodash's _.set(). Writing a parser that detects that foo.0.bar is different from foo[0].bar is extremely complicated. Read more about field names.