Implementing a validation library isn’t all that hard. Neither is adding all of those extra features that make your validation library much better than the rest.

This article will continue implementing the validation library we started implementing in the previous part of this article series. These are the features that are going to take us from a simple proof of concept to an actual usable library!

• Part 1: The Basics
• Part 2: The Features
• Part 3: The Experience (Coming up next week)

Only Show Validation On Submit

Since we’re validating on all change events, we’re showing the user error messages way too early for a good user experience. There are a few ways we can mitigate this.

The first solution is simply providing the submitted flag as a returned property of the useValidation hook. This way, we can check whether or not the form is submitted before showing an error message. The downside here is that our “show error code” gets a bit longer:

<label>
  Username
  <br />
  <input {...getFieldProps('username')} />
  {submitted && errors.username && (
    <div className="error">{errors.username}</div>
  )}
</label>

Another approach is to provide a second set of errors (let’s call them submittedErrors), which is an empty object if submitted is false, and the errors object if it’s true. We can implement it like this:

const useValidation = config => {
  // as before
  return {
    errors: state.errors,
    submittedErrors: state.submitted ? state.errors : {},
  };
}

This way, we can simply destructure out the type of errors that we want to show. We could, of course, do this at the call site as well — but by providing it here, we’re implementing it once instead of inside all consumers.

• See CodeSandbox demo showing how submittedErrors can be used.

Show Error Messages On-Blur

A lot of people want to be shown an error once they leave a certain field. We can add support for this, by tracking which fields have been “blurred” (navigated away from), and returning an object blurredErrors, similar to the submittedErrors above.

The implementation requires us to handle a new action type — blur, which will be updating a new state object called blurred:

const initialState = {
  values: {},
  errors: {},
  blurred: {},
  submitted: false,
};

function validationReducer(state, action) {
  switch (action.type) {
    // as before
    case 'blur':
      const blurred = { 
        ...state.blurred, 
        [action.payload]: true 
      }; 
      return { ...state, blurred };
    default:
      throw new Error('Unknown action type');
  }
}

When we dispatch the blur action, we create a new property in the blurred state object with the field name as a key, indicating that that field has been blurred.

The next step is adding an onBlur prop to our getFieldProps function, that dispatches this action when applicable:

getFieldProps: fieldName => ({
  // as before
  onBlur: () => {
    dispatch({ type: 'blur', payload: fieldName });
  },
}),

Finally, we need to provide the blurredErrors from our useValidation hook so that we can show the errors only when needed.

const blurredErrors = useMemo(() => {
    const returnValue = {};
    for (let fieldName in state.errors) {
      returnValue[fieldName] = state.blurred[fieldName]
        ? state.errors[fieldName]
        : null;
    }
    return returnValue;
  }, [state.errors, state.blurred]);
return {
  // as before
  blurredErrors,
};

Here, we create a memoized function that figures out which errors to show based on whether or not the field has been blurred. We recalculate this set of errors whenever the errors or blurred objects change. You can read more about the useMemo hook in the documentation.

• See CodeSandbox demo

Time For A Tiny Refactor

Our useValidation component is now returning three sets of errors — most of which will look the same at some point in time. Instead of going down this route, we’re going to let the user specify in the config when they want the errors in their form to show up.

Our new option — showErrors — will accept either “submit” (the default), “always” or “blur”. We can add more options later, if we need to.

function getErrors(state, config) {
  if (config.showErrors === 'always') {
    return state.errors;
  }
  if (config.showErrors === 'blur') {
    return Object.entries(state.blurred)
      .filter(([, blurred]) => blurred)
      .reduce((acc, [name]) => ({ ...acc, [name]: state.errors[name] }), {});
  }
  return state.submitted ? state.errors : {};
}
const useValidation = config => {
  // as before
  const errors = useMemo(
    () => getErrors(state, config), 
    [state, config]
  );

  return {
    errors,
    // as before
  };
};

Since the error handling code started to take most of our space, we’re refactoring it out into its own function. If you don’t follow the Object.entries and .reduce stuff — that’s fine — it’s a rewrite of the for...in code in the last section.

If we required onBlur or instant validation, we could specify the showError prop in our useValidation configuration object.

const config = {
  // as before
  showErrors: 'blur',
};
const { getFormProps, getFieldProps, errors } = useValidation(config);
// errors would now only include the ones that have been blurred

• See CodeSandbox demo

Note On Assumptions

“Note that I’m now assuming that each form will want to show errors the same way (always on submit, always on blur, etc). That might be true for most applications, but probably not for all. Being aware of your assumptions is a huge part of creating your API.”

Allow For Cross-Validation

A really powerful feature of a validation library is to allow for cross-validation — that is, to base one field’s validation on another field’s value.

To allow this, we need to make our custom hook accept a function instead of an object. This function will be called with the current field values. Implementing it is actually only three lines of code!

function useValidation(config) {
  const [state, dispatch] = useReducer(...);
  if (typeof config === 'function') {
    config = config(state.values);
  }
}

To use this feature, we can simply pass a function that returns the configuration object to useValidation:

const { getFieldProps } = useValidation(fields => ({ 
  password: {
    isRequired: { message: 'Please fill out the password' },
  },
  repeatPassword: {
    isRequired: { message: 'Please fill out the password one more time' },
    isEqual: { value: fields.password, message: 'Your passwords don't match' }
  }
}));

Here, we use the value of fields.password to make sure two password fields contain the same input (which is terrible user experience, but that’s for another blog post).

• See CodeSandbox demo that doesn’t let the username and the password be the same value.

Add Some Accessibility Wins

A neat thing to do when you’re in charge of the props of a field is to add the correct aria-tags by default. This will help screen readers with explaining your form.

A very simple improvement is to add aria-invalid="true" if the field has an error. Let’s implement that:

const useValidation = config => {
  // as before
  return {
    // as before
    getFieldProps: fieldName => ({
      // as before
      'aria-invalid': String(!!errors[fieldName]),
    }),
  }
};

That’s one added line of code, and a much better user experience for screen reader users.

You might wonder about why we write String(!!state.errors[fieldName])? state.errors[fieldName] is a string, and the double negation operator gives us a boolean (and not just a truthy or falsy value). However, the aria-invalid property should be a string (it can also read “grammar” or “spelling”, in addition to “true” or “false”), so we need to coerce that boolean into its string equivalent.

There are still a few more tweaks we could do to improve accessibility, but this seems like a fair start.

Shorthand Validation Message Syntax

Most of the validators in the calidators package (and most other validators, I assume) only require an error message. Wouldn’t it be nice if we could just pass that string instead of an object with a message property containing that string?

Let’s implement that in our validateField function:

function validateField(fieldValue = '', fieldConfig, allFieldValues) {
  for (let validatorName in fieldConfig) {
    let validatorConfig = fieldConfig[validatorName];
    if (typeof validatorConfig === 'string') {
      validatorConfig = { message: validatorConfig };
    }
    const configuredValidator = validators[validatorName](validatorConfig);
    const errorMessage = configuredValidator(fieldValue);

    if (errorMessage) {
      return errorMessage;
    }
  }
  return null;
}

This way, we can rewrite our validation config like so:

const config = {
  username: {
    isRequired: 'The username is required',
    isEmail: 'The username should be a valid email address',
  },
};

Much cleaner!

Initial Field Values

Sometimes, we need to validate a form that’s already filled out. Our custom hook doesn’t support that yet — so let’s get to it!

Initial field values will be specified in the config for each field, in the property initialValue. If it’s not specified, it defaults to an empty string.

We’re going to create a function getInitialState, which will create the initial state of our reducer for us.

function getInitialState(config) {
  if (typeof config === 'function') {
    config = config({});
  }
  const initialValues = {};
  const initialBlurred = {};
  for (let fieldName in config.fields) {
    initialValues[fieldName] = config.fields[fieldName].initialValue || '';
    initialBlurred[fieldName] = false;
  }
  const initialErrors = validateFields(initialValues, config.fields);
  return {
    values: initialValues,
    errors: initialErrors,
    blurred: initialBlurred,
    submitted: false,
  };
}

We go through all fields, check if they have an initialValue property, and set the initial value accordingly. Then we run those initial values through the validators and calculate the initial errors as well. We return the initial state object, which can then be passed to our useReducer hook.

Since we’re introducing a non-validator prop into the fields config, we need to skip it when we validate our fields. To do that, we change our validateField function:

function validateField(fieldValue = '', fieldConfig) {
  const specialProps = ['initialValue'];
  for (let validatorName in fieldConfig) {
    if (specialProps.includes(validatorName)) {
      continue;
    }
    // as before
  }
}

As we keep on adding more features like this, we can add them to our specialProps array.

• See CodeSandbox demo

Summing Up

We’re well on our way to create an amazing validation library. We’ve added tons of features, and we’re pretty much-thought leaders by now.

In the next part of this series, we’re going to add all of those extras that make our validation library even trend on LinkedIn. Stay tuned!