While the most beginner-friendly approach to form validation in Vue.js might be through template-based forms, a much more flexible way is to validate the model instead. Model-based validation tends to be easier to understand and change for larger apps, moving the visual clutter from the template to the model and reducing it significantly. The most well-known library to accomplish this in Vue is through Vuelidate.

Installation

As usual, Vuelidate can be installed from NPM or Yarn.

# Yarn
$ yarn add vuelidate

# NPM
$ npm install vuelidate --save

Then, in your app bootstrap, enable the Vuelidate plugin.

import Vue from 'vue';
import Vuelidate from 'vuelidate';
import App from 'App.vue';

Vue.use(Vuelidate);

new Vue({
  el: '#app',
  render: h => h(App)
});

Field Validation

To validate fields, add a definition object for properties in your data model to the validations property of your component. Then import validation functions from vuelidate/lib/validations (or custom ones you create). You will then be able to bind to the data property via v-model as usual. Validation information will be stored in this.$v[propertyName].

$v’s Schema:

$v[propertyName]: {
  $dirty: boolean, // Whether or not this field has been interacted with yet.
  $invalid: boolean, // If this field is current valid or invalid, regardless of whether or not it is "dirty"
  $error: boolean, // Shortcut for $invalid && $dirty
  $pending: boolean, // Whether or not there is a pending validation (for async validations)
  $each: object // Holds all the validations for looped models.
  [YourValidationName]: boolean // The state of the validations defined on this property validations. (ie. if you have the 'required' validator, there would be a boolean 'required' property here, and so forth.)
  [Nested]: object // You can nest values in your model here as well, and the validation properties will be added to each nested field.
}

<template>
  <form>
    <input type="text" v-model="emailValue"/>
    <p v-if="!$v.emailValue.required">The email field is required!</p>
    <p v-if="!$v.emailValue.email">The input must be a proper email!</p>
  </form>
</template>

<script>
import { required, email } from 'vuelidate/lib/validations'

export default {
  data() {
    return {
      emailValue: ''
    }
  },

  validations: {
    emailValue: {
      required,
      email
    }
  }
}
</script>

Built-In Validators

Vuelidate provides these validators by default. They can be imported from vuelidate/lib/validators. If you’re using Webpack 2 or Rollup with tree shaking, any validators not used in your project will not be bundled with it.

• required() - This field cannot be empty.
• minLength(length: number) - Minimum length of the field. Works on strings and arrays.
• maxLength(length: number) - Maximum length of the field. Works on strings and arrays.
• between(min, max) - Requires a number to be between the minimum and maximum values.
• alpha() - Only allows alphabetic characters.
• alphaNum() - Only allows alphanumeric characters.
• email() - Allow only valid emails.
• sameAs(fieldName: string | getFieldName: function -> string) - Allows you to require the input to be the same as another field, specified by fieldName or a function.
• or(validators…) - Valid when at least one of the specified validators is valid.
• and(validators…) - Valid when all of the specified validators are valid.

Custom Validatiors

A custom validator in Vuelidate is simply a function that returns a boolean or a promise that resolves to a boolean.

If you want to ensure that fields contain only the word “tom”, you might write a validator like this.

export const TomValidator = (value, component) => {
  return value === 'tom';
}

...
<script>
import { TomValidator } from './tom-validator';
export default {
  data() {
    return {
      inputField: ''
    }
  },

  validators: {
    inputField: {
      TomValidator
    }
  }
}
</script>

You might want to go a step further and allow a string to be specified that must be matched. In that case, just create a higher-order function that returns the validator function, like so.

export const MatchValidator = (stringToMatch) => {
  return (value, component) => value === stringToMatch;
}

...
<script>
import { MatchValidator } from './match-validator';
export default {
  data() {
    return {
      inputField: ''
    }
  },

  validators: {
    inputField: {
      MatchValidator: MatchValidator('rupert')
      // Only allows inputField to contain 'rupert.' One might Wonder why the field even exists then...
    }
  }
}
</script>