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Formik is a React/React Native package used for handling forms; it keeps track of form values, errors, and events, and handles form submissions. Formik eliminates the work involved in setting up a state for form fields, allowing you to focus more on other aspects of development.

Yup is a JavaScript schema builder for validating or parsing values. It allows you to model complex or inter-dependent validations using built-in validators or custom validations using regular expressions.

The Yup schema allows you to create validation schema/rules that values should follow. You can create a Yup validation schema by calling Yup.object().shape(). You’ll pass the schema object as a parameter with the schema rules as the value for the field keys. The schema has different datatypes: string, numbers, date, tuple, arrays, objects, booleans, and mixed. The mixed method allows you to create a schema that matches all data types or the ones you configured. Next, you’ll learn about the different Yup validation in React methods you can apply to schema types.

Formik allows easy integration with Yup for validating form values and ensures that the submitted data is error-free and matches a predetermined schema. The following sections will cover how to use Formik and Yup to validate forms in a simple React Application.

To create a React app, you’ll need to have Node.js installed. In your terminal, run the following command:

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Once you create the application, update the App.css file with the following styles:

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The App.js file has a simple sign-up form controlled by Formik. In order to validate Forms in React, inject the Yup form validation schema into the Formik object:

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In the code block above, we can see the Yup and Formik validation schema for the sign-up form on line 10. This creates a set of rules that each form field will follow.

On line 27, we have the useFormik hook with the initial values for the form state, the validation schema created with Yup and the onSubmit event for the form. Next, we linked the Formik values to the input fields. Using the First Name form field as a reference, we connected the field value from Formik to the value attribute and for the input to create a controlled component. Finally, we passed the handleSubmit function from Formik to the onSubmit event for the form.

Validation messages are tips that help the user understand which characters are valid for a specific form field, making them incredibly useful in React form validation. In addition, they serve as a guide for the user in resolving form errors. Validation messages are accessible through Formik errors for each field. We can see this example on line 61. The First Name field validation message can be accessed from formik.errors.firstName.

Yup has some built-in validators that we can implement. As seen in the code block above, we used some built-in validators in the validation schema. As discussed in the previous section, there are different datatypes for a Yup Schema. For each data type, there are different validation methods that can be chained to it. An exception is a boolean datatype that can be either true or false.

We can create our custom validation rules by using the matches() method for string schema. This accepts a regular expression and the validation message as a value:

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The schema rule above will check if a value is a string and verify that the string value is not empty. Next, it will check to make sure that the minimum length for the string characters is eight. Finally, the following three validation rules for the value are customized using a regular expression:

This validation can be applied in real-life scenarios for creating a strong password, as seen in the code sample in the App.js file:

The Formik validation package also comes with built-in components that let us control the form state and events. In this section we’ll examine how to create a dynamic form using Formik form components and Yup. First, we’ll set up a form that allows us to create a list of items:

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In the code block above, we have the Yup schema, which is an array type of strings. As we can see from the Yup validation schema, the array values are required; we cannot have an empty string as an array value.

Next, we imported the Formik component from the Formik package; this wraps the form. Since we’re working with arrays/dynamic form values, we used the FieldArray component, which helps with array manipulations. On line 32, we have the FieldArray, which is used to render each form field and button. The render props from the FieldArray had helper props passed to them. This prop allowed us to mutate the array values, as seen in the "Add New Item" button, where we used the push() method from the helpers to add new values at the end of the array. Also, the "remove" button calls the remove() method on click, with the item's index to remove passed as a parameter:

Finally, to test your dynamic form, you can use the code sample in this codesandbox.

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