React formik form validation: How to initially have submit button disabled

You can add validateOnMount and set it to true

const formik = useFormik({
    initialValues: {
      validateOnMount: true, // <================= Add this
      firstName: "",
      lastName: "",
      email: ""
    },
    validationSchema: Yup.object({
      firstName: Yup.string()
        .max(15, "Must be 15 characters or less")
        .min(3, "Must be at least 3 characters")
        .required("Required"),
      lastName: Yup.string()
        .min(3, "Must be at least 3 characters")
        .max(20, "Must be 20 characters or less")
        .required("Required"),
      email: Yup.string()
        .email("Invalid email address")
        .required("Required")
    }),
    onSubmit: values => {
      handleSubmit(values);
    }
  });

Use on button

disabled={!formik.isValid}

If you want to keep the submit button disabled initially when the form loads, you can use the use the dirty : boolean property of Formik something as below:

disabled={!formik.dirty}

If you want to keep the submit button disabled until all the field values are valid then you can use isValid: boolean which works as below:

Returns true if there are no errors (i.e. the errors object is empty) and false otherwise.

So you may use it as:

disabled={!formik.isValid}

Now, if you want the submit button to be disabled until all the fields are valid and if the fields values have been changed from their initial values then you would have to use both of the above attributes in conjunction as below:

disabled={!(formik.isValid && formik.dirty)}

To have the button initially disabled just check if the touch object is empty and keep it this way until all the fields are validated with !isValid

disabled={!isValid || (Object.keys(touched).length === 0 && touched.constructor === Object)}

Formik keeps track of field values and errors however exposes them for your use, this used to be done via formProps using the render props pattern however now seems to be part of the formik variable returned by the useFormik hook.

I would recomend to start by removing the initial values to a constant. Then you need to access the formik's error object. I have not done this using the new hook syntax, however, looking at the docs I would expect "formik.errors" to work (this is exposed in formProps.errors using render props). Finally the submit buttion disabled should be a check that either formik.values is equal to the initial values OR the errors object is not empty.