register() in react-hook-form v7 does not accept a string but string literal. The literal value is one of the field name in the form:

interface IFormValues {
  firstName: string;
  lastName: string;
const { register, handleSubmit } = useForm<IFormValues>();

When you assign a generic type parameter to the useForm hook, the register expects to receive either firstName or lastName literal, a string or anything else will throw a type error.


According to the official example, if you are using useFieldArray and the fields are created dynamically using map, you need to assert the name as a const before passing to the register, this is because of type widening when you mix string literal with the index number:

<input key={} {...register(`test.${index}.test` as const)}  />

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