Why "he" does not write React.FC on each function?

Question

I learn typescript, and I dont understand why he write in the first Function React.FC and in other function he does not write "React.FC". Why?

exp:

...
const Login: React.FC<Props> = ({ signIn }) => {

 const Add = (id: number) => {
   ...
  };

 return (
  <div>
   <p>Hello</p>
  </div>
)
};

Why I dont write React.FC in Add function ?

Answer 1

Combining with Typescript I actually use FC only if I want to pass and extract children from props because it is the best way to type them. If we use your example:

Without children it can be like:

interface Props {
  signIn: string;
}

const Login = ({ signIn }: Props) => {

With children you can still do it the same way:

interface Props {
  signIn: string;
  children: React.ReactNode; //or something similar, there are few options
}

const Login = ({ signIn, children }: Props) => {

or you can use FC here and skip children in your Props interface:

interface Props {
  signIn: string;
}

const Login: React.FC<Props> = ({ signIn, children }) => {

Answer 2

React.FC is (and is only) used to enforce the types for a React functional component - a function that takes in props (a single argument, which is an object) and returns a JSX.Element.

Add is not a React component, but a plain function. There's no information on how it's used, but because its argument is (id: number), it's clearly not a component - so it shouldn't be typed as React.FC.

(You don't even need React.FC on components anyway - it's just a convention that some prefer)