Uncertain about the unique JavaScript method of defining a function

I am exploring various ways of defining a function, but I recently stumbled upon a method that seems unfamiliar to me. As a newcomer in this field, can someone help me understand how this specific function declaration works and when it should be used? The presence of a ":" after the function name is puzzling to me!

const formSubmitHandler: SubmitHandler<IFormInputs> = () => {}

(I wonder if this has something to do with typescript)

import './App.css';
import { useForm, SubmitHandler } from 'react-hook-form'

type IFormInputs = {
  email: string,
  password: string
}

const formSubmitHandler: SubmitHandler<IFormInputs> = (data: IFormInputs) => {
  console.log('the form data is', data);
}


const App = () => {
  const { register, handleSubmit, watch, formState: {errors}} = useForm<IFormInputs>()

  console.log(errors, 'this is an error explanation');
  console.log(watch('email'), 'watch the email variable');

  return (
    <div style={{padding: '2rem', border: '1px black solid'}}>
    <form onSubmit={handleSubmit(formSubmitHandler)}>
      <input defaultValue='<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="7b1e031a160b171e550f1e080f553b1c161a121755181416">[email protected]</a>' {...register('email')}/>
      <br />
      {errors.password && <span>This field is required</span>}
      <br />
      <input {...register('password', { required: true })}/>
      <br />
      <br />
      <input type="submit" />
    </form>
    </div>
  )
}


export default App;  

Answer №1

Here is a snippet of TypeScript code:

const formSubmitHandler: SubmitHandler<IFormInputs> = (data: IFormInputs) => {
  console.log('the form data is', data);
}

This code declares the formSubmitHandler constant with the type SubmitHandler<IFormInputs>, which involves a generic type argument (IFormInputs). The SubmitHandler is assumed to define a function type, as demonstrated below:

type SubmitHandler<DataType> = (data: DataType) => void;

The value assigned to this constant is an arrow function that takes a single parameter data of type IFormInputs.

If we were to write this code in JavaScript without type annotations, it would look like this:

const formSubmitHandler = (data) => {
  console.log('the form data is', data);
}

(It's important to note that both versions of the code rely on Automatic Semicolon Insertion, but there should be a semicolon at the end of the assignment statement. This could be seen as a typo or misunderstanding rather than a deliberate choice to use ASI, especially considering the semicolon included after the console.log statement.)

Answer №2

The concept of the anonymous function, often referred to as a fat arrow function (()=>{}), bears resemblance to lambda functions found in other programming languages. One key distinction is that it does not possess a .this value.

These types of functions provide a convenient means of returning computed values without the additional complexity associated with named functions or methods.

If you wish to delve deeper into this topic, feel free to explore this resource

Fireship provides an insightful comparison between various functions and their respective purposes in a video available here.

In Typescript, it is possible to designate the assigned variable name for an anonymous function as an arrow function. Consider a scenario where we have a compute function that adds two values:

Javascript:

const add = (a, b) =>{
  return a+b
}

Typescript

const add = (a:number, b:number) =>{
  return a+b
}

Although 'add' initially lacks a static type and can be inferred, opting for strict type declarations is recommended for clarity. This involves defining a type for add as a function with parameters specified followed by the expected return type:

type Add = (a: number, b: number) => number;

const add: Add = (a, b) => {
  return a + b;
};

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