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HOOKS or useState Hook

HOOKS or useState Hook

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Megha Khatri
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4 min read

Table of contents

Introduction of hooks?

  • it allow you to use state and other react features without written a class.

  • Hooks allow function components to have acess to state and other react features like life cycle methods.

  • Because of this, class components are generally no longer needed.

  • Hooks wew added to react in 16.8(February 16, 2019).

What is Hooks?

  • Hooks are basically functions.

  • Hooks allow us to "Hook" into react features such as state and lifecycle methods.

HOOKS RULES:-

  • Hooks can only be called inside react function component.

  • Hooks can only be called at top level of a component.

  • We should not call hooks inside loops, conditions or nested function.

  • Only call hooks from react function, we sholud not call hooks from regular javascript function.

  • hooks cannot be conditional.

  • React relies on the order in which hooks are called.

  • Hooks are not used for class based component.

CUSTOM HOOKS:-

If you have stateful logic that needs to be reused in several components, you can build your own custom hooks.

When to use a hooks

  • if you write function component, and then you want to add some state to it,previously yod do this by converting it to a class.

  • But, now you can do it by using a hook inside the existing function component.

Hooks available

There a lots of built in hooks are available.

  • useState

  • useEffect

  • useContext

  • useRef

  • useReducer

  • usseCallback

  • useMemo

  • Custom Hooks

useState hook

  • Hook state is the new way of declaring a state in react app.

  • Hook uses useState() functional component for setting and retriving state.

  • The react useState Hook allows us to track state in a function component.

  • State generally refers to data or properties that need to be tracing in an application.

    Import useState:-

  • To us ethe useState Hooks, we first need to import it into our component.

  • At the top of your component, import the useState Hooks.

  • import { useState } from "react";

  • Notice that we are destruction useState from react as it is a named export.

Intialize useState:-

  • We intialize our state by calling useState in our function component.

  • useState accepts an intial state and return two value:

    1. The current state.

    2. A function that updates the state.

  • Intialize state at the top of the function component.

  • Notice that again, we are destructuring the return valures from useState.

import { useState } from 'react'
function User() {
  const [name,setname] = useState("riya")
  let changename = () => {
    setname("siya")
  }
  return (
    <div>
      <h1>{name}</h1>
      <button onClick={changename}>click</button>
    </div>
  )
}

Update state:-

  • To update our state, we use our state updater function.

  • We should never directly update state, Ex: color="red" is not allowed.

What can state hold:-

  • The useState Hooks can be used to track of string, number, boolean,array,object and any combination of these

  • We could create multiple state hooks to track individual values.

function car(){
    const [brand,setBrand] = useState("Ford");
    const [model,setModel] = useState("Mustang");
    const [color,setcolor] = useState("white");
}

Object in useState:-

function User() {
  const obj = {
    name:"Megha",
    age:23,
    isYoung:true
  }
  const [person,setPerson] =useState(obj);
  const changeState = () => {
    setPerson({
      name:"Megha Khatri",
      age:12,
      isYoung:false
    })
  }
  return (
    <div>
      <h1>{person.name}</h1>
      <h1>{person.age}</h1>
      <h1>{person.isYoung.toString()}</h1>
      <button onClick={changeState}>change state</button>
    </div>
  )
}

Update only one state:-

  • When state is updated, the entire state gets overwritten.

  • when if we only want to update the name of a person?

  • if we only called setPerson({name:"Megha Khatri"}), this would remove the age,height and isYoung from our State.

  • We can use the JS spread operator(...) to help us.

function User() {
  const obj = {
    name:"Megha",
    age:23,
    isYoung:true
  }

  const [person,setPerson] =useState(obj);
  const changeState = () => {
    setPerson(previousState => {
        return {
                ...previousState,
                name:"Megha Khatri"
                }
    });

  return (
    <div>
      <h1>{person.name}</h1>
      <h1>{person.age}</h1>
      <h1>{person.isYoung.toString()}</h1>
      <button onClick={changeState}>change state</button>
    </div>
  )
}

Spread Operator:-

The JS spread operator (...) allows us to quickly copy all or part of an existing array or object into another array or object

Updating object in state:-

  • Because we need the current value of state, we pass a function into our setPerson function. This function receives the previous value.

  • We then return an object, spreading the previousState and overwritten only the name.

useState with Array:-

function User() {
  let num=[1,2,3,4,5,6];
  let [number,setNumber]= useState(num);
  const changeList= () =>{
    setNumber([11,22,33,44,55,66]);
  }
  return (
    <div>
      <ul>
        {number.map((n, i) => <li key={i}>{n}</li>)}
      </ul>
      <button onClick={changeList}>Change</button>
    </div>
  )
}

Add a Randdom number in array:-


function User() {
    let num=[1,2,3,4,5,6];
    let [number,setNumber]= useState(num);
    const changeList= () =>{
      setNumber(priviousState =>{
        return [
          ...priviousState,
          Math.floor(Math.random()*50)
        ]
      });

    }
    return (
      <div>
        <ul>
          {number.map((n, i) => <li key={i}>{n}</li>)}
        </ul>
        <button onClick={changeList}>Change</button>
      </div>
    )
  }

HAPPY LEARNING...

react js