In this tutorial, I'm going to give you a step-by-step guide on understanding the main concepts of React which introduce the concept of state and lifecycle in a React component.

State is Memory of a Component

Let's have a brief on what State is.

As a result of an interaction, components frequently need to update what's on the screen. Typing into the form updates the input box, clicking "next" on an image carousel changes the picture displayed and clicking "purchase" adds a product to the shopping cart. Components must "remember" information such as the current input value, the current picture, and the shopping cart. This type of component-specific memory is referred to as a state in React.

Full Tutorial Code

These are the code that you will achieve once you are done with the tutorial. My advice is, to read it until finished and start doing again using the code editor.

import React, { useState, useEffect } from "react";

function Clock() {
  const [time, setTime] = useState(new Date().toString());

  useEffect(() => {
    console.log("component mounted or updated");
    const interval = setInterval(showDate, 1000);

    return () => {
      console.log("cleanup of Interval");
      clearInterval(interval);
    };
  }, [time]);

  function showDate() {
    console.log(new Date().toString('en-GB'));
    setTime(new Date().toString());
  }

  return <h1>{time}</h1>;
}

export default Clock;

First Step

To begin, you can start creating your own react app using the command line or can directly go to CodeSandbox if you want to skip using the command line which is faster. CodeSandbox is an online code editor and prototype tool that speeds up the creation and sharing of web apps where you can directly deploy your app without any hustle.

First, create a new folder inside the src folder named components. Then, create a JavaScript file called Clock.js which in React is the component of Clock.

Create a function Clock Component.

Export this component by typing export default Clock below the function.

function Clock() {}

export default Clock;

Let's create another function in this component, which will be the showDate() function.

After have showDate(), we'll have a <h1> tag to make it large or other tags you prefer and this <h1> will have the date of {new Date().toString()}.

return <h1>{new Date().toString()}</h1>;

Then we call this function with the setInterval(). And have the setInterval as in every one second. Let's call the function first.

function showDate() {
 return <h1>{new Date().toString()}</h1>;
}

setInterval(showDate, 1000);

And this showDate() function will be called in every one second.

Alright, and another thing going do is to have also console.log() above the return method. We console.log() the Date function to see whether its running or not in our console. Like below:

function showDate() {
 console.log(new Date().toString());
 return <h1>{new Date().toString()}</h1>;
}

setInterval(showDate, 1000);

And have to also return a JSX because every component returns a JSX.

Return a JSX below the setInterval() method.

Type return (<div> </div>) as below and let's call the showDate() function inside the <div> JSX.

function Clock() {
 return (
   <div>
     <h1>Hello from Clock component!</h1>
     <div>{showDate()}</div>
   </div>
 );
}

You can also use an empty fragments <> </> to display the JSX like this:

function Clock() {
 return (
  // empty fragment
   <>
     <h1>Hello from Clock component!</h1>
     <div>{showDate()}</div>
   </>
  // empty fragment
 );
}

If you are not seeing anything happen on your screen, go to App.js and import our Clock Component, then use the Clock Component by adding the <Clock />. Like this:

App.js

import Clock from "./components/Clock";

function App() {
  return (
    <>
      <Clock /> {*to use the Clock Component*}
      <p>Hello from App.js</p>
    </>
  );
}

export default App;

Now we will see what is actually happening when we use the Clock component. We see the time and here in the Clock component, you will see that we have a setInterval which is being called every one second.

So, if I go to the inspect element (F12 hotkey) and if I go to console, then in the console you will see that the time is getting updated every second. This is what we will achieve so far:

function Clock() {
  return (
    <div>
      <h1>Hello from Clock component!</h1>
      <div>{showDate()}</div>
    </div>
  )
}

function showDate() {
 console.log(new Date().toString());
 return <h1>{new Date().toString()}</h1>;
}
setInterval(showDate, 1000);

export default Clock;

Time not updated on the screen

But then, on the screen this time not getting updated every second. Only inside the console does it get updated every second.

And that is where we have to use the State.

A state will be a local state of a component. So, whenever a State is changed, a component will re-render and it will render the updated JSX.

So, what we can do is, we need to import a State.

What to do is in the Clock.js Component. Let's have an import React from 'react'; at the top of the Clock() function.

And in order to use a State, there is a hook called useState. To use the useState hook, you need to import the state variable from 'react'. Like below:

import React, { useState } from "react";

Alright, now let's create a constant. To use a State in a function component, you need to create a constant and you have to use an array [] like structure, and then you will have a useState(). Like this:

const [] = useState();

And here you need to give the initial value of your state => useState(initial value). So, my initial value will be the same which is the Date() function.

const [] = useState(new Date().toString())

Put inside useState(), and here I will have the time and I will have a setTime. Whenever you use a useState(), it actually gives you a time variable and it also gives a function to update that variable.

const [time, setTime] = useState(new Date().toString())

So now, we are going to change line return <div>{showDate()}</div>; and have a {time} here and save it. Then start seeing a time on the screen and this is the time.

function Clock() {
 return (
   <div>
     <h1>Hello from Clock component!</h1>
     <div>{time}</div>
   </div>
 );
}

Now, what I'm going to do is I need to update that time.

So whenever this setInterval makes a call to the showDate() in every one second instead of, returning this return <h1>{new Date().toString()}</h1>;

Next, I'm going to update my state.

To Update the State

So want to update my state, I'm going to do a setTime() and this setTime() is going to update the state.

So, let's type setTime(new Date().toString()) and save it.

So, every second my showDate() function is being called function showDate() { and whenever it is called I changed the state variable.

So, you change the state variable with the help of set function; [setTime] = ... This is a set function setTime(new Date().toString()); and as soon as I do a setTime() with a new updated time new Date().toString(), so my state useState() gets changed and whenever the states get change, the component is re-rendered and that's where we will see that the screen is now updating.

We can see now on the screen the time is running every second and also from the console.

If error occurs, make sure that the showDate() is inside the Clock() function. Like below:

import React, { useState } from "react";

function Clock() {
 const [tick, setTick] = useState(new Date().toString());
 setInterval(showDate, 1000);

 return (
   <>
     <h1>Hello from Clock component!</h1>
     <div>{tick}</div>
   </>
 );

 function showDate() {
   console.log(new Date().toString());
   setTick(new Date().toString());
 }
}

export default Clock;

This showDate() function works as updating in every one second, and we use showDate() function inside the Clock function to update the time every one second, to do that, we use state variable which is setTime(new Date().toString()). Earlier in showDate() function we put return <h6>{date}</h6>; because whenever you use a useState(), it actually gives you a time Variable and it also gives a function to update that variable which using set.

Thats why we change from return <h6>{date}</h6>; to setTime(new Date().toString()).

Now on the screen, the time is running every one second and also from the console.

import React, { useState } from "react";

function Clock() {
 const [time, setTime] = useState(new Date().toLocaleTimeString("en-GB"));
 setInterval(updateTime, 1000);

 return (
   <>
     <h2>{time}</h2>
   </>
 );

 // this function used to update the time every one second
 function updateTime() {
   console.log(new Date().toLocaleTimeString("en-GB"));
   setTime(new Date().toLocaleTimeString("en-GB"));
 }
}

export default Clock;

useState Hook

Now, what is this useState() is and what is this [time] and [setTime] is.

Let's have a state, I'm gonna have a constant and now I'm going to create a state variable of name.

So, you will give it as a name, and then whenever you want to update this name, you will update it with the help of a setName function.

const [name, setName]

For example, you can also do the same thing whenever you want to have a flag. Alright, and now you want to update this flag value so you will have a setFlag

const [flag, setFlag]

Alright, so you will use the useState() as const [flag, setFlag] = useState() and then you will set the initial value.

So, my initial value of the flag is true. so let's have a useState(true).

In this way, you will create and use the State.

Lifecycle using the useEffect

Now, what is the lifecycle?

So whenever you have a component, the component also has lifecycles.

So the lifecycles will be that you want to execute something whenever your component is mounted or whenever your component is first time rendered or you want to do something whenever your component is getting updated or whenever your component is destroyed.

💡 Mounting is the phase in which our React component mounts on the DOM (i.e., is created and inserted into the DOM). This method is called just before a component mounts on the DOM or the render method is called. After this method, the component gets mounted.

For example, the Clock component is inserted into the DOM.

We use lifecycle because we want the clock keeps updating. So, need to use the useEffect() function

So, in those cases, you will use a lifecycle.

To use a life cycle method in the function component, we make use of a useEffect hook.

import React, { useState, useEffect } from "react";

So, this way I will use an useEffect hook. And now what I'm going to do here is that I'm going to write a useEffect and this useEffect will have two arguments, useEffect(args1, args2)

The first argument will be a function or a side effect that you want to run and the second argument will be a dependency array. I will use an arrow function as the first argument.

useEffect(() => {})

And the second argument will be the dependency array. So this dependency array will depend on these State variables.

So, now what I'm going to do here is that let's have a console.log() and I'm going to have here a string of "component mounted or updated".

Alright, and remove the dependency array and save it.

 useEffect(() => {
   console.log("component mounted or updated");
 });

I'll save it and now let's go to the inspect element and I will go to the console and here I will just remove this function because we don't want and it's polluting our console. Let's comment out console.log(new Date().toString());

Alright, now I will refresh my page. And you will see that as soon as I refresh my page, My component is mounted and my state is also getting updated.

So, the state is getting updated because this state variable is getting updated.

So, if I don't want that this gets updated every time, what I can do here is that I have a dependency array []

 useEffect(() => {
   console.log("component mounted or updated");
 }, []);

And if I give it a dependency in this array of a state where able then in that case the useEffect() will run only if the state variable changes.

So, what I'm going to do is I'm going to have a flag here:

useEffect( ... ;}, [flag]);

Then, save it, and now will refresh the page.

So, you now will see that even though your time is getting changed in every one second, your useEffect() useEffect( ... ;}, [time]); is not running because your useEffect() is now dependent on another state variable which is a time.

So, that means whenever a time is changed in that case, the useEffect() will run.

So, if the flag is changing from true to false, then you will see that the useEffect will run if I change this flag to true again, then you will see that the useEffect will run.

So this way this is a lifecycle of a component, whenever a component is mounted or whenever a component is updated. you want to run some side effects.

Now, there is a scenario that whenever a component is destroyed you want to run effect of unmounting that whenever the component is unmounted you want to do something.

So, in that case, what we can do is that let's have a constant and gonna have an interval and I will remove/cut the setInterval()

So now I have the interval here and this interval depends on the time.

So, I'm gonna copy this {time} and gonna have the {time} here at useEffect dependency array:

useEffect( ... ;}, [time]);

So, you can see that every time this showDate() function is running my component is getting updated.

So, that's why we see a lot components updated on console logs.

Now, let's assume that you have a subscription to this interval.

Now, you want to clear this subscription whenever your component is unmounted or whenever your component is not visible on your screen.

So what we can do here, inside this useEffect() function is that we can have a clean function. Which will have a return and this will be an arrow function. Alright and here, inside this arrow function, you can do your cleanup.

So, let me have a console.log("cleanup of Interval") and below gonna have the clearInterval method. And let gonna clear this interval. I'm going to add the constant interval into the clearInterval(interval). Like below:

() => {
     console.log("cleanup every one second");
     clearInterval(setInterval(showDate, 1000));
   };

Now see that whenever a component is mounted. the first thing that will happen is the cleanup function will run whenever this component is mounted or whenever a component is unmounted.

useEffect(() => {
  console.log("component keep mounted or updated");

  return () => {
    console.log("cleanup of Interval");
    clearInterval(setInterval(updateTime, 1000));
  };
}, [time]);

Dependency Array

If we remove the dependency, which is [time], in the console will look like this, not update the interval.

component keep mounted or updated
cleanup of Interval
component keep mounted or updated

It is not running because the useEffect() is now dependent on another state variable which is a time.

But if we put dependency array [time], then the console will look like this:

cleanup of Interval
component keep mounted or updated
cleanup of Interval
component keep mounted or updated
cleanup of Interval
component keep mounted or updated
cleanup of Interval
component keep mounted or updated
cleanup of Interval

Then, in that case, the useEffect() will run only if the state variable changes. The second argument is will be the dependency array. So, this dependency array will depend on these State variables.

So, this is how you will do the State and the lifecycle in the React components.

CodeSandbox

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