Understanding React `setState`

Originally published by Kingsley Silas at https://css-tricks.com

React components can, and often do, have state. State can be anything, but think of things like whether a user is logged in or not and displaying the correct username based on which account is active. Or an array of blog posts. Or if a modal is open or not and which tab within it is active.

React components with state render UI based on that state. When the state of components changes, so does the component UI.

That makes understanding when and how to change the state of your component important. At the end of this tutorial, you should know how setState works, and be able to avoid common pitfalls that many of us hit when when learning React.

Workings of `setState()`

setState() is the only legitimate way to update state after the initial state setup. Let’s say we have a search component and want to display the search term a user submits.

Here’s the setup:

import React, { Component } from 'react'
 
class Search extends Component {
  constructor(props) {
    super(props)
 
    state = {
      searchTerm: ‘’
    }
  }
}

We’re passing an empty string as a value and, to update the state of searchTerm, we have to call setState().

setState({ searchTerm: event.target.value })

Here, we’re passing an object to setState(). The object contains the part of the state we want to update which, in this case, is the value of searchTerm. React takes this value and merges it into the object that needs it. It’s sort of like the Search component asks what it should use for the value of searchTerm and setState() responds with an answer.

This is basically kicking off a process that React calls reconciliation. The reconciliation process is the way React updates the DOM, by making changes to the component based on the change in state. When the request to setState() is triggered, React creates a new tree containing the reactive elements in the component (along with the updated state). This tree is used to figure out how the Search component’s UI should change in response to the state change by comparing it with the elements of the previous tree. React knows which changes to implement and will only update the parts of the DOM where necessary. This is why React is fast.

That sounds like a lot, but to sum up the flow:

We have a search component that displays a search term
That search term is currently empty
The user submits a search term
That term is captured and stored by setState as a value
Reconciliation takes place and React notices the change in value
React instructs the search component to update the value and the search term is merged in

The reconciliation process does not necessarily change the entire tree, except in a situation where the root of the tree is changed like this:

// old

<div>

  <Search />

</div>

 

// new

<span>

  <Search />

</span>

All <div> tags become <span> tags and the whole component tree will be updated as a result.

The rule of thumb is to never mutate state directly. Always use setState() to change state. Modifying state directly, like the snippet below will not cause the component to re-render.

// do not do this

this.state = {

  searchTerm: event.target.value

}

Passing a Function to `setState()`

To demonstrate this idea further, let’s create a simple counter that increments and decrements on click.

Let’s register the component and define the markup for the UI:

class App extends React.Component {

 

state = { count: 0 }

 

handleIncrement = () => {

  this.setState({ count: this.state.count + 1 })

}

 

handleDecrement = () => {

  this.setState({ count: this.state.count - 1 })

}

  render() {

    return (

      <div>

        <div>

          {this.state.count}

        </div>

        <button onClick={this.handleIncrement}>Increment by 1</button>

        <button onClick={this.handleDecrement}>Decrement by 1</button>

      </div>

    )

  }

}

At this point, the counter simply increments or decrements the count by 1 on each click.

But what if we wanted to increment or decrement by 3 instead? We could try to call setState() three times in the handleDecrement and handleIncrement functions like this:

handleIncrement = () => {

  this.setState({ count: this.state.count + 1 })

  this.setState({ count: this.state.count + 1 })

  this.setState({ count: this.state.count + 1 })

}

 

handleDecrement = () => {

  this.setState({ count: this.state.count - 1 })

  this.setState({ count: this.state.count - 1 })

  this.setState({ count: this.state.count - 1 })

}

If you are coding along at home, you might be surprised to find that doesn’t work.

The above code snippet is equivalent to:

Object.assign( 

  {},

  { count: this.state.count + 1 },

  { count: this.state.count + 1 },

  { count: this.state.count + 1 },

)

Object.assign() is used to copy data from a source object to a target object. If the data being copied from the source to the target all have same keys, like in our example, the last object wins. Here’s a simpler version of how Object.assign() works;

let count = 3

 

const object = Object.assign({},

  {count: count + 1},

  {count: count + 2},

  {count: count + 3}

);

 

console.log(object);

// output: Object { count: 6 }

So instead of the call happening three times, it happens just once. This can be fixed by passing a function to setState(). Just as you pass objects to setState(), you can also pass functions, and that is the way out of the situation above.

If we edit the handleIncrement function to look like this:

handleIncrement = () => {

  this.setState((prevState) => ({ count: prevState.count + 1 }))

  this.setState((prevState) => ({ count: prevState.count + 1 }))

  this.setState((prevState) => ({ count: prevState.count + 1 }))

}

…we can now increment count three times with one click.

In this case, instead of merging, React queues the function calls in the order they are made and updates the entire state ones it is done. This updates the state of count to 3 instead of 1.

Access Previous State Using Updater

When building React applications, there are times when you’ll want to calculate state based the component’s previous state. You cannot always trust this.state to hold the correct state immediately after calling setState(), as it is always equal to the state rendered on the screen.

Let’s go back to our counter example to see how this works. Let’s say we have a function that decrements our count by 1. This function looks like this:

changeCount = () => {

  this.setState({ count: this.state.count - 1})

}

What we want is the ability to decrement by 3. The changeCount() function is called three times in a function that handles the click event, like this.

handleDecrement = () => {

  this.changeCount()

  this.changeCount()

  this.changeCount()

}

Each time the button to decrement is clicked, the count will decrement by 1 instead of 3. This is because the this.state.count does not get updated until the component has been re-rendered. The solution is to use an updater. An updater allows you access the current state and put it to use immediately to update other items. So the changeCount() function will look like this.

changeCount = () => {

  this.setState((prevState) => {

    return { count: prevState.count - 1}

  })

}

Now we are not depending on the result of this.state. The states of count are built on each other so we are able to access the correct state which changes with each call to changeCount().

setState() should be treated asynchronously — in other words, do not always expect that the state has changed after calling setState().

Wrapping Up

When working with setState(), these are the major things you should know:

Update to a component state should be done using setState()
You can pass an object or a function to setState()
Pass a function when you can to update state multiple times
Do not depend on this.state immediately after calling setState() and make use of the updater function instead.

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In the article, we discuss the similarities between native mobile development and development using React Native. We also touch upon where they differ and how to bridge the gaps. Read on.

A brief introduction to React Native

Let’s briefly set the context first. We will briefly touch upon what React Native is and how it differs from earlier hybrid frameworks.

React Native is a popular JavaScript framework that Facebook has created. You can use this open-source framework to code natively rendering Android and iOS mobile apps. You can use it to develop web apps too.

Facebook has developed React Native based on React, its JavaScript library. The first release of React Native came in March 2015. At the time of writing this article, the latest stable release of React Native is 0.62.0, and it was released in March 2020.

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Performance: It delivers optimal performance.
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Are you wondering whether React Native is just another of those hybrid frameworks like Ionic or Cordova? It’s not! React Native is fundamentally different from these earlier hybrid frameworks.

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Near-native user experience: React Native provides several native components, and these are platform agnostic.
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Due to these factors, React Native offers many more advantages compared to those earlier hybrid frameworks. We now review them.

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What is React.js?

Facebook built and maintains React.js, an open-source JavaScript library for designing development tools. React.js is used to create single-page applications (SPAs) that can be used in conjunction with React Native to develop native cross-platform apps.

React vs React Native

React Native is a platform that uses a collection of mobile-specific components provided by the React kit, while React.js is a JavaScript-based library.
React.js and React Native have similar syntax and workflows, but their implementation is quite different.
React Native is designed to create native mobile apps that are distinct from those created in Objective-C or Java. React, on the other hand, can be used to develop web apps, hybrid and mobile & desktop applications.
React Native, in essence, takes the same conceptual UI cornerstones as standard iOS and Android apps and assembles them using React.js syntax to create a rich mobile experience.

What is the Average React Developer Salary?

In the United States, the average React developer salary is $94,205 a year, or $30-$48 per hour, This is one of the highest among JavaScript developers. The starting salary for junior React.js developers is $60,510 per year, rising to $112,480 for senior roles.

* React.js Developer Salary by Country

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In context of software developer wage rates, the United States continues to lead. In high-tech cities like San Francisco and New York, average React developer salaries will hit $98K and $114per year, overall.

However, the need for React.js and React Native developer is outpacing local labour markets. As a result, many businesses have difficulty locating and recruiting them locally.

It’s no surprise that for US and European companies looking for professional and budget engineers, offshore regions like India are becoming especially interesting. This area has a large number of app development companies, a good rate with quality, and a good pool of React.js front-end developers.

As per Linkedin, the country’s IT industry employs over a million React specialists. Furthermore, for the same or less money than hiring a React.js programmer locally, you may recruit someone with much expertise and a broader technical stack.

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Conduct thorough candidate research, including portfolios and areas of expertise.
Before you sit down with your interviewing panel, do some homework.
Examine the final outcome and hire the ideal candidate.

Why is React.js Popular?

React is a very strong framework. React.js makes use of a powerful synchronization method known as Virtual DOM, which compares the current page architecture to the expected page architecture and updates the appropriate components as long as the user input.

React is scalable. it utilises a single language, For server-client side, and mobile platform.

React is steady.React.js is completely adaptable, which means it seldom, if ever, updates the user interface. This enables legacy projects to be updated to the most new edition of React.js without having to change the codebase or make a few small changes.

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Is there a market for React.js programmers?
The need for React.js developers is rising at an unparalleled rate. React.js is currently used by over one million websites around the world. React is used by Fortune 400+ businesses and popular companies such as Facebook, Twitter, Glassdoor and Cloudflare.

Final thoughts:

As you’ve seen, locating and Hire React js Developer and Hire React Native developer is a difficult challenge. You will have less challenges selecting the correct fit for your projects if you identify growing offshore locations (e.g. India) and take into consideration the details above.

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See getting started guide, demo, docs, roadmap | Join #react-starter-kit chat room on Gitter | Visit our sponsors:

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Follow the getting started guide to download and run the project (Node.js >= 8.16.2)
Check the code recipes used in this boilerplate, or share yours

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The master branch of React Starter Kit doesn't include a Flux implementation or any other advanced integrations. Nevertheless, we have some integrations available to you in feature branches that you can use either as a reference or merge into your project:

feature/redux (PR) — isomorphic Redux by Pavel Lang (see how to integrate Redux) (based on master)
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