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Understanding React Constructors with Components: How does constructor work in react?

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22nd Jun, 2023
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Understanding React Constructors with Components: How does constructor work in react?

What Is a Constructor in React?

React is a popular JavaScript library used for building user interfaces. When working with components in React, constructors play a crucial role. In React, a constructor is a unique method called when a component is created and initialised. It is typically used for two primary purposes — initialising state and binding event handlers.

The constructor method is defined within a class-based component using the constructor() syntax. It sets the component’s initial state by assigning values to the state object. It is particularly useful when defining initial data or variables used within the component.

This blog will delve into the concept of constructors in React JS and explore how they function within components. Gain a solid understanding of React constructors and their significance in building robust and dynamic applications. 

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Why Are Constructors Used in React Components?

Constructors in React components are used for two main reasons:

  • Initialisation of State: Constructors provide a means to initialise the state within a component, establishing the initial values to be used and manipulated. This feature proves especially valuable when setting defaults or retrieving data from an API.
  • Binding of Event Handlers: Constructors serve the purpose of binding event handlers to the component instance. In JavaScript, the value of “this” depends on its invocation. Binding event handlers in the constructor ensure the proper “this” context, enabling access to the component’s properties and methods. It becomes crucial when handling events and executing actions within the component. 

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Syntax of a Constructor in React

The syntax of a constructor in React follows the structure of a class method:

constructor(props) {
  // Initialisation code and state setup

You may perform any necessary initialisation tasks within the constructor and set up the component’s initial state. The super(props) line calls the parent class’s constructor (React.Component) and passes the props to it. It is required when defining a constructor in a React component.

Initialising State With the Constructor

The constructor in React provides an opportunity to perform additional setup or initialisation tasks beyond just initialising state. It can bind methods, set up event listeners, or perform other necessary setup operations.

For example, if you have a custom method called handleClick that needs to be bound to the component’s instance, you can bind it within the constructor:

constructor(props) {
  this.state = {
    // Initialise state properties
  this.handleClick = this.handleClick.bind(this);

By binding the method to the component instance, you ensure that the method retains the correct ‘this’ context when invoked.

Additionally, the constructor can also be used to subscribe to external data sources or set up event listeners that need to be established during component initialisation. It allows for greater control and customisation in the component’s setup phase. 

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Binding Event Handlers With the Constructor

Binding event handlers with the constructor in React is a common practice to ensure the correct ‘this’ context when the event is triggered. Here’s how you can bind event handlers using the constructor:

Define the constructor method within your component class:

constructor(props) {
  // Bind event handlers here
  this.handleClick = this.handleClick.bind(this);

Within the constructor, use the bind() method to bind the event handler function to the component’s instance (this):

constructor(props) {
  this.handleClick = this.handleClick.bind(this);

Implement the event handler function elsewhere in your component:

handleClick() {
  // Event handling logic goes here

By binding the event handler in the constructor, you ensure that when the event occurs and the handler is invoked, it will have the correct this context, allowing you to access the component’s properties and methods within the handler.

Setting Default Props With the Constructor

In React, default props can be set using the defaultProps property or the constructor. Here’s how you can set default props using the constructor:

Define the constructor method within your component class:

constructor(props) {
  // Set default props here
  this.state = {
    value: props.defaultValue

Access the default prop values within the constructor using the props parameter passed to the constructor. Assign the default prop values to the desired state properties or perform any necessary operations.

In the example above, the defaultValue prop is accessed through props.defaultValue and used to set the initial state value of the value. Setting default props in the constructor allows for greater customisation and flexibility in defining initial values and behaviours for your React components.

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Best Practices for Using Constructors in React

Here are some recommended practices for using constructors effectively in React components:

  • Use constructors judiciously: Constructors may not always be necessary in modern React versions. Evaluate whether you truly need a constructor or can initialise state directly or use hooks like useState().
  • Prioritise super(props) in React: Ensure that super(props) is called as the first line in the constructor to invoke the parent class constructor and pass the props.
  • Bind event handlers appropriately: Bind event handlers in the constructor to guarantee the correct ‘this’ context, or consider using arrow functions or class properties as alternatives.
  • Avoid complex operations in constructors: Focus on initialising state and binding event handlers in the constructor, avoiding excessive logic or heavy operations. Extract complex logic into separate methods or functions if needed.
  • Minimise side effects and API calls: Constructors should primarily handle initialisation tasks, so it’s best to avoid making API calls or performing significant side effects within them. Use lifecycle methods or hooks like useEffect() for such purposes.
  • Explore alternative approaches: Explore alternative patterns like class properties, arrow functions, or hooks to simplify your code and reduce reliance on constructors.

Common Mistakes With Constructors in React

Here are some common mistakes to avoid when using constructors in React JS:

  • Forgetting to call super(props) in React: Remember to include super(props) as the first line in the constructor to invoke the parent class constructor properly.
  • Neglecting to bind event handlers: Make sure to bind event handlers within the constructor or use arrow function syntax to maintain the correct ‘this’ context.
  • Overusing constructors: Constructors may not always be necessary for modern React. Consider if you need a constructor or if state initialisation and setup can be handled more easily using class properties or hooks.
  • Avoiding heavy operations in constructors: Constructors should focus on initialisation tasks rather than performing complex operations, API calls, or severe side effects. Use lifecycle methods or hooks like useEffect() for such purposes.
  • Keeping the constructor concise: Limit the logic and operations within the constructor to state initialisation and event handler binding. Extract complex logic into separate methods or functions if needed.
  • Updating state correctly: If you modify the state within the constructor, use this.setState() instead of directly mutating the state object to ensure proper state updates.

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Alternatives to Using Constructors in React

There are a few alternatives to using constructors in React JS that can simplify your code:

  • Class properties: With class properties, you can directly initialise state and bind event handlers within the class body, eliminating the need for a constructor. This approach simplifies your code by removing the extra constructor method. Class properties use a concise syntax and can improve the readability of your component code.
  • Functional components with hooks: Hooks, introduced in React 16.8, provide a more functional and streamlined approach to working with state and side effects in functional components. The useState hook lets you declare and manage the state within a functional component. The useEffect hook handles side effects, such as API calls or subscriptions. Hooks allow you to write more concise and readable code without needing class components or constructors.
  • ES6 class methods: Another alternative is to define event handlers as arrow functions within the class. Arrow functions automatically bind this to the component instance, eliminating the need for explicit binding in the constructor. This simplifies the code removing the boilerplate code of binding event handlers, thus, ensuring the event handlers always have the correct ‘this’ context.


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Constructors in React JS provide a way to initialise component state and bind event handlers. They play a vital role in the component’s lifecycle and help ensure proper functionality and data handling. By grasping the concept of constructors and their usage, you will be better equipped to build efficient and maintainable React applications. 

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Frequently Asked Questions



Pavan Vadapalli

Blog Author
Director of Engineering @ upGrad. Motivated to leverage technology to solve problems. Seasoned leader for startups and fast moving orgs. Working on solving problems of scale and long term technology strategy.

Frequently Asked Questions (FAQs)

1How do constructors function in React?

Constructors in React function as unique methods that are called during the creation and initialisation of components, allowing for state initialisation and event handlers' binding.

2Do React components require constructors?

React components do not necessarily require constructors, as state can be initialised directly within the component class or by using state hooks such as useState().

3Why are component constructors called only once in React JS?

Component constructors in React JS are called only once during the initial creation of the component. This ensures that the initialisation logic and state setup occur just once in the component's lifecycle.