Gaurav Singhal

How to Iterate Through a Component's Children in React Typescript

Gaurav Singhal

Aug 18, 2020
5 Min read
30,203 Views

Aug 18, 2020
5 Min read
30,203 Views

Web Development

Front End Web Development

Client-side Framework

React

Introduction

React does not come with a type-check system out of the box. When working on a complex or huge enterprise-level application, you probably want to write better code that can be unit tested, type-checked, and debugged easily. TypeScript is a compiled superset of JavaScript that can do just that.

In this guide, you will get a quick overview of how to use TypeScript with React and learn to enforce type checks inside a React component.

Creating Components With TypeScript Type Checking

Start by creating the main App component.

1class App extends React.Component<{}, AppState> {
2  //...
3}

tsx

For this example, you will fetch a list of users from an endpoint and display it on the frontend. To type check the user entity, create an interface called User in the User.interface.tsx file.

1export default interface User {
2  id: number;
3  name: string;
4  avatar: string;
5}

tsx

Next, create an AppState interface to type check the state in the App component.

Add a "allowSyntheticDefaultImports": true line in the tsconfig file to use the import statements similar to regular jsx.

1import React from "react";
2
3import UserInterface from "User.interface.tsx";
4
5interface AppState {
6  users: UserInterface[];
7}
8
9class App extends React.Component<{}, AppState> {
10  constructor(props: any) {
11    super(props);
12
13    this.state = {
14      users: [],
15    };
16  }
17
18  render() {
19    // ...
20  }
21}

tsx

To render a single user on the web page, create a User component.

1import React, { Component } from "react";
2
3import UserInterface from "../User.interface.tsx";
4
5const User = ({ id, name, avatar }: UserInterface) => (
6  <div className="user-container" data-id={id}>
7    <img src={avatar} className="user-avatar" />
8    <span className="user-name">{name}</span>
9  </div>
10);
11
12export default User;

tsx

Fetching Data And Assigning State

In the componentDidMount lifecycle method, get the users list from the backend server using the browser Fetch API. As shown below, pass the user's entity URL to the fetch method, and once the JSON response is ready to be used, set the users in the state.

1class App extends React.Component<{}, AppState> {
2  constructor(props: any) {
3    super(props);
4
5    this.state = {
6      users: [],
7    };
8  }
9
10  componentDidMount() {
11    const URL = `some-user-endpoint-for-fetching-users`; // in a real world example this would be a valid URL
12    fetch(URL)
13      .then((res) => res.json())
14      .then((data) =>
15        this.setState({
16          users: data.users,
17        })
18      );
19  }
20
21  render() {
22    // ...
23  }
24}

tsx

Using Map To Iterate Over a User's State

The map method can be used to iterate over an array. To leverage type checking, use the UserInterface to type check each child in the user array.

1// ...
2
3import User from "./components/User";
4
5class App extends React.Component<{}, AppState> {
6  constructor(props: any) {
7    super(props);
8
9    this.state = {
10      users: [],
11    };
12  }
13
14  componentDidMount() {
15    const URL = `some-user-endpoint-for-fetching-users`;
16    fetch(URL)
17      .then((res) => res.json())
18      .then((data) =>
19        this.setState({
20          users: data.users,
21        })
22      );
23  }
24
25  render() {
26    return (
27      <div className="users">
28        {this.state.users.map((user: UserInterface) => (
29          <User {...user} />
30        ))}
31      </div>
32    );
33  }
34}

tsx

Below is a shorthand version for assigning the props.

1<User {...user} />

jsx

The above syntax infers the same as follows.

1<User id={user.id} name={user.name} avatar={user.avatar} />

jsx

Iterating Through A Component's Children

Iterating through a component's children is the same as any other array. Use the map method in this case as well. To type check the child, use React.ReactElement type, along with the type for the child's props. In this case, it is UserInterface.

1import React from "react";
2import UserInterface from "User.interface.tsx";
3
4interface AppProps {
5  children: React.ReactElement<UserInterface>[];
6}
7
8class App extends React.Component<AppProps, {}> {
9  render() {
10    return this.props.children.map(
11      (child: React.ReactElement<UserInterface>) => (
12        <div>{child.props.name}</div>
13      )
14    );
15  }
16}

tsx

Conclusion

TypeScript is an excellent utility that can be used to create a type-checked codebase. This will help in debugging further issues and save a lot of time in solving runtime errors during the app development lifecycle. For iterating over an array, use the map, forEach, reduce, filter, etc. methods; each method has a different purpose. To fetch data from an external source, you can use Axios, as well.

That's it from this guide. Keep learning.

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