Category: React Answers

Sometimes, we want to render child components in a stateless React function component with TypeScript.

In this article, we’ll look at how to render child components in a stateless React function component with TypeScript.

Use Children with React Stateless Functional Component in TypeScript

We can pass in the interface or type alias into the generic type argument of React.FunctionComponent to set the type for ur props.

As long as the alias or interface has the children prop, we can use the children prop.

For instance, we can write:

const Foo: React.FunctionComponent<FooProps> = props => (
  <div>
    <p>{props.bar}</p>
    <p>{props.children}</p>
  </div>
);

FooProps has the bar and children entries, so we can reference both in our component.

React.FC is the shorthand for React.FunctionComponent .

Before React 16.8, we use the React.StatelessComponent type instead.

For instance, we can write:

const Foo: React.StatelessComponent<{}> = props => (
  <div>{props.children}</div>
);

or:

const Foo : React.StatelessComponent<FooProps> = props => (
  <div>
    <p>{props.propInMyProps}</p>
    <p>{props.children}</p>
  </div>
);

React.SFC is the shorthand for React.StatelessComponent.

Conclusion

We can pass in the interface or type alias into the generic type argument of React.FunctionComponent to set the type for ur props.

As long as the alias or interface has the children prop, we can use the children prop.

Sometimes, we want to fix the ‘Cannot Read Property ‘map’ of Undefined’ error in a React component.

In this article, we’ll look at how to fix the ‘Cannot Read Property ‘map’ of Undefined’ error in a React component.

Fix the ‘Cannot Read Property ‘map’ of Undefined’ Error in a React Component

We may get this if we try to call map on something that doesn’t have a map method.

This may happen to something that we expect to be an array, but it’s not.

Therefore, we should check if the value we’re calling map on is an array before doing anything.

For instance, we can do that by writing:

if (Array.isArray(this.props.data)) {
  const commentNodes = this.props.data.map((comment) => {
    return (
      <div>
        <h1>{comment.title}</h1>
      </div>
    );
  });
}

We called Array.isArray to check if this.props.data is an array.

If it is, then we call map to map the data prop to h1 elements.

Conclusion

We may get the ‘Cannot Read Property ‘map’ of Undefined’ error if we try to call map on something that doesn’t have a map method.

This may happen to something that we expect to be an array, but it’s not.

Therefore, we should check if the value we’re calling map on is an array before doing anything.

Sometimes, we want to disable buttons in our React components.

In this article, we’ll look at how to disable buttons in our React components.

Disable Button with React

We can disable a button with React by setting the disabled prop of the button.

For instance, we can write:

<button disabled={!this.state.value} />

We can use it in a component by writing:

class ItemForm extends React.Component {
  constructor() {
    super();
    this.state = { value: '' };
    this.onChange = this.onChange.bind(this);
    this.add = this.add.bind(this);
  }

  add() {
    this.props.addItem(this.state.value);
    this.setState({ value: '' });
  }

  onChange(e) {
    this.setState({ value: e.target.value });
  }

  render() {
    return (
      <div>
        <input
          type="text"
          value={this.state.value}
          onChange={this.onChange}
          placeholder='item name'
        />
        <button
          disabled={!this.state.value}
          onClick={this.add}
        >
          Add
        </button>
      </div>
    );
  }

We pass in the value state to let us enter the data that we want into the input field.

Then we check that in disabled prop of the button.

This way, the button is disabled if we have nothing inputted into the form field.

Conclusion

We can disable a button with React by setting the disabled prop of the button.

Sometimes, we want to make requests with file data in the request payload.

In this article, we’ll look at how to make requests with file data in the request payload.

Send Multipart Form Data with Axios in a React Component

We can send form data with the FormData constructor.

We can pass that straight into the Axios post method.

For instance, we can write:

import React from 'react'
import axios, { post } from 'axios';

class App extends React.Component {

  constructor(props) {
    super(props);
    this.state ={
      file:null
    }
    this.onFormSubmit = this.onFormSubmit.bind(this)
    this.onChange = this.onChange.bind(this)
    this.fileUpload = this.fileUpload.bind(this)
  }

  onFormSubmit(e){
    e.preventDefault()
    const url = 'http://example.com/upload';
    const formData = new FormData();
    formData.append('file', this.state.file);
    const config = {
      headers: {
        'content-type': 'multipart/form-data'
      }
    }
    post(url, formData, config);
      .then((response) => {
        console.log(response.data);
      })
  }

  onChange(e) {
    this.setState({ file: e.target.files[0 ]})
  }

  render() {
    return (
      <form onSubmit={this.onFormSubmit}>
        <h1>File Upload</h1>
        <input type="file" onChange={this.onChange} />
        <button type="submit">Upload</button>
      </form>
   )
  }
}

We have a file input, where we set the file input to the file that’s submitted in the onChange method.

We save the selected file object as the value of the file state.

Then when we click the Upload button, onFormSubmit is run.

In the method, we created a FomrData instance.

Then we append our file into the FormData instance.

We also set the header so that we indicate that we’re sending form data.

Once we did that, we proceed with our file upload.

Conclusion

We can send form data with the FormData constructor.

And we can pass that straight into the Axios post method.

React supports synthetic events in both the capturing and bubbling phases.

Therefore, our click events are propagated to the parent elements unless we stop them from doing so explicitly.

So if we have:

<root>
  <div onClick={this.handleAllClickEvents}>
    <foo>
      <bar>
        <target />
      </bar>
    </foo>
  </div>
</root>

We can listen to click events from child component with the handleAllClickEvents method.

Within it, we can write:

handleAllClickEvents(event) {
  const target = event.relatedTarget;
  const targetId = target.id;
  switch(targetId) {
    case 'foo':
      //...
    case 'bar':
      //...
  }
}

We get the click target with the event.relatedTarget property.

Then we get the click target’s ID with the id property.

Then we can check the IDs as we wish.

Difference Ways to Set the Initial State in a React Class Component

There’re multiple ways to set the initial state of a component in a class.

One way is standard and the other isn’t.

The standard way would be:

class App extends Component {
  constructor(props) {
    super(props);
    this.state = {
      name: 'james'
    }
  }

  render() {
    return  <div>{this.state.name}</div>
  }
}

We set it in the constructor with the this.state assignment.

The non-standard way is:

class App extends Component {
  state = {
    name: 'John'
  }

  render() {
    return  <div>{this.state.name}</div>
  }
}

This isn’t standard JavaScript syntax.

However, we can use it with TypeScript or a Babel plugin to transform it back to standard JavaScript.

This also sets the this.state instance variable.

Pass Props to a Component Passed as a Prop

If we want to pass props to a component that has been passed in as a prop, then we need to clone with the React.cloneElement method.

For instance, we can write:

const GrandChild = props => <div>{props.count}</div>;

class Child extends React.Component{
  constructor(){
    super();
    this.state = { count: 1 };
    this.updateCount= this.updateCount.bind(this);
  }

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

  render(){
    const Comp = this.props.comp;
    return (
      <div>
        {React.cloneElement(Comp, { count: this.state.count })}
        <button onClick={this.updateCount}>+</button>
      </div>
    );
  }
}

class Parent extends React.Component{
  render() {
    return(
      <Child comp={<GrandChild />} />
    )
  }
}

We have the GrandChild component which displays the count prop.

Then we have the Child prop to let us update the count state.

We pass that into the component that’s been passed in from the comp prop with React.cloneElement .

We got to clone it so that we return a version of the component that’s writable.

The 2nd argument has the prop name and value that we want to pass in respectively.