React Todo App

Continuation...

The React main site page shows a component for a Todo app. Today we discuss that code.

The Todo List

The list code is a good place to start. This is the code that creates each line item shown in the GUI using a template.

class TodoList extends React.Component {
  render() {
    return (
      <ul>
        {this.props.items.map(item => (
          <li key={item.id}>{item.text}</li>
        ))}
      </ul>
    );
  }
}

ReactDOM.render(
  <TodoApp />,
  document.getElementById('todos-example')
);

Of interest is this code:

 {
   this.props.items
    .map(item => (
      <li key={item.id}>
        {item.text}
      </li>
    ))
  }

Lets break it down...

There is a property named 'items'
We know 'items' should be an array due to the .map function call.
".map" is just a way of saying "for each item in the array send my code an 'item'.

Angular does templating a bit differently it uses html attributes for the iteration.
React, because it can; mixes JavaScript and HTML inside brackets {...}. Remember brackets indicate a code block.

For each item we see this template.

<li key={item.id}>
    {item.text}
 </li>

The code above creates a HTMLLineItem element with a "key" of "item.id" and the text it shows will be "item.text".
This means the items property must look like this:

let items = [];
items=[...items, {id:1, text:"abc"];
items=[...items,{id:2, text:"abcd"}]

//etc....

References:

The TODO App before render()

class TodoApp extends React.Component {
 constructor(props) {
  super(props);
  this.state = { 
    items: [], text: '' 
  };
 this.handleChange = 
  this.handleChange.bind(this);
 this.handleSubmit = 
  this.handleSubmit.bind(this);
}

References:

This code:

this.handleChange = 
  this.handleChange.bind(this);

Sets up a variable named handleChange for this class.
It then; tells react, that I want it to bind to 'this' which means 'this' class object. This allows the HTML code to 'see' this variable.

Reference:

Javascript bind

The TODO app render code:

render() {
 return (
 <div>
  <h3>TODO</h3>
  <TodoList items= 
   {this.state.items} 
  />
 <form onSubmit= 
  {this.handleSubmit}>
    <label htmlFor="new-todo">
       What needs to be done?
    </label>
    <input
      id="new-todo"
      onChange=
       {this.handleChange}
      value=
       {this.state.text}
    />
    <button>
     Add #
     {this.state.items.length + 1}
   </button>
 </form>
</div>
);}

...

References:

And the Event Handlers:

handleChange(e) {
 this.setState(
  {text: e.target.value }
 );
}
handleSubmit(e) {
 e.preventDefault();
if 
 (this.state.text.length === 0) {
      return;
 }
const newItem = {
  text: this.state.text,
  id: Date.now()
};
this.setState(state => (
 {items:
   state.items.concat(newItem),
   text: ''
 })

...

References:

Summary:

The TODO App has two parts
- The list template
- The user interaction
If you study this application and really get to know what it's doing, your on your way to becoming a React developer.

JWP2021 React Todo App

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John Peters

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