为什么当我尝试使用 props 传递的函数时 preventDefault 停止工作？

Question

I have a component with a form and when I use this line of code the function passed by props works.

<form onSubmit={this.props.onCreateNewItem.bind(this)}>

If I try to create another function before the render that will run onCreateNewItem it stops working.

<form action="" onSubmit={this.onChangeTest}>

onChangeTest(event){
    event.preventDefault();
    this.props.onCreateNewItem();
  } 

If I run the code with out trying to call the function then the preventDefault works. If I try to run the function I get this error: "TypeError: Cannot read property 'preventDefault' of undefined." I tried to bind this and then preventDefault works but it does not run the function.

this.props.onCreateNewItem.bind(this);

I am confused because I can get the preventDefault to work without calling the function and I can get the function to work if I call it directly in the form but I can not get them both to work together.

Answer 1

You have to bind 'onChangeTest' method. Currently the this inside onChangeTest is referring to the form object not React component.

 class App extends React.Component { constructor(props) { super(props); this.onChangeTest = this.onChangeTest.bind(this); } onChangeTest(event){ event.preventDefault(); this.props.onCreateNewItem(); } render() { return ( <form action="" onSubmit={this.onChangeTest}> <button type="submit">Submit</button> </form> ); } } /// function onCreateNewItem() { console.log("onCreateNewItem called") } const rootElement = document.getElementById("root"); ReactDOM.render(<App onCreateNewItem={onCreateNewItem} />, rootElement);

 <script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script> <div id="root"></div>