[英]Unhandled Runtime Error TypeError: Cannot read properties of undefined (reading 'length')
[英]NextJS - Unhandled Runtime Error TypeError: Cannot read properties of undefined (reading 'length') when converting a class function to hook function
我正在尝试将 class function 文件转换为挂钩 function 但每当在字段中输入文本时都会收到以下错误。
未处理的运行时错误类型错误:无法读取未定义的属性(读取“长度”)
我想我已经转换了所有东西,但找不到什么 - 我错过了什么吗?
带有上述错误的新钩子代码:
import React, { useState } from "react";
function Todo(props) {
const [state, setState] = useState({ items: [], text: "" });
const handleChange = (e) => {
setState({ text: e.target.value });
};
const handleSubmit = (e) => {
e.preventDefault();
if (state.text.length === 0) {
return;
}
const newItem = {
text: state.text,
id: Date.now(),
};
setState((state) => ({
items: state.items.concat(newItem),
text: "",
}));
};
return (
<div>
<form onSubmit={handleSubmit}>
<label htmlFor="new-todo">What needs to be done?</label>
<input id="new-todo" onChange={handleChange} value={state.text} />
<button>Add #{state.items.length + 1}</button>
</form>
<TodoList items={state.items} />
</div>
);
}
function TodoList(props) {
return (
<ul>
{props.items.map((item) => (
<li key={item.id}>{item.text}</li>
))}
</ul>
);
}
export default Todo;
以前的 class 函数文件被转换为上面的钩子文件:
import React from "react";
class Todo extends React.Component {
constructor(props) {
super(props);
this.state = { items: [], text: "" };
this.handleChange = this.handleChange.bind(this);
this.handleSubmit = this.handleSubmit.bind(this);
}
render() {
return (
<div>
<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>
<TodoList items={this.state.items} />
</div>
);
}
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: "",
}));
}
}
class TodoList extends React.Component {
render() {
return (
<ul>
{this.props.items.map((item) => (
<li key={item.id}>{item.text}</li>
))}
</ul>
);
}
}
export default Todo;
非常感谢!
发生这种情况是因为在 handleChange 中你用 { text: e.target.value } 重写整个 state (状态挂钩的工作方式与基于 class 的组件状态不同)你应该试试这个:
const handleChange = (e) => {
setState((prevState) => {...prevState, text: e.target.value });
};
该问题很可能是由于此代码块造成的。
const handleChange = (e) => {
setState({ text: e.target.value });
// mutating state as this will overwite the current values with
// {text: e.target.value }
};
更好的方法是使用 rest 语法,如下所示。
const handleChange = (e) => {
setState((prev) => { return {...prev, [text]: e.target.value }});
// and now when you update text, it shouldnt overwrite your items or
//vice versa
};
如果我必须转换,那么我会这样做。
import React, { useState } from "react";
const Todo = () => {
const [items, setItems] = useState([]);
const [text, setText] = useState("");
const handleChange = (e) => {
setText(e.target.value);
};
const handleSubmit = (e) => {
e.preventDefault();
if (text.length === 0) {
return;
}
const newItem = {
text,
id: Date.now()
};
setItems((prevItems) => [...prevItems, newItem]);
// clear the input box after adding.
setText("");
};
return (
<div>
<form onSubmit={handleSubmit}>
<label htmlFor="new-todo">What needs to be done?</label>
<input id="new-todo" onChange={handleChange} value={text} />
<button>Add #{items.length + 1}</button>
</form>
<TodoList items={items} />
</div>
);
};
const TodoList = (props) => {
const { items } = props;
return (
<ul>
{items.map((item) => (
<li key={item.id}>{item.text}</li>
))}
</ul>
);
};
export default Todo;
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