Warning: Encountered two children with the same key

当我们提交一个空数据时，react循环，可能会出现错误警告。

Warning: Encountered two children with the same key, ``. Keys should be unique so that components maintain their identity across updates. Non-unique keys may cause children to be duplicated and/or omitted — the behavior is unsupported and could change in a future version.
in ul (at TodoList.js:33)
in TodoList (at src/index.js:5)

我在这里展示一下，我写的具体代码。

render() {
		
		return (
			<Fragment>
				<div>
					<label htmlFor='inputArea'>请输入内容</label>
					<input 
						id='inputArea'
						className='input'
						value={this.state.inputValue} 
						onChange={this.handleInputChange}
					/>
					<button onClick={this.handleBtnClick}>提交</button>
				</div>
				<ul>
					{this.getTodoItem()}
				</ul>
			</Fragment>
		);
	}

getTodoItem() {
	return this.state.list.map((item,index) => {
		return (
			<TodoItem 
				key={item}//这里我使用的是item作为key值
				content={item} 
				index={index}
				deleteItem={this.handleItemDelete}
			/>
		)
	})
}

使用item作为key值，会出现错误，所以，我将key值改成index，测试之后，未发现错误警告，所以，是key值导致这个错误警告

getTodoItem() {
	return this.state.list.map((item,index) => {
		return (
			<TodoItem 
				key={index}//这里的key值改成index
				content={item} 
				index={index}
				deleteItem={this.handleItemDelete}
			/>
		)
	})
}