问为什么在本例中空字符串可以工作，而this.state.foo不能呢？

EN

import React, { Component } from 'react'
import classnames from 'classnames'

class TodoTextInput extends Component {
  state = {
    text: this.props.text
  }

  handleSubmit = e => {
    const text = e.target.value.trim()
    if (e.which === 13) {
      this.props.onSave(text)
      this.setState({
        text: ''
      })
    }
  }

  handleChange = e => {
    this.setState({
      text: e.target.value
    })
  }

  handleBlur = e => {
    if (!this.props.newTodo) {
      this.props.onSave(e.target.value)
    }
  }

  render() {
    return (
      <input className={
             classnames({
               edit: this.props.editing,
               'new-todo': this.props.newTodo
               })
             }
             type='text'
             autoFocus='true'
             value={this.state.text}
             onChange={this.handleChange}
             placeholder={this.props.placeholder}
             onKeyDown={this.handleSubmit} />
    )
  }
}

export default TodoTextInput

text: this.props.text || ''