Javascript 在类中使用If-else。

我们可以在Javascript类中使用

if-else语句吗

我在有状态的课堂上学习

在课堂上，我做了类似的事情

import React, { Component } from 'react';
import Aux from '../../HOC/Aux.js';
import Burger from '../../burger/burger.js'
import BuildControls from '../../burger/Build-Control/build-controls.js';
class BurgerBuilder extends Component {
state = {
    ingredient: {
      salad: 0,
      bacon: 0,
      cheese: 0,
      meat: 0
    },
    totalPrice: 4,
    purchaseable: false
  }

//Something 

if (this.state.totalPrice != 4) {
  let newState = {
    ...this.state.ingredient
  }
  this.setState({ingredient: newState})
}

但后来它开始抛出这个错误

Unexpected token (63:4)

  61 | 
  62 | 
> 63 | if (this.state.totalPrice != 4) {
     |     ^
  64 |   let newState = {
  65 |     ...this.state.ingredient
  66 |   }

如果我们可以使用If-else语句，那么您知道这为什么不起作用吗？
您可以将该逻辑移动到一个方法
setIngredientState
，并从
构造函数中调用它

代码：

您可以将该逻辑移动到方法
setIngredientState
，并从
构造函数中调用它

代码：

使用组件时，应在函数中使用if语句

您可以在安装组件时执行此操作，也可以在组件更新时执行此操作，如下所示：

*/ This will only occur once, at the moment the component is mounted */
componentDidMount() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

// This will happen every time a prop or state changes, which triggers a re-render of the component. This can trigger a recursive loop if you're not careful
componentDidUpdate() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

render() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

当组件呈现如下方式时，也可以设置if语句：

*/ This will only occur once, at the moment the component is mounted */
componentDidMount() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

// This will happen every time a prop or state changes, which triggers a re-render of the component. This can trigger a recursive loop if you're not careful
componentDidUpdate() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

render() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

另请注意，您不应该使用
=在Javascript中比较值时，最好使用
==在处理组件时，应该在函数中使用if语句

您可以在安装组件时执行此操作，也可以在组件更新时执行此操作，如下所示：

*/ This will only occur once, at the moment the component is mounted */
componentDidMount() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

// This will happen every time a prop or state changes, which triggers a re-render of the component. This can trigger a recursive loop if you're not careful
componentDidUpdate() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

render() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

当组件呈现如下方式时，也可以设置if语句：

*/ This will only occur once, at the moment the component is mounted */
componentDidMount() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

// This will happen every time a prop or state changes, which triggers a re-render of the component. This can trigger a recursive loop if you're not careful
componentDidUpdate() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

render() {
  if (this.state.totalPrice != 4) {
    let newState = {
      ...this.state.ingredient
    }
    this.setState({ingredient: newState})
  }
}

另请注意，您不应该使用
=在Javascript中比较值时，最好使用
==
if-else语句
应该位于
函数中或
构造函数内部
至少是这样。否。如果需要条件，请在函数中执行。
If-else语句
应在
函数中
或在
构造函数中
至少。否。如果需要条件，请在函数中执行。