我有一个按钮,点击它时应该将按钮颜色更改为红色,我这样做是通过更改状态来更新组件的类来使它.red类因此更新颜色,4s后它会变回normal.Yet不知何故它不更新和重新呈现组件.我的代码:

import React from "react";
import ReactDom from "react-dom";

const app = document.getElementById("app");
class Layout extends React.Component{
constructor(props){
    super(props);
    this.users =[
            {
                name:"user1",
                world:"88",
            },{
                name:"user12",
                world:"882",
            },{
                name:"user13",
                world:"883",
            },{
                name:"user14",
                world:"884",
            },{
                name:"user14",
                world:"884",
            },{
                name:"user15",
                world:"885",
            },{
                name:"user16",
                world:"886",
            },{
                name:"user17",
                world:"8867",
            },{
                name:"user18",
                world:"8868",
            }
    ];
    this.ulist = this.users.map((user, i) => {
        var cName = i<5 ? "active":"nonActive";
        return <div key = {i} className = {cName}><h4>{user.name}</h4><p>{user.world}</p></div>;
    });
    this.state = {
        lastUser:4,
        firstUser:0,
        errorUp:"",
        errorDown: "",
    }; …

因此,我想使用Js创建一个简单的货币转换器,我想通过创建列表与5个可用硬币(美元,欧元,GPB,澳元和日元),用户必须从第一个列表中选择他想要转换的硬币和在第二个清单上,他必须选择他想要的硬币货币.这就是我迄今所做的:

function calculate() {
  var amount = parseFloat(document.getElementById("amount").value);
  var select = document.getElementById("select");
  var select1 = document.getElementById("select1");
  var result = document.getElementById("result");

  if (select.value === "USD") && (select1.value === "USD")  {
    result.value = (amount * 1);
  }
  if (select.value === "USD") && (select1.value === "EUR")  {
    result.value = (amount * 0.88);
  }
  if (select.value === "USD") && (select1.value === "AUD")  {
    result.value = (amount * 1.37);
  }
  if (select.value === "USD") && (select1.value === "GBP")  {
    result.value = (amount * 0.69);
  } …

简短的问题

使用 Laravel 的 Eloquent 模型；如何按具有morphMany多态关系的列对查询进行排序？

较长的问题

假设我有给定的表结构：

// table: images
+ -- + ------------ + -------------- + ----- + ----------- +
| id | imageable_id | imageable_type | views | upload_date |
+ -- + ------------ + -------------- + ----- + ----------- +
| 1  | 1            | User           | 100   | 2016-16-06  |
| 2  | 3            | User           | 200   | 2016-16-06  |
+ -- + ------------ + -------------- + ----- + ----------- +

// …