小编Isi*_*sse的帖子

我正在尝试在 macos (10.14.3) 上设置 Visual Studio 2019，但是在尝试构建默认的 Web MVC 项目时，我收到此依赖项错误：

    Target _CheckForNETCoreSdkIsPreview:
    /usr/local/share/dotnet/sdk/3.0.100-preview3-010431/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.RuntimeIdentifierInference.targets(151,5): message NETSDK1057: You are using a preview version of .NET Core. See: https://aka.ms/dotnet-core-preview
Target ResolvePackageAssets:
    /usr/local/share/dotnet/sdk/3.0.100-preview3-010431/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(228,5): error MSB4018: The "ResolvePackageAssets" task failed unexpectedly.
    /usr/local/share/dotnet/sdk/3.0.100-preview3-010431/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(228,5): error MSB4018: System.TypeLoadException: Could not resolve type with token 0100003f from typeref (expected class 'NuGet.Packaging.Core.PackageDependency' in assembly 'NuGet.Packaging, Version=5.0.0.4, Culture=neutral, PublicKeyToken=31bf3856ad364e35')
    /usr/local/share/dotnet/sdk/3.0.100-preview3-010431/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(228,5): error MSB4018:   at Microsoft.NET.Build.Tasks.ResolvePackageAssets+CacheWriter.GetPlatformPackageExclusions () [0x000d7] in <9524e6fbf1724027a475d15b01fab866>:0 
    /usr/local/share/dotnet/sdk/3.0.100-preview3-010431/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(228,5): error MSB4018:   at Microsoft.NET.Build.Tasks.ResolvePackageAssets+CacheWriter..ctor (Microsoft.NET.Build.Tasks.ResolvePackageAssets task, System.IO.Stream stream) [0x0009b] in <9524e6fbf1724027a475d15b01fab866>:0 
    /usr/local/share/dotnet/sdk/3.0.100-preview3-010431/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(228,5): …

我在JavaScript的第一天,我有一个我不明白的事情.我有这段代码:

var a = [1, 2, 3, 5, 8];
var b = [];
for(var i=0; i<a.length; i++){
    b.push(a[i]);
}
console.log(b);

输出是:

[ 1, 2, 3, 5, 8 ]
=> undefined

你能解释一下undefined的含义吗？

我正在尝试学习对钩子的反应，在这一点上，我完全不知道问题是什么。该列表正确呈现，但是当我按下按钮时，我的renderCardList（）中出现“ TypeError：cards.map不是一个函数”。任何帮助将不胜感激。

当我在课堂上使用状态时它起作用

import React, { useState } from 'react';
import RentalCard from './RentalCard';

const RentalCardList = () => {
    const [cards, setCards] = useState([
        { name: 'Anne`s', price: 234 },
        { name: 'John`s', price: 83 },
        { name: 'Mary`s', price: 733 }
    ]);

    const renderCardList = () => {
        return cards.map(c => (
            <RentalCard key={Math.random()} name={c.name} price={c.price} />
        ));
    };

    const add = obj => {
        setCards(cards.push(obj));
        console.log(cards);
    };

    return (
        <div className="container">
            <section id="rentalListing">
                <h1 className="page-title">Our hot listings</h1> …