预计会返回箭头值; function array-callback-return。为什么？ [英] Expected to return a value in arrow; function array-callback-return. Why?

问题描述

我遇到一些问题，理解为什么我在这段反应代码上收到编译警告

I'm having some issues understanding why I'm getting a compile warning on this piece of my react code

fetch('/users')
        .then(res => res.json())
        .then(data => {
            data.map(users => {
                console.log(users);
            });
        });

我得到的警告是预计会在箭头函数中返回一个值array-callback-return

但是我仍然从我的 / users ，它们分别打印到控制台。对象是：

However I'm still get the json object values from my /users, and they are printed to the console individually. The object is:

    {
        id: 1,
        username: "Foo"
    }, {
        id: 2,
        username: "Bar"
    }

我错过了一个返回语句，或者我错过了 map 如何在之后返回值.then（）？我不清楚为什么会出现编译警告。

Am I missing a return statement, or am I missing something with how map returns values after a .then()? I'm unclear on why the compile warning would appear at all.

推荐答案

data.map function（检查 Array.map 规范： https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map ）转换一个数组（在您的情况下，数据）到新数组。此映射由 data.map 的参数定义。 data.map 的参数（回调函数），在你的情况下是箭头函数 users => {console.log（users）;} ，必须返回一个值。通过为每个数组元素返回一个值， data.map 定义了映射。

data.map function (check Array.map specification: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map) converts one array (data in your case) to a new array. This mapping is defined by the argument of data.map. The argument of data.map (the callback function), in your case the arrow function users => {console.log(users);}, must return a value. By returning a value for each array element is how data.map defines the mapping.

但在你的情况下，回调函数不会返回任何内容，只需 console.log s。在您的情况下，您可以使用 data.forEach （ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach ）因为你不使用 Array.map 功能。

But in your case the callback function does not return anything, just console.logs. In your case you can use data.forEach (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach) as you don't use Array.map functionality.

注意：如果您决定使用 data.map 你应该有一个单数（而不是复数）名称作为回调的参数： data.map（user => {console.log（user）;}）; 现在为每个用户调用。

NOTE: If you decide to use data.map you should have a singular (rather than plural) name as the argument of callback: data.map(user => {console.log(user);}); which is now called for each user.

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