JSX不会将表达式中的整数评估为布尔值 [英] JSX doesn't evaluate integer in expression as boolean

问题描述

我习惯于这样编写渲染可选组件:

I'm used to write render optional Components like this:

var Foo = React.createClass({
  render: function() {
    var length = 0;
    return <div>Foo {length && <Bar />}</div>;
  }
}); 

此速记if在 if/else中提到在JSX指南中作为立即调用的函数表达式.但是，自从我最新的React更新以来，它开始渲染0而不是null.

This shorthand if is mentioned in the if/else in JSX guide as immediately-invoked function expression. However, since my latest update of React, it started to render 0 instead of null.

这是一个jsfiddle

为什么会这样?

推荐答案

&&运算符首先计算左手表达式，如果左手表达式求值虚假，则返回该值.左手表达，而无需进一步评估.

The && operator evaluates the left-hand expression first, and if the left-hand expression evaluates to something falsy it returns the value of the left-hand expression without evaluating further.

因此，(0 && "Bar")的计算结果为0，然后将其呈现为字符串.如果所有伪造的值都在渲染中被丢弃，则无法在React中打印0，例如length is { 0 }仅打印length is.

So (0 && "Bar") evaluates to 0 which is then rendered as a string. If all falsy values were discarded in the rendering then there would be no way to print a 0 in React, for example length is { 0 } would only print length is.

但是，如果将false，null和undefined用作子级，则会被React渲染器丢弃，它是

However false, null and undefined are discarded by React renderer if used as a child, and it's exactly for this use case:

length is { 0 } // length is 0
length is { NaN} // length is NaN
length is { null } // length is
length is { false } // length is
length is { undefined } // length is

您需要&&运算符的左侧表达式来返回这三个中的一个，最简单的是布尔值:

You need the left-hand expression of your && operator to return one of those three, the simplest being a boolean:

( !!length && "Bar" ) // evaluates to false, doesn't print
( (length > 0) && "Bar" ) // evaluates to false, doesn't print
( (length != 0) && "Bar" ) // evaluates to false, doesn't print
( Boolean(length) && "Bar" ) // evaluates to false, doesn't print

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