为什么我的等式比较使用=（单个等于）才能正常工作？ [英] Why doesn't my equality comparison using = (a single equals) work correctly?

问题描述

我正在尝试检查字符串是否为空，小于或等于9位数，或最多10位数。但它始终跟在 else if（str.length< = 9）。

I'm trying to check if a string is blank, less than or equal to 9 digits, or up to 10 digits. But it always follows the else if (str.length <= 9).

if (str = ''){
    console.log("The string cannot be blank");
} else if (str.length <= 9) {
    console.log("The string must be at least 9 characters long");
} else if (str.length <= 10) {
    console.log("The string is long enough.");
}

无论我投入什么，我总是得到字符串长度必须至少为9个字符。为什么？

No matter what I put in, I always get The string must be at least 9 characters long. Why?

推荐答案

= 总是分配。平等比较是 == （松散，强制尝试进行匹配的类型）或 === （无类型）强制）。

= is always assignment. Equality comparison is == (loose, coerces types to try to make a match) or === (no type coercion).

所以你想要

if (str === ''){
// -----^^^

不

// NOT THIS
if (str = ''){
// -----^

当你执行 if（str =''）时会发生什么是赋值 str =''已完成，然后是结果值（'' ）经过测试，实际上是这样的（如果我们忽略了一些细节）：

What happens when you do if (str = '') is that the assignment str = '' is done, and then the resulting value ('') is tested, effectively like this (if we ignore a couple of details):

str = '';
if (str) {

由于''是JavaScript中的 falsy 值，该检查将为false，如果（str.length< = 9） else c>步骤。从那时起， str.length 是 0 ，这就是代码所采用的路径。

Since '' is a falsy value in JavaScript, that check will be false and it goes to the else if (str.length <= 9) step. Since at that point, str.length is 0, that's the path the code takes.

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