Javascript 当字符串可能为空时，在 Node JS 中确定字符串长度
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/10635727/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Determining string length in Node JS when string may be null
提问by Amateur Programmer by Night
I'm trying to learn Node and have the function:
我正在尝试学习 Node 并具有以下功能：
this.logMeIn = function(username,stream) {
if (username === null || username.length() < 1) {
stream.write("Invalid username, please try again:\n\r");
return false;
} else {
....etc
and I'm passing it
我正在通过它
if (!client.loggedIn) {
if (client.logMeIn(String(data.match(/\S+/)),stream)) {
I've tried both == and ===, but I'm still getting errors as the username is not detecting that it is null, and username.length() fails on:
我已经尝试了 == 和 ===，但我仍然收到错误，因为用户名没有检测到它为空，并且 username.length() 失败：
if (username === null || username.length() < 1) {
^
TypeError: Property 'length' of object null is not a function
I'm sure that Node won't evaluate the second part of the || in the if statement when the first part is true - but I fail to understand why the first part of the if statement is evaluating to false when username is a null object. Can someone help me understand what I've done wrong?
我确信 Node 不会评估 || 的第二部分 在 if 语句中，当第一部分为真时 - 但我不明白为什么当用户名是空对象时，if 语句的第一部分评估为假。有人可以帮助我了解我做错了什么吗？
采纳答案by Engineer
You're passing String(data.match(/\S+/))as usernameargument, so when data.match(/\S+/)is null, you get "null"not nullfor username, as:
你String(data.match(/\S+/))作为username参数传递，所以当data.match(/\S+/)is 时null，你得到"null"not nullfor username，因为：
String(null) === "null"
So you need to change your condition:
所以你需要改变你的条件：
if( username === null || username === "null" || username.length < 1 )
回答by Amberlamps
lengthis an attribute, not a function. Try username.length
length是一个属性，而不是一个函数。尝试username.length
回答by Ritu
Try
尝试
if( username === null || username.toString().length < 1 )
if( username === null || username.toString().length < 1 )
I used if( username === null || username.length < 1 ) and it failed at length check.
我使用了 if( username === null || username.length < 1 ) 并且它在长度检查中失败了。
回答by jmar777
If you require a non-empty string, you can do a simple "truthy" check that will work for null, undefined, '', etc:
如果你需要一个非空字符串，你可以做一个简单的“truthy”检查，会为工作null，undefined，''，等：
if (username) { ... }
With that approach, you don't even need the .lengthcheck. Also, lengthis a property, not a method.
使用这种方法，您甚至不需要.length支票。此外，length是一个属性，而不是一个方法。
Edit: You have some funkiness going on. I think you need to start with how you're passing in your username - I don't think that your String(data.match(/\S+/))logic is behaving the way that you're expecting it to (credit to @Engineer for spotting this).
编辑：你有一些时髦。我认为你需要从你如何传递你的用户名开始——我认为你的String(data.match(/\S+/))逻辑并不像你期望的那样（归功于@Engineer 发现了这一点）。
Your match expression is going to return one or two types of values: nullor an Array. In the case that it's null, as @Engineer pointed out, you end up passing in "null"as a string, which should resultantly pass your username check later on. You should consider revising this to:
您的匹配表达式将返回一种或两种类型的值：null或Array. 在它为空的情况下，正如@Engineer 所指出的那样，您最终会"null"以字符串形式传入，因此稍后应该会通过您的用户名检查。您应该考虑将其修改为：
if (!client.loggedIn) {
var matches = data.match(/\S+/);
if (client.logMeIn(matches ? matches[0] : '',stream)) {
Regarding .lengthbeing equal to 1in all cases - that doesn't honestly make a lot of sense. I would recommend adding a lot of console.log()statements to try and figure out what's going on.
关于在所有情况下.length都等于1- 老实说，这并没有多大意义。我建议添加很多console.log()语句来尝试弄清楚发生了什么。
