Determining string length in Node JS when string may be null

Question

Determining string length in Node JS when string may be null

I'm trying to learn Node and have the function:

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

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:

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?

javascript
string
node.js
if-statement
null

Answer 1

length is an attribute, not a function. Try username.length

Answer 2

If you require a non-empty string, you can do a simple "truthy" check that will work for null, undefined, '', etc:

if (username) { ... }

With that approach, you don't even need the .length check. Also, length is a property, not a method.

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).

Your match expression is going to return one or two types of values: null or 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:

if (!client.loggedIn) {
    var matches = data.match(/\S+/);
    if (client.logMeIn(matches ? matches[0] : '',stream)) {

Regarding .length being equal to 1 in 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.

Answer 3

You're passing String(data.match(/\S+/)) as username argument, so when data.match(/\S+/) is null, you get "null" not null for username, as:

String(null) === "null"

So you need to change your condition:

if( username === null || username === "null" || username.length < 1 )