NULL if statement excecuted wrong in node.js

Question

I'm using Node.js, I've made a function that looks like this:

function makeTitleWith(name) {
   if (name === null) {
         return "Hello, welcome to my app!"
   } else {
         return "Hello, welcome " + name + "!"
   }
}

And when I call it with a variable thats null, it returns "Hello, welcome null!". I'm new pretty new to JS, and this is driving me insane. I've also tried == instead of === but it makes no difference.

@T.J.Crowder Sorry, it executes the other block. Not the === null one. I fixed my question now. — user109321948492842303
– user109321948492842303, Commented Jul 23, 2019 at 8:30
Then please add a minimal reproducible example that shows the actual problem. — Andreas
– Andreas, Commented Jul 23, 2019 at 8:32
@andymccullough Yes I have also tried checking for undefined, but it didnt change anything. And if the value im passing to the function would be undefined, it would log to the console as undefined instead of null right? — user109321948492842303
– user109321948492842303, Commented Jul 23, 2019 at 8:32
Can you please post here what the console.log(name) returns? — Sunny Parekh
– Sunny Parekh, Commented Jul 23, 2019 at 8:43

T.J. Crowder · Accepted Answer · 2019-07-23 08:39:51Z

2

when I call it with a variable thats null, it returns "Hello, welcome null!"

That means you're not calling it with name set to null, you're calling it with name set to "null" (or an unlikely second possibility I'll cover later). "null" is not equal to null (either == or ===).

Example:

function makeTitleWith(name) {
   if (name === null) {
         return "Hello, welcome to my app!"
   } else {
         return "Hello, welcome " + name + "!"
   }
}
console.log(makeTitleWith("null"));
// or more likely:
var n = String(null);
console.log(makeTitleWith(n));

You probably want to fix where you're calling it, since that "null" is probably the result of converting null to string.

The unlikely second possibility: You're calling it with name set to an object that, when converted to string, converts to "null", like this:

function makeTitleWith(name) {
   if (name === null) {
         return "Hello, welcome to my app!"
   } else {
         return "Hello, welcome " + name + "!"
   }
}

var n = {
    toString() {
        return "null";
    }
};
console.log(makeTitleWith(n));

I very much doubt that's what's going on.

edited Jul 23, 2019 at 8:39

answered Jul 23, 2019 at 8:31

T.J. Crowder

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6 Comments

BenM Over a year ago

"Hello, welcome " + name + "!" will coerce null into a string anyway.

T.J. Crowder Over a year ago

@BenM - It would, but it won't given the OP's code, which checks for null and takes another branch.

user109321948492842303 Over a year ago

This was the case, what would be the best way to prevent this from happening? Should i write my own toString function, which checks for the value being null and preventing it from being changed to string (and causing a disaster in my code), or is there some existing way that everyone uses?

T.J. Crowder Over a year ago

@user109321948492842303 - Do the null check before converting to string, e.g. x = y === null ? null : String(y); or similar. You could make your own toNonNullString function or similar, but if you're doing your validation early, it isn't really needed.

Dov Rine Over a year ago

As an aside, you can catch null, undefined, empty string, and false with: if(name)

|

Sagar Agrawal · Accepted Answer · 2019-07-23 09:42:20Z

0

I would recommend you can modify the above code like this (more like defensive coding) and expect/enforce the caller to send the string value as input param to the function.

if(typeof name == "string"){
    return "Hello, welcome " + name + "!";
}
return "Hello, welcome to my app!";

Also note, if the caller is calling the function with null or empty object, in both the cases, typeof operator returns object. So based on your case I don't think that the method was called with null or empty object.

answered Jul 23, 2019 at 9:42

Sagar Agrawal

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