Setting up variable to return function value or false

Question

I wasn't sure how to properly word the title for this question but jumping right into it,

Using the the ? : operator I setup a JavaScript variable and an if statement like so:

var test = callsAFunction ? test : false;

if I do this, will test equal whatever the function returns here? Or will it be undefined since it's "Technically" not set yet?

What I'm trying to do is set a variable to either what data it gets back and is run through a function, or set it to Boolean false so it will process through my Boolean check as an error. I figured this approach was best but I'm running into issues determining how to go about this. Below are my code snippets:

var hash = self.getHashFromMasterRecords(d[0]);

if(hash === false){
                done("Can't find master record hash", self.requestData, self);
            }

Obviously it needs to turn false as a Boolean in order to pass as an error. I want to do this without having to call the function more than 1 time for this sequence.

So if I do var hash = self.getHashFromMasterRecords(d[0]) ? hash : flase; would hash return the value? Or undefined?

Thank you ahead of time, if I need to post more info let me know.

Have you tried it? Anyway var hash = self.getHashFromMasterRecords(d[0]) || false; is probably what you are looking for — Rhumborl
– Rhumborl, Commented Dec 15, 2014 at 14:27
That's what I was thinking of but couldn't get it past the tip of my tongue. Thank you sir. — dhershman
– dhershman, Commented Dec 15, 2014 at 14:30

Mike · Accepted Answer · 2014-12-15 14:29:10Z

2

The syntax:

var a = (b) ? c : d;

Will set a equal to c if b is evaluates to something truthy, and d if b evaluates to something falsy. If I understand you correctly, you want to set a variable to the return value of a function, and if that function fails, set it to some default value. To do that, use ||:

var a = b() || 'error occurred!';

In the line above, if b() evaluates to something falsy, a will get the value of error occurred!. If b() evaluates to something truthy, it will get the return value of b().

answered Dec 15, 2014 at 14:29

Mike

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dhershman Over a year ago

Thank you for the clarification I was thinking along those lines but could not get it fully on of the tip of my tongue.

DoctorMick · Accepted Answer · 2014-12-15 14:30:17Z

1

It would be undefined. As long as your function returns a falsey value when it returns you can use the or (||) operator:

var hash = self.getHashFromMasterRecords(d[0]) ?? false;

Although, if that is a concrete example there is little value in the above code as you'd already have a falsey value by just storing the result from the function.

answered Dec 15, 2014 at 14:30

DoctorMick

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