What is the proper way to check for existence of variable in an EJS template (using ExpressJS)?

Question

On the EJS github page, there is one and only one simple example: https://github.com/visionmedia/ejs

Example

<% if (user) { %>
    
&         


        

Answer 1

I had same issue, and luckily, I found that there is also a short-circuit function in JS (I knew there was one in Ruby and some other languages).

On my server/controller side (this is from Node.js/Express):

return res.render('result', {survey_result : req.session.survey_result&&req.session.survey_result.survey }); 

See what I did there? The && which follows after a possibly undefined variable (i.e. request.session.survey_result object, which might or might not have form data) is the short-circuit notation in JS. What it does is only evaluate the part that follows the && if the part to the left of the && is NOT undefined. It also does not throw an error when the left part IS undefined. It just ignores it.

Now, in my template (remember that I passed the object req.session.survey_result_survey object to my view as survey_result ) and then I rendered fields as:

I used short-circuit there also, just for safe-keeps. 
When I tried it with previously suggested ways, just: 
<% if (typeof survey_result !== undefined) { %>
... 
<% } %>
Sometimes, it would still try to evaluate the properties within the IF statement...Maybe someone can offer an explanation as to why?
Also, I wanted to correct that undefined needs to be without the single quotes, as I saw done in previous examples. Because the condition will never evaluate to true, as you are comparing a String value 'undefined' with a datatype undefined. 

                                                        

              
                
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 Name:
        
 <%=survey_result&&survey_result.name%>
    
    

        
 Dojo Location:
        
 <%=survey_result&&survey_result.dojo_loc%>
    
    

        
 Favourite Language:
        
 <%=survey_result&&survey_result.fave_lang%>