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) { %>
    <h2><%= user.name %></h2>
<% } %>

This seems to be checking for the existence of a variable named user, and if it exists, do some stuff. Duh, right?

My question is, why in the world would Node throw a ReferenceError if the user variable doesn't exist? This renders the above example useless. What's the appropriate way to check for the existence of a variable? Am I expected to use a try/catch mechanism and grab that ReferenceError?

ReferenceError: user is not defined
    at IncomingMessage.anonymous (eval at <anonymous> (/usr/local/lib/node/.npm/ejs/0.3.1/package/lib/ejs.js:140:12))
    at IncomingMessage.<anonymous> (/usr/local/lib/node/.npm/ejs/0.3.1/package/lib/ejs.js:142:15)
    at Object.render (/usr/local/lib/node/.npm/ejs/0.3.1/package/lib/ejs.js:177:13)
    at ServerResponse.render (/usr/local/lib/node/.npm/express/1.0.7/package/lib/express/view.js:334:22)
    at Object.<anonymous> (/Users/me/Dropbox/Projects/myproject/server.js:188:9)
    at param (/usr/local/lib/node/.npm/connect/0.5.10/package/lib/connect/middleware/router.js:146:21)
    at pass (/usr/local/lib/node/.npm/connect/0.5.10/package/lib/connect/middleware/router.js:162:10)
    at /usr/local/lib/node/.npm/connect/0.5.10/package/lib/connect/middleware/router.js:152:27
    at Object.restrict (/Users/me/Dropbox/Projects/myproject/server.js:94:5)
    at param (/usr/local/lib/node/.npm/connect/0.5.10/package/lib/connect/middleware/router.js:146:21)

I understand that I could make this error go away by simply adding a "user" local variable in my server code, but the whole point here is that I want to check for the existence of such variables at runtime using your every day if/else nullcheck type pattern. An exception for a variable that doesn't exist seems ridiculous to me.

Answer 1

The same way you would do it with anything in js, typeof foo == 'undefined', or since "locals" is the name of the object containing them, you can do if (locals.foo). It's just raw js :p

Answer 2

Try prepending the variable with locals

Example: if(locals.user){}

Answer 3

<% if (locals.user) { %>

 // Your logic goes here 

<% } %>

Answer 4

To check if user is defined, you need to do that:

<% if (this.user) { %>
   here, user is defined
<% } %>

Answer 5

You can create a view helper which checks for "obj === void 0", this one is for express.js:

res.locals.get = function() {
    var args = Array.prototype.slice.call(arguments, 0);
    var path = args[0].split('.');
    var root = this;
    for (var i = 0; i < path.length; i++) {
        if(root[path[i]] === void 0) {
            return args[1]?args[1]:null;
        } else {
            root = root[path[i]];
        }
    };
    return root;
}

Then use it in the view like

<%- get('deep.nested.non.existent.value') %>  //returns: null
<%- get('deep.nested.non.existent.value', "default value") %> //returns: default value

Answer 6

I've come across the same issue using node.js with mongoose/express/ejs when making a relation between 2 collections together with mongoose's populate(), in this case admins.user_id existed but was related to an inexistant users._id.
So, couldn't find why:

if ( typeof users.user_id.name == 'undefined' ) ...

was failing with "Cannot read property 'name' of null" Then I noticed that I needed to do the checking like this:

if ( typeof users.user_id == 'undefined' ) ...

I needed to check the "container" of 'name', so it worked!
After that, this worked the same:

if ( !users.user_id ) ...  

Hope it helps.

Answer 7

Came to this page for an answer but I came up with a shorter inline syntax for it which is:

 <h2><%= user.name ? property.escrow.emailAddress : '' %></h2>

Answer 8

For your if statement you need to use typeof:

<% if (typeof user == 'object' && user) { %>

<% } %>

Answer 9

What I do is just pass a default object I call 'data' = '' and pass it to all my ejs templates. If you need to pass real data to ejs template, add them as property of the 'data' object.

This way, 'data' object is always defined and you never get undefined error message, even if property of 'data' exist in your ejs template but not in your node express route.

Answer 10

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:

<table>
    <tr>
        <td> Name:</td>
        <td> <%=survey_result&&survey_result.name%></td>
    </tr>
    <tr>
        <td> Dojo Location:</td>
        <td> <%=survey_result&&survey_result.dojo_loc%></td>
    </tr>
    <tr>
        <td> Favourite Language:</td>
        <td> <%=survey_result&&survey_result.fave_lang%></td>
    </tr>

I used short-circuit there also, just for safe-keeps.

When I tried it with previously suggested ways, just:

<% if (typeof survey_result !== undefined) { %>
... <!-- some js and/or html code here -->
<% } %>

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.