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.

Estis

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

Apograph

I'm beginning to wonder if I've got a bigger problem on my hands. This throws a Reference Error too: <% alert('test'); %> – Estis

@Aashay alert is part of a browser's window object, and not available in node.js. The alternative is console.log, though I don't know if it's available in an ejs template unless you pass it in via locals. – Bragi

@Bragi In this case I'm looking specifically for EJS, so it's on the browser page. But, like TJ said, typeof foo == 'undefined' works. I learned that I could also add a simple !!foo if foo has been defined but is null or empty. – Estis

Is there a shorthand for this? Why doesn't EJS simply honor the if (foo) case? – Tolentino

is locals.foo the same as foo? – Kondon

I have the same problem. I have to check if(locals.foo) instead of just checking if(foo). – Od

if (locals.user){ } FTW – Cromorne

<% if (locals.foo) { %> Here is the variable: <%= foo %> <% } %> – Biondo

This is wrong. In ejs, if you do a type check or any check on an undefined variable, it throws an error. Every time. The above answer typeof foo == 'undefined' is incorrect. – Asymptomatic

Answer 2

Try prepending the variable with locals

Example: if(locals.user){}

Catsup

Answer 3

<% if (locals.user) { %>

 // Your logic goes here 

<% } %>

Bartram

Answer 4

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

Antimasque

not sure why this was downvoted, its rather elegant in my opinion. – Neon

What is the difference between <%= and <%-? – Steerage

@Steerage As per the documentation, under "Tags": <%= HTML escapes the value first; <%- outputs the unescaped value. So <b> would become a bold tag when using <%-, but simply the text "<b>" when using <%= – Piety

Thank you! I had no idea <%- was even a valid tag. – Steerage

I know it's a long dated post, but how can I implement this ? I tried on my own, but can't find how to properly convert this to a helper. thank you. – Grube

Answer 5

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

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

Cytoplasm

This does not work in the most up-to-date EJS library (1.0.0). Although valid Javascript, EJS will refuse to act as expected. – Dessiatine

Does not work for me. Even though I have defined foo, else clause is still evaluated. Tested with EJS 2.3.4 EDIT: using locals in place of this works – Ulloa

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.

Purpleness

Anyway, can't believe there so few official documentation on EJS... they don't update their documentation since 2010!!! – Purpleness

Answer 7

This may help somebody.

<p><%= locals?.message %></p>

or

<p><%= locals.message || '' %></p>

Sulphanilamide

Answer 8

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>

Choosy

This is dangerous to use, because if the object 'user' does not have a 'name' property, the ejs rendering will fail. – Helenhelena

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.

Denys

Answer 10

For your if statement you need to use typeof:

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

<% } %>

Ephemeral

Answer 11

Update 2022

Use the locals object

This will print the variable if it exists

<%= locals?.yourvariable %>

You can also do

This will run a condition if the variable exists

<% if(locals.yourvariable) { %>
<% } %>

Estelleesten

Answer 12

The safest and only solution that worked for me

<% if (typeof user !== 'undefined' && typeof user.name !== 'undefined') { %>
    <h2><%= user.name %></h2>
<% } %>

Or a short version

<% (typeof user !== 'undefined' && typeof user.name !== 'undefined') ? user.name : '' %>

Rebecarebecca

Answer 13

Here is the solution you've been looking for:

I just fixed it now.

Code:

<% if(typeof(username)==='undefined') username='Login' %>

Output:

If username exists, then it will be shown as it is. Otherwise username will contain the string 'Login'.

Grader

Answer 14

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.

Hertzog

Answer 15

You can use this trick :

user.name // stops code execution,if user is undefined
(scope.user||0).name // === undefined

where scope is parent object of user

Tobias

Answer 16

if you plan to use the same object often, you could use a function like this:

<% function userObj(obj){
    //replace user with what you name your object
    if(typeof user === 'object'){
        let result = user;
        if(obj){
            obj = obj.split('.');
            for(let i = 0; i < obj.length; i++){
                if(obj[i] && obj[i].trim() !== '' && result[obj[i]]){
                    result = result[obj[i]];
                }else{
                    result = false;
                    break;
                }
            }
        }
        return result;
    }
    return false;
} %>

usage:

<% if(userObj('name')){
    <h2><%= userObj('name') %></h2>
} %>

Allergic

Answer 17

I have come up with another solution.

<% if(user){ %>
   <% if(user.name){ %>
     <h1><%= user.name %></h1>
<%}}%>

I hope this helps.

Assumed

You can join both conditionals in just one evaluation: if (user && user.name) – Breathless

Answer 18

I read somewhere using locals isn't perfect so I came up with something like this, for example:

in express file:

const toRender = {
 user: false,
};
//...
if (userExists) {
 toRender.user = true;
 toRender.data = userData;
}
res.render("site", {info: toRender});

and in ejs template:

<% if (info.user) { %>
    <h2><%= info.data.name %></h2>
<% } %>

Aba

Answer 19

I wanted to use one template and conditionally display a bit more data.

I tried <? more_data = more_data ?? false ?>, which is how I prefer to do it in typescript (and other languages that support it). This would fix it in PHP, as php is pretty null-tolerable at set-time, but less so at read-time (works great).

After reading some answers here..

<? more_data = locals.more_data ?? false ?>

Jameyjami

Answer 20

UPDATE 2024

I know it has been answered for a long time, but since none of the answers above did the trick, this is what worked for me :

<% if(typeof myVariable !== 'undefined') { %>
    // Variable exists
<% } %>

Lubricate

Answer 21

I found what I believe to be a better solution on Joe Andrieu's blog:

<%= user.name ? user.name : '' %>

This applies in more situations than the other answers (i.e., when you need to check if a single property exists).

Herakleion