How would you check for undefined property in ejs for node.js?

Question

What is the best way to check for an undefined property in an ejs template?

(I'm using the node.js package by TJ Holowaychuk)

Example:

var tpl = '<% if (foo) { %>foo defined<% } else { %>foo undefined<% } %>';
console.log(ejs.render(tpl, { locals: { bar: "baz" } }));

I'd expect this to render "foo undefined". It does throw an foo undefined error instead.

I know that this is not supposed to be an issue, since this is expected behavior in the tests. Is there an easy way to avoid this?

The only solution I found is using the hasOwnProperty method.

var tpl = '<% if (hasOwnProperty("foo")) { %>foo defined<% } else { %>foo undefined<% } %>';
console.log(ejs.render(tpl, { locals: { bar: "baz"} }));

This doesn't throw any errors.

Is there a better way to keep the template clean? Or why does it throw this error?

Answer 1

Another way to test for a property is to reference it indirectly via the locals object. Using your example:

var tpl = '<% if(locals.foo){ %>foo defined<% }else{ %>foo undefined<% } %>';
console.log(ejs.render(tpl, { locals: { bar: "baz"} }));

Answer 2

I would use typeof, as in if (typeof foo == 'undefined'). I use the typeof operator with the string "undefined", whereas some people might do a direct comparison with the undefined global variable. I prefer this method because it is protected against some terrorist JS library developer changing the value of the global variable, leaving your code broken.

This could also be expressed as a ternary, which some view as "cleaner" due to the absence of curlies:

var tpl = '<% (typeof foo != "undefined" ? %>foo defined<% : %>foo undefined<% ) %>';

Answer 3

Simplest, and cleanest in my opinion:

<%= (!!locals.foo)?foo:'' %>