NodeJS Express app.locals are not directly accessible in function - node.js

Question

This is a weird kind of a question but follow me along. I have a nodejs server with express application. Inside the application, I set my locals as follows:
var moment = require('moment');
app.locals.moment = moment;
The ejs is being rendered as:
exports.page = function (req, res) {
res.render('first-page');
};
Then, in my ejs, I have the following code:
<%
if (!moment) {
throw new Error('moment is not defined');
}
function formatDate(date) {
return moment(date).format();
}
%>
<p><%= formatDate(1435856054045); %></p>
The interesting that happens is that moment does not raise the exception. Thus, it is defined in the scope of ejs, just as documentation says. However, an exception is raised by ejs saying that moment is not defined at formatDate. If I change formatDate to the following, everything works.
function formatDate(date) {
return locals.moment(date).format();
}
My question is how are the functions, defined in ejs, are scoped and which context is applied to them. Does ejs apply a different context to the function than to the floating javascript? I'm assuming it does something like formatDateFunctionPointer.call(ejsScope, ...);

Answer 1

The problem becomes clear when you have ejs output the generated function (to which a template is compiled):
with (locals || {}) {
if (!moment) {
throw new Error('moment is not defined');
}
function formatDate(date) {
return moment(date).format();
}
...
}
The problem is that your formatDate function is hoisted to outside the with block; inside that block, moment is actually locals.moment, so your test to see if it exists works.
However, when you can formatDate, it's not run within the context of the with block, and therefore, moment doesn't exist (but locals.moment does, as you already found out).
Here's a standalone example of the problem:
var obj = { test : 123 };
with (obj) {
if (test !== 123) throw new Error('test does not equal 123');
function showTest() {
console.log('test', test);
}
showTest();
}
One way to resolve this is to use a function expression:
<%
if (typeof moment === 'undefined') {
throw new Error('moment is not defined');
}
var formatDate = function(date) {
return moment(date).format();
};
%>
<p><%= formatDate(1435856054045); %></p>
(it also fixes your test to see if moment is actually defined)
Or you can set the EJS _with option to false.