Merging files with Node.js

I can't seem to solve this particular problem. Here's what I'm supposed to do:

"Complete the implementation of the merge method in utilities.js file. You should be able to pass in a string with placeholders with percent signs (%) surrounding them. The second parameter should be an object with values to be inserted in to the placeholders. Look at index.js to see how it should work."

Here is the index.js file:

var utilities = require("./utilities");

var mailValues = {};

mailValues.first_name = "Janet";

var emailTemplate = "Hi %first_name%! Thanks for completing this code challenge :)";

var mergedContent = utilities.merge(emailTemplate, mailValues);

//mergedContent === "Hi Janet! Thanks for completing this code challenge :)";

Here is the utilities.js file with the work I've already completed:

function merge(content, values) { for(var mailValues in values) { content = content.replace("{%" + mailValues + "%}", values[mailValues]); } return content; }

module.exports.merge = merge;

So, how do I solve this?