Inject JS global var in Jade template

I have a form that is created in Jade ans then compiled. I need to pass the Label name with Javascript. So the javascript var will look like this :

var trans = {"formFirstName": "First Name"}

in the form.jade currently its 

.field.first_name
  label First Name
  .input-wrapper
    input(type="text")

But I want First Name to be the variable. I am very new in jade and node. I have no idea how to do that.

Replace the text "First Name" with #{formFirstName} . For more samples, refer to this link http://naltatis.github.io/jade-syntax-docs/#text

.field.first_name
  label #{formFirstName}
  .input-wrapper
    input(type="text")