The basic principle is this: You create a template in Excel. This can be formatted as you wish, contain formulae etc. In this file, you put placeholders using a specific syntax (see below). In code, you build a map of placeholders to values and then load the template, substitute the placeholders for the relevant values, and generate a new .xlsx file that you can then serve to the user.

Placeholders

Placeholders are inserted in cells in a spreadsheet. It does not matter how those cells are formatted, so e.g. it is OK to insert a placeholder (which is text content) into a cell formatted as a number or currecy or date, if you expect the placeholder to resolve to a number or currency or date.

Scalars

Simple placholders take the format ${name}. Here, name is the name of a key in the placeholders map. The value of this placholder here should be a scalar, i.e. not an array or object. The placeholder may appear on its own in a cell, or as part of a text string. For example:

| Extracted on: | ${extractDate} |

might result in (depending on date formatting in the second cell):

| Extracted on: | Jun-01-2013 |

Here, extractDate may be a date and the second cell may be formatted as a number.

Columns

You can use arrays as placeholder values to indicate that the placeholder cell is to be replicated across columns. In this case, the placeholder cannot appear inside a text string - it must be the only thing in its cell. For example, if the placehodler value dates is an array of dates:

| ${dates} |

might result in:

| Jun-01-2013 | Jun-02-2013 | Jun-03-2013 |

Tables

Finally, you can build tables made up of multiple rows. In this case, each placeholder should be prefixed by table: and contain both the name of the placeholder variable (a list of objects) and a key (in each object in the list). For example:

| Name                 | Age                 |
| ${table:people.name} | ${table:people.age} |

If the replacement value under people is an array of objects, and each of those objects have keys name and age, you may end up with something like:

| Name        | Age |
| John Smith  | 20  |
| Bob Johnson | 22  |

If a particular value is an array, then it will be repeated accross columns as above.

Generating reports

To make this magic happen, you need some code like this:

var XlsxTemplate = require('xlsx-template');

// Load an XLSX file into memory
fs.readFile(path.join(__dirname, 'templates', 'template1.xlsx'), function(err, data) {

    // Create a template
    var template = new XlsxTemplate(data);

    // Replacements take place on first sheet
    var sheetNumber = 1;

    // Set up some placeholder values matching the placeholders in the template
    var values = {
            extractDate: new Date(),
            dates: new Date("2013-06-01"), new Date("2013-06-02"), new Date("2013-06-03"),
            people: [
                {name: "John Smith", age: 20},
                {name: "Bob Johnson", age: 22}
            ]
        };

    // Perform substitution
    template.substitute(sheetNumber, values);

    // Get binary data
    var data = template.generate();

    // ...

});

At this stage, data is a string blob representing the compressed archive that is the .xlsx file (that's right, a .xlsx file is a zip file of XML files, if you didn't know). You can send this back to a client, store it to disk, attach it to an email or do whatever you want with it.

Caveats

The spreadsheet must be saved in .xlsx format. .xls, .xlsb or .xlsm won't work.
Merged cells are complicated. Things may go wrong if you have placeholders inside, to the left of, or above merged cells.
Placeholders only work in simple cells, not data tables or pivot tables or other such things.