1.0.3 • Public • Published

Xlsx smart templates DataFill

The library takes an existing .xlsx file and populates the extracted data from provided JSON, into the designated cells, which we call templates. If the extracted data is an array - it is expanded and occupies as many cells, as the size of the data. In both dimensions.

Each template follows a specific format and structure, and it defines these crucial aspects:

  • Where is the data coming from?
  • How is the data extracted?
  • How are the values for the cells extracted from the data?
  • How are the cell styles tweaked?

All these in the context of the provided JavaScript object. If you've alrady know the story - check the API.

Few examples that explain it all

Consider the following JSON:

	"title": "An exemplary title",
	"rows": [
			"header": "Row 1",
			"data": [11, 12, 13, 14, 15]
		}, {
			"header": "Row 2",
			"data": [21, 22, 23, 24, 25]
		}, {
			"header": "Row 3",
			"data": [31, 32, 33, 34, 35]

A cell with the following content: {{ | | title }} will be expanded into An exemplary title after the data is filled. More information on the template format will be given in a second.

Consider this cell content: {{ | rows | header }}. It’ll be expanded into three cells, in a column. Like this:

1 Row 1
2 Row 2
3 Row 3

The template defines that the data should be extracted from rows - which results in an array of 3 objects, and the values for the target cells - 3, of course - should be extracted using the header path inside each of the entries in the extracted data.

So far, the data was extracted from the root of the initially provided JavaScript object. However, each template can refer another one, taking the data extracted from it, as a basis for its own processing.

Nested blocks. Consider this spreadsheet:

1 {{ | rows | header }} {{ A1 | 1 * data | }}

The template in A1 is clear - it expands into the range A1:A3 as expected. The template in B1, however, introduces two new, interesting aspects. First, it refers another template - the one in A1, and second - it gives strange notion of how the data is extracted - 1 * data, instead of just data.

The second one is simple - since the general data extraction form is <rows> * <columns> and, for example rows (in A1) is a shorthand for rows * 1, so the 1 * data instructs the engine to expand the retrieved data horizontally (i.e. in many columns, but a single row), rather than vertically.

The first one - the reference - means that the B1 will not extract the data directly from the provided JS object, but rather - from the data already extracted from the referred template. And this happens for each data entry extracted from the referred template.

Replaying in "slow-mo", the whole data extract & placement process will look like:

  1. The engine processes A1 template:
    1. Extracts the data, resulting in an array of 3 objects (exactly the one referred by rows property).
    2. From each of these object, a value is extracted using the header property, resulting in the following array: [“Row 1”, “Row 2”, “Row 3”].
    3. The values are placed from the template’s cell (A1) downwards.
  2. For each of the objects in the array, extracted in [1.a], all dependent templates - in this case B1 are processed. We’ll show the processing for only the { “header”: “Row 1”, “data”: [...]} object:
    1. Data is extracted using the data property from the provided reference object, resulting in an array of 5 numbers.
    2. Since there is no additional value extractor - the data is used as is, for filling the cells - in this case in the range B1:F1.
    3. The same process is repeated for all three of the objects extracted in step [1.b].

The resulting table will look like:

1 Row 1 11 12 13 14 15
2 Row 2 21 22 23 24 25
3 Row 3 31 32 33 34 35

If you want to get a real feel about the power of the engine — check the multi-dimensional example, showing how an automatically generated 5D data can get expanded throughout the sheet, by a simple template.

Great! That’s it!

One more thing... As the general syntax of the data extraction suggests - there is another, more elegant way to achieve the same result. The template in B1 could have be written in the following form: {{ | rows * data | }}. Quite natural to write, and should be clear, by now, why it leads to the same result.

There is one more heavy lifting task that the engine does - it automatically merges cells, if the referring template turns to occupy more than one cell in the same dimension. In other words, if the template in B1 was written as {{ A1 | data | }}, (i.e. without the 1* part), this would instruct the engine to grow the data vertically. But, the data from A1 template, already grows vertically, so the engine will have to make the A1 cells “bigger”, i.e. occupying more rows. The result will look like this:

1 Row 1 11
2 12
3 13
4 14
5 15
6 Row 2 21
7 22

... at least, these are the first 7 rows of it. Cells A1:A5 will be merged, just like A6:A10, and A11:A15.

Hope it is clear by now. Check this and the other examples.

How to use it

The actual access to a XLSX notebook is delegated to an external library, through a so-called accessor, and there is currently one implementation, based on xlsx-populate library. Check the API to see how a custom one can be implemented.

Considering the existing accessor implementation, the use of xlsx-datafill is quite simple:

// Open the notebook and create the accessor for it
const wb = await XlsxPopulate.fromFileAsync(path);
const xlsxAccess = new XlsxPopulateAccess(wb, XlsxPopulate);

// Create an instance of XlsxDataFill and provide custom
// options.
const dataFill = new XlsxDataFill(xlsxAccess, { 
  callbacksMap: ... // Some custom handlers.
// Make the actual processing of `data`.

// The data in populated inside the `wb`, so it can be used.

Note: The template definitions are overwritten by the actual data, so don’t expect to be able to run fillData() with different data.

Check the template options section for more information on how to configure the XlsxDataFill instance.

Refer to the examples folder, as well as to the API documentation for more and deeper documentation.

Template format

The general format of each template is like follows:

{{ <reference cell>
 | <iterators>
 | <extractor>
 | <padding>
 | <styling> }}

Both the surrounding {{mustache}} brackets, and the | separator symbol are configurable, via XlsxDataFill constructor’s options.

The meaning of each field is:

Field Meaning
reference The address of a cell, to be used as a reference for data extraction. If empty - the provided object’s root is considered. In both cases this is referred as template data root in the rest of the description.
iterators JSON paths, determining how the data should be extracted from the template data root, and it follows the form
<row data path> * <column data path>.
The <row path> is applied on the template data root, while the <col path works on the result of <row path> extraction. If one needs the data to grow vertically (i.e. only as a column), the form 1 * <col path> is allowed, in which case <col path> works directly on the template data root.
Can be empty, if the template data root itself should be used.
extractor A JSON path, determining how the value that needs to be written in the cell(s) should be extracted from the data, provided by the iterators.
Can be omitted, in which case the iterators’ provided data is taken as a whole.
padding A : delimited pair specifying how many cells on each direction row:column need to be added for each new entry from the extracted data. Can be omitted.
styling A comma-delimited styling pairs of the format <style name>=<extractor>, setting each cell’s style (with the given name), to the value extracted from iterator’s data, using the extractor as a JSON path. E.g. fill='red' will set the fill style of each of the cells to red.
It is allowed to pass a handler as value (e.g. fill=:dangerColor will set fill color to the value returned by dangerColor handler). Also, any of the options can be overridden, by providing it here with a ! as prefix - e.g. !mergeCells=false will set this options to false only for this template.

The JSON path, mentioned above, refers to the ability to provide a full path of properties from the template data root instead of just one property. So valid paths are, for example: rows, genres.fiction, data[0].name, etc. Check the lodash’s get helper, because this is what is used.

In order to add additional flexibility, one can reference a user-provided function (via template options) for both iterators and extractors, including those used for styling. If a JSON-path component is suffixed with :<handler name> (e.g. data:dataFix, or even just :dataGive), the result of invoking the corresponding handler is used. The expected definition of such handler is:

 * @param {object} data The data base for the current context. 
 * @param {Cell} cell A target cell, if applicable.
 * @returns {*} The required value.
function myHandler(data, cell);

Few things need to be clarified. First, the context (i.e. this) provided is the options object, as provided upon XlsxDataFill instantiation.

The data object is the one that corresponds to the given context. For example, in the following template:

{{ A1 | rows:hRows * data:hData | :hNumber }}

All three handlers hRows, hData and hNumber will be invoked with different data argument - hRows will receive whatever rows extracted, hData will receive, whatever hRows returned (!), and hNumber will be given whatever hData returned.

In other words:

Handlers are applied after, and their result taken instead of, whatever data is extracted with the JSON path.

Another interesting thing is styling. It is quite straightforward, however. Each named style, as recognized by accessor’s setStyle() method, is referred and the value is extracted in the usual way (JSON path + handler).

Template options

Here are the options and their defaults.

    templateRegExp: new RegExp(/\{\{([^}]*)\}\}/),
    fieldSplitter: "|",
    joinText: ",",                     
    mergeCells: true,
    duplicateCells: false,
    followFormulae: false,
    copyStyle: true,
    callbacksMap: {
        '': data => _.keys(data),
        $: data => _.values(data)

Check the detailed description in the API. It is worth noting the mergeCells and duplicateCells behavior.

First, they both have the same set of possible values: true, false, ”both”, ”vertical”, ”horizontal” – in which direction the cells need to be merged/duplicated. As expected true and ”both” have the same meaning.

Second, cells duplication is valid only when merging is disabled, in other words, if mergeCells == true, duplicateCells is ignored. Given these options:

mergeCells: "vertical",
duplicateCells: true

Is interpreted as follow:

  • If during data expansion a value in higher dimension occupies more than one cell in a column, they are merged – because these are vertical.
  • If a value from higher dimension occupies more than one cell in a row, then duplication options is taken into account and same value is duplicated on all these cells.

Formulae handling

As formulas are a key Excel feature, so xlsx-datafill is trying to keep them alive and meaningful. As a basic rule Raw formulas are kept as they are, only those put in a template format, are handled.

So, the template format has slightly different version:

  • iterators determine how the formula will be populated, and is one of the following values: both, rows, cols or none. The latter can be replaced with an empty value.
  • extractor is the actual formula and must start with = so the engine recognizes it as such.
  • reference should be present, otherwise an error will be issued. If a non-referenced formula is needed - just don’t use the template format.

Two operations are performed during data population of the referenced template - formula alteration and formula population. How this happens depends on the value inside iterators field:

  • When none is selected, the formula is not populated - The ranges inside it are expanded to match the size of the referenced data block.
  • When cols is selected, the formula is populated across the columns of referenced data block, while each range inside the formula is expanded across the rows of the data block.
  • When rows is selected, the population and expanding processes are reversed - the formula is populated across rows, and the ranges inside are expanded across columns.
  • Finally, when both is selected - the formula is not expanded - it is just populated across the same area of cells, as the referenced data block.

It is important to note, that the formula population starts from the cell with the formula template, or one with the same offset, as the current data block - review the nested blocks concept for more information.

Few examples

Consider the following template (and the same data as the previous examples):

1 {{ | rows | header }} {{ | rows * data | }}
4 Formula: {{ B1 || =SUM(B1:B1) }}

Will result in the following output table:

1 Row 1 11 12 13 14 15
2 Row 2 21 22 23 24 25
3 Row 3 31 32 33 34 35
4 Formula: 345

With the formula inside B4 being altered to =SUM(B1:F3). The skipped iterators field is synonym of none, therefore - no population, just expansion.

If the formula template was this one: {{ B1 | cols | =SUM(B1:B1) }}, then the resulting table would be this:

1 Row 1 11 12 13 14 15
2 Row 2 21 22 23 24 25
3 Row 3 31 32 33 34 35
4 Formula: 63 66 69 72 75

The formula was populated across the columns, with each one being expanded across rows, i.e. the formula in C4 will be =SUM(C1:C3).

Specifying both as iterators keyword will result in the same size (3x5 in this example) table. It does make sense if it includes anchored references, i.e a template like this {{ B1 | both | =B1 * $A$5 }} will result in a 3x5 table, starting from B4, with each value from B1:F3 being multiplied by the value in A5. For example the formula in C4 would be C1 * $A$5.

Some important notes

There are several specifics to be kept in mind:

  • The iterators usually resolve to an array. If it resolves to an object, and still some iteration is expected — it can be converted to an array of object’s values, using the $ handler (i.e. appending :$). If you need the keys (as opposed to object’s values) — append : to the last iterator, i.e. utilizing an empty extraction handler. Both of these default handlers can be overridden with the provided options.
  • If during value extraction, the result is an array — it is automatically joined, using the (configurable) joinText from the options.
  • No matter what part of the cell’s value the template definition occupies, at the end — the whole cell is overwritten with the resolved value(s). Since, this is not a simple find-and-replace, and the cells need duplication — it is not as trivial as expected. A possible workaround is to use handlers to append whatever is needed for each cell.
  • The xlsx-populate library is not a dependency, because (potentially in the future) other accessors can be used, so don’t expect it to be there if you just refer xlsx-datafill.


Any help is appreciated! Check the list in the repo.

Thank you!


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