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====== How to write a Javascript expression generator? ======


===== About =====

This howto will show you how to write an ''expression'' for a [[:docs:generator:expression|expression generator]].

An [[:docs:generator:expression|expression generator]] generates data from another column based on an ''expression''.


===== Example =====

This example generate a times table (known also a time dimension in analytics).

It's using a:
    *[[howto:generator:sequence_time|time sequence]]: a [[:docs:generator:sequence|sequence]] that generates a [[docs:data_type:date_time|date]] (ie time at the day level)
  * and expressions to generate the next columns:
      * month
      * year



<unit>
With the following [[:docs:resource:generator|generator resource]]
<file yaml generator/expression--generator.yml>
kind: generator
spec:
  MaxRecordCount: 10
  Columns:
    # Generate 10 days sequentially
    - name: d_date
      comment: A date
      Type: date
      data-supplier:
        type: sequence
        arguments:
          start: 2025-06-06 # by default, today
    # Generate the month of the year from the date with an expression
    - name: d_moy
      comment: the month number in year
      Type: Integer
      data-supplier:
        type: expression
        arguments:
          column-variable: d_date
          expression: "d_date.getMonth()+1"
    # Generate the year from the date with an expression
    - name: d_year
      comment: The year number
      Type: Varchar
      Precision: 4
      data-supplier:
        type: expression
        arguments:
          column-variable: d_date
          expression: "d_date.getFullYear()"


</file>
You would get the following data:
<code bash>
tabul data print generator/expression--generator.yml@howto
</code>
<console>
d_date       d_moy   d_year
----------   -----   ------
2025-06-05       6   2025
2025-06-04       6   2025
2025-06-03       6   2025
2025-06-02       6   2025
2025-06-01       6   2025
2025-05-31       5   2025
2025-05-30       5   2025
2025-05-29       5   2025
2025-05-28       5   2025
2025-05-27       5   2025
</console>
</unit>

===== What are Expressions ?=====

Expressions are Javascript expressions.


Tabulify pass the data as native javascript [[docs:data_type:data_type|data type]] in a variable with the name of the column.
You don't need to instantiate the variables, Tabulify do.



Note:
  * For [[docs:data_type:date_time|date/timestamp/time data type]], we create a [[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date|Date]] as this is the only temporal data type of javascript.
  * For all numbers (Double, Integer, ...) , we create a [[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number|Number]] as this is also the only number data type of javascript.

Example for ''2009/10/01'' and the column name ''date'', Tabulify would create the `date` variable like this:
<code javascript>
date = new Date("2009","10","01")
</code>
And if you want the month, you expression would use [[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/getMonth|getMonth]] :
<code javascript>
date.getMonth()+1
</code>



===== Develop your expression  in the Devtool or Node Console =====

You can test your expression quickly with:
  * your [[#browser devtool console]] (F12 > Console)
  * or the [[#Node console]]

==== Browser Devtool Console ====

Illustration of Devtool and a [[#time|time expression to get the month]]

{{:docs:generator:javascript_expression_console_devtool.jpg|}}


==== Node Console ====

Illustration of [[https://nodejs.org/en|Node]] in the terminal and a [[#time|time expression to get the month]]

{{:howto:generator:node_console_expression_generator_illustration.png|}}

===== Snippet of expressions =====



This section is showing snippet of expression (Javascript).


==== Time ====



  * Month where ''x'' represents the ''column name'' value of the parent columns
<code javascript>
x.getMonth()+1 // Month
</code>
  * Month names. The below snippet use a ''date'' column as parent and lookup an array to get the month name.
<code javascript>
var monthNames = ['Jan', 'Feb', 'Maa', 'Apr', 'Mei', 'Jun', 'Jul', 'Aug', 'Sep', 'Okt', 'Nov', 'Dec'];
monthNames[x.getMonth()]
</code>
  * Year number
<code javascript>
x.getFullYear()
</code>
==== Email ====
Email String concatenation where:
      * ''x'' would be a name from the first parent column
      * ''y'' would be a company name from the second parent column
      * ''z'' would be a TLD domain extension (''.com'', ...)
<code javascript>
x+'@'+y+'.'+z
</code>