mapcomplete/Docs/CalculatingExtraTags.md

Extra, automatically created tags

In some cases, it is useful to have some tags calculated based on other properties.

Some useful tags are available by default (e.g. _lat, _lon, _country) and are always available (have a lookt at CalculatedTags.md to see an overview).

It is also possible to calculate your own tags - but this requires some javascript knowledge.

Before proceeding, some warnings:

• DO NOT DO THIS AS BEGINNER
• Only do this if all other techniques fail. This should not be done to create a rendering effect, only to calculate a specific vaue
• THIS MIGHT BE DISABLED WITHOUT ANY NOTICE ON UNOFFICIAL THEMES. As unofficial themes might be loaded from the internet, this is the equivalent of injecting arbitrary code into the client. It'll be disabled if abuse occurs.

In the layer object, add a field calculatedTags, e.g.:

  "calculatedTags": {
    "_someKey": "javascript-expression",
    "name": "tags.name ?? tags.ref ?? tags.operator"
  }