Expression

The value for any layout property, paint property, or filter may be specified as an expression. An expression defines a formula for computing the value of the property using the operators described below. The set of expression operators provided by MapLibre GL includes:

• Element
• Mathematical operators for performing arithmetic and other operations on numeric values
• Logical operators for manipulating boolean values and making conditional decisions
• String operators for manipulating strings
• Data operators, providing access to the properties of source features
• Camera operators, providing access to the parameters defining the current map view

Expressions are represented as JSON arrays. The first element of an expression array is a string naming the expression operator, e.g. "*"or "case". Subsequent elements (if any) are the arguments to the expression. Each argument is either a literal value (a string, number, boolean, or null), or another expression array.

Data expression: a data expression is any expression that access feature data -- that is, any expression that uses one of the data operators:get,has,id,geometry-type, or properties. Data expressions allow a feature's properties to determine its appearance. They can be used to differentiate features within the same layer and to create data visualizations.

Camera expression: a camera expression is any expression that uses the zoom operator. Such expressions allow the the appearance of a layer to change with the map's zoom level. Camera expressions can be used to create the appearance of depth and to control data density.

Composition: a single expression may use a mix of data operators, camera operators, and other operators. Such composite expressions allows a layer's appearance to be determined by a combination of the zoom level and individual feature properties.

Example expression:

FillLayer fillLayer = new FillLayer("layer-id", "source-id");
fillLayer.setProperties(
  fillColor(
    interpolate( linear(), zoom(),
      stop(12, step(get("stroke-width"),
        color(Color.BLACK),
        stop(1f, color(Color.RED)),
        stop(2f, color(Color.WHITE)),
        stop(3f, color(Color.BLUE))
      )),
      stop(15, step(get("stroke-width"),
        color(Color.BLACK),
        stop(1f, color(Color.YELLOW)),
        stop(2f, color(Color.LTGRAY)),
        stop(3f, color(Color.CYAN))
      )),
      stop(18, step(get("stroke-width"),
        color(Color.BLACK),
        stop(1f, color(Color.WHITE)),
        stop(2f, color(Color.GRAY)),
        stop(3f, color(Color.GREEN))
      ))
    )
  )
);

Inheritors