Expressions

Expressions allow for defining predicates or computing new variables based on existing data. The expression language is based on JavaScript, but provides only a limited set of features, guaranteeing secure execution.

Expressions can be used with the "filter" and "formula" transforms.

Usage

All basic arithmetic operators are supported:

(1 + 2) * 3 / 4

The current data object is provided as the datum object. Its properties (fields) can be accessed by using the dot or bracket notation:

datum.foo + 2

If the name of the property contains special characters such as ".", "!", or " " (a space) the bracket notation must be used:

datum['A very *special* name!'] > 100

Conditional operators

Ternary operator:

datum.foo < 5 ? 'small' : 'large'

And an equivalent if construct:

if(datum.foo < 5, 'small', 'large')

Provided constants and functions

Common mathematical functions are supported:

(datum.u % 1e-8 > 5e-9 ? 1 : -1) *
  (sqrt(-log(max(1e-9, datum.u))) - 0.618) *
  1.618

The full list of built-in constants and functions are documented in vega-expression.