Custom fuction in vega expression

I’m wondering if it is possible to add a new function to be used in vega expressions like this

...
"transform": {"type": "formula", "field": "myNewField", "expr": "myFunction(datum.mySourceField)"},
...

After looking at how things are done in vega/vega-expression, I tried the following but it did not work:

// Include vega.js and dependencices

window.myFunctionInRealLife = function(value) {
  return value * 42;
}
vg.parse.expr.codegen.functions["myFunction"] = function(args) {
        var a = args.map(vg.parse.expr.codegen);
        return 'window.myFunctionInRealLife('+a[0]+')';
    }

// Use myFunction in a vega spec

It would be nice to see a temporal workaround and a proper solution in future releases.

Just in case: my specific goal is to calculate centroids for geopolygons after they have been projected (i.e convert layout_path to some [x,y]). This can be done using d3.path.centroid().

[jheer]

Good question! The this context bound to the function includes a context property that is the runtime context object for the Vega View. To explore this yourself, go to the Vega home page, open the console, and try this:

vega.expressionFunction('foo', function() { console.log('foo!', this); });
var spec = {signals: [{name:'bar', update:'foo()'}]};
var view = new vega.View(vega.parse(spec)).run();

Use this.context.dataflow to get a reference to the current View instance.

[jheer]

The original issue was specific to Vega 2.

In Vega 3, you can add new expression functions using vega.expressionFunction, a method exported by vega-parser. The signature is:

expressionFunction(name, fn, visitor)

• The name argument is a string indicating the name of the function as used in the Vega expression language.
• The fn argument is the actual function to invoke.
• The visitor argument is an expression AST visitor which can be used to perform dependency analysis (e.g., for scale or data source lookups). For most basic functions no visitor is needed, in which case this argument can be omitted.

New expressions must be added before parsing a spec that uses the custom function. Also this is a global operation, and all subsequent Vega views will have access to the function.

We will update the documentation with the above information; closing out this issue now.