Expose access to `vega.expressionFunction`

In https://github.com/vega/vega-embed/blob/next/src/embed.ts#L348, we already expose formatLocale and timeFormatLocale as opts for Vega embed.

To let people specify custom expression function, I think we should expose expressionFunction in our opts as well.

Since vega.expressionFunction(name[, fn, visitor]) has a slightly more complicated signature than formatLocale(definition) , timeFormatLocale(definition), we need to define opt.expressionFunction’s structure.

I think {<name>: <fn>, <name2>: <fn2>} as the short form and {<name>: {fn: <fn>, visitor: <visitor>}, ...} as the full form probably makes sense.

Basically, we’ll add

expressionFunction: ExpressionFunction

to EmbedOptions’s interface where the type is

type ExpressionFunction = Record<string, Function | {fn: Function, visitor: Function}>;

@domoritz @jheer – any opinion?

cc: @zacharybys

[kanitw]

For the prop name, should we do expressionFunction: {...} or expressionFunctions: {...} or sth else ?

Basically the function is vega.expressionFunction, but the object passed in technically can represent multiple expressionFunction.

There is also expr: for “Custom Vega Expression interpreter” (which is basically the same as expr: in vega.View).

I think I prefer expressionFunction: so that the name exactly matches the function so it’s easier to know that it’s a wrapper for the same thing.

[domoritz]

Okay. I’m convinced to have the shorthand. The design makes sense to me.