Full scripting is only available to Enterprise plan developers.
Developers on the free plan have access to limited javascript that is executed only for the game's primary owner. It is useful for testing and debugging.
To use it, use the custom javascript action. All javascript is evaled in the context of the the match and the action specifically.
Examples of custom javascript commands.
console.log(thisCard)
console.log(thisCard.variable)
console.log(parseVariable('$variable'))
console.log(thisMatch.variable)
console.log(thisCard.effects)
For client side scripting you have full access to the DOM as well as included libraries Lodash, jQuery, threejs.