const jsonata = require('@elastic.io/jsonata-moment');

const PASSTHROUGH_BODY_PROPERTY = 'elasticio';

function handlePassthrough(message: any) {
  if (message.passthrough && Object.keys(message.passthrough)) {
    if (PASSTHROUGH_BODY_PROPERTY in message.body) {
      throw new Error(`${PASSTHROUGH_BODY_PROPERTY} property is reserved \
            if you are using passthrough functionality`);
    }
    const result = JSON.parse(JSON.stringify(message)); // Otherwise subsequent calls to Jsonata transform with same message would fail
    result.body.elasticio = {};
    Object.assign(result.body.elasticio, result.passthrough);
    return result;
  }
  return message;
}

/**
 * This method will be called from elastic.io platform providing following data
 *
 * @param msg incoming message object that contains ``body`` with payload
 * @param cfg configuration that is account information and configuration field values
 * @param context this of action or trigger, optional

 */
export function jsonataTransform(msg: any, cfg: any, context: any) {
  const { expression } = cfg;
  const compiledExpression = jsonata(expression);
  if (context && context.getFlowVariables) {
    compiledExpression.assign('getFlowVariables', () => context.getFlowVariables());
  }
  const handledMessage = handlePassthrough(msg);
  const passthrough = handledMessage.body[PASSTHROUGH_BODY_PROPERTY];
  compiledExpression.assign('getPassthrough', () => passthrough);
  return compiledExpression.evaluate(handledMessage.body);
}