How to use the vega-expression.codegen function in vega-expression

export default function parseDateTime(dateTime: string | number | DateTime) {
  if (typeof dateTime === 'number' || typeof dateTime === 'string') {
    return new Date(dateTime);
  }

  const expression = dateTimeExpr(dateTime, true) as string;
  const code = codegen({ globalvar: 'window' })(parse(expression)).code as string;
  // Technically the "code" here is safe to eval(),
  // but we will use more conservative approach and manually parse at the moment.
  const isUtc = code.startsWith('Date.UTC');

  const dateParts = code
    .replace(/^(Date[.]UTC|new[ ]Date)\(/, '')
    .replace(/\)$/, '')
    .split(',')
    .map((chunk: string) => Number(chunk.trim())) as [
    number, // year
    number, // month
    number, // date
    number, // hours
    number, // minutes
    number, // seconds
    number, // milliseconds

export function toPredicateFunction(filters: ShelfFilter[]) {
  const expr = '(' +
    filters.map(f => {
      return fieldFilterExpression(f, false); // Do not use inrange as it is not included in the main Vega Expression
    }).join(')&&(') +
  ')';
  const ast = vegaExpression.parse(expr);
  const codegen = vegaExpression.codegen({
    whitelist: ['datum'],
    globalvar: 'global'
  });
  const value = codegen(ast);

  return new Function('datum', `return ${value.code};`) as (d: object) => boolean;
}

for (var name in functionContext) { fn[name] = thisPrefix + name; }
  return fn;
}

// Export code generator and parameters
export var codegenParams = {
  blacklist:  ['_'],
  whitelist:  ['datum', 'event', 'item'],
  fieldvar:   'datum',
  globalvar:  function(id) { return '_[' + stringValue('$' + id) + ']'; },
  functions:  buildFunctions,
  constants:  constants,
  visitors:   astVisitors
};

export var codeGenerator = codegen(codegenParams);

// register Vega-Lite selection functions
expressionFunction('vlSelectionTest', selectionTest, selectionVisitor);
expressionFunction('vlSelectionResolve', selectionResolve, selectionVisitor);

// Export code generator and parameters
export const codegenParams = {
  blacklist:  ['_'],
  whitelist:  ['datum', 'event', 'item'],
  fieldvar:   'datum',
  globalvar:  function(id) { return '_[' + stringValue(SignalPrefix + id) + ']'; },
  functions:  buildFunctions,
  constants:  constants,
  visitors:   astVisitors
};

export var codeGenerator = codegen(codegenParams);