GraphQL Node.js: determine what types are used within a query

Given a valid document string representing some GraphQL operation, you can parse the string into an AST.

import { parse, validate } from 'graphql' 

const document = parse(someDocumentString)
// if you want to validate your document to verify it matches your schema
const errors = validate(schema, document)

AFAIK, there's no utility function for getting an array of the types in a document, if that's what you're asking for, but you can just traverse the AST and gather whatever information from it. As an example, here's how GraphiQL does just that to generate a map of variables to their corresponding types:

import { typeFromAST } from 'graphql'

export function collectVariables(schema, documentAST) {
  const variableToType = Object.create(null);
  documentAST.definitions.forEach(definition => {
    if (definition.kind === 'OperationDefinition') {
      const variableDefinitions = definition.variableDefinitions;
      if (variableDefinitions) {
        variableDefinitions.forEach(({ variable, type }) => {
          const inputType = typeFromAST(schema, type);
          if (inputType) {
            variableToType[variable.name.value] = inputType;
          }
        });
      }
    }
  });
  return variableToType;
}