traverson/lib/transforms/fetch_resource.js

Hypermedia API/HATEOAS client for Node.js and the browser

'use strict';

var minilog = require('minilog')
  , log = minilog('traverson')
  , abortTraversal = require('../abort_traversal')
  , isContinuation = require('../is_continuation')
  , httpRequests = require('../http_requests');

var nextTickAvailable = process &&
  Object.hasOwnProperty.call(process, 'nextTick');

/*
 * Execute the next step in the traversal. In most cases that is an HTTP get to
 *the next URL.
 */

function fetchResource(t, callback) {
  if (isContinuation(t)) {
    convertContinuation(t, callback);
  } else {
    fetchViaHttp(t, callback);
  }
}

fetchResource.isAsync = true;

/*
 * This is a continuation of an earlier traversal process.
 * We need to shortcut to the next step (without executing the final HTTP
 * request of the last traversal again.
 */
function convertContinuation(t, callback) {
  log.debug('continuing from last traversal process (walker)');
  if (nextTickAvailable) {
    return process.nextTick(function() { // de-zalgo continuations
      callback(t);
    });
  }
  return callback(t);
}

function fetchViaHttp(t, callback) {
  // always check for aborted before doing an HTTP request
  if (t.aborted) {
    return abortTraversal.callCallbackOnAbort(t);
  }
  httpRequests.fetchResource(t, function(err, t) {
    log.debug('fetchResource returned');
    if (err) {
      if (!err.aborted) {
        log.debug('error while executing http request');
        log.error(err);
      }
      return t.callback(err);
    }
    callback(t);
  });
}

module.exports = fetchResource;