{"componentChunkName":"component---src-templates-post-template-js","path":"/posts/en/2016-07-05-return-callback/","webpackCompilationHash":"a3ac22c47fe06e2afd02","result":{"data":{"markdownRemark":{"id":"8526f8c6-7122-5bf4-95cc-7a6b96f747b3","html":"

Problem

\n

You want to be sure that a callback function is not accidentally called twice or more.

\n

Ingredients

\n\n

Directions

\n
    \n
  1. \n

    Given: a function that accepts a callback function and calls it in two different places.

    \n
    function someFunction(callback) {\n  let result = {};\n  try {\n    result = someOtherFunction();\n  } catch(error) {\n    callback(error, null);\n  }\n  callback(null, result);\n}
    \n
    2. \n
  2. \n

    Now assume that someOtherFunction() produces an error. What happens?\nThe error is catched and callback(error, null) is executed. But additionally\ncallback(null, result) is called as well. This is most likely not the intention.

    \n
    function someFunction(callback) {\n  let result = {};\n  try {\n    result = someOtherFunction();  // (1) Produces an error\n  } catch(error) {\n    callback(error, null);         // (2) Gets called\n  }\n  callback(null, result);          // (3) Gets called as well\n}
    \n
    3. \n
  3. \n

    Solution: add return after calling callbacks.

    \n
    function someFunction(callback) {\n  let result = {};\n  try {\n    result = someOtherFunction();  // (1) Produces an error\n  } catch(error) {\n    callback(error, null);         // (2) Gets called\n    return;\n  }\n  callback(null, result);          // (3) Doesn't get called\n  return;\n}
    \n
    4. \n
  4. \n

    Even better: add return before the call of the callback function.

    \n
    function someFunction(callback) {\n  let result = {};\n  try {\n    result = someOtherFunction();  // (1) Produces an error\n  } catch(error) {\n    return callback(error, null);  // (2) Gets called\n  }\n  return callback(null, result);   // (3) Doesn't get called\n}
    \n
    5. \n
  5. Voilá, now if someOtherFunction() produces an error, the second callback is not called.
    6. \n
\n

Alternative recipes

\n","fields":{"slug":"/posts/en/2016-07-05-return-callback/","tagSlugs":["/tag/java-script/"]},"frontmatter":{"date":"2016-07-05T08:00:00.000Z","description":null,"tags":["JavaScript"],"title":"Return after calling a callback"}}},"pageContext":{"isCreatedByStatefulCreatePages":false,"slug":"/posts/en/2016-07-05-return-callback/"}}}