{"componentChunkName":"component---src-templates-post-template-js","path":"/posts/en/2016-07-13-promisify/","webpackCompilationHash":"a3ac22c47fe06e2afd02","result":{"data":{"markdownRemark":{"id":"605e3503-bd8a-574f-93cb-bcd8c835e595","html":"

Problem

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You want to create a generic function to promisify asynchronous functions.

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Ingredients

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Directions

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    \n
  1. \n

    Given: an asynchronous function …

    \n
    const add = (x, y, z, callback) => {\n  setTimeout(() => {\n    callback(    // Remember:\n      null,      // First argument: the error\n      x + y + z  // Second argument: the result\n    );\n  }, 5000);\n}
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    2. \n
  2. \n

    … that we call like this …

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    add(2,3,4, (error, result) => {\n  if(error) {\n    console.error(error);\n  } else {\n    console.log(result));\n  }\n}
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    3. \n
  3. \n

    … but what we really want is this:

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    const promisifiedAdd = promisify(add);\npromisifiedAdd(2,3,4)\n  .then(result => console.log(result))\n  .catch(error => console.error(error));
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    4. \n
  4. \n

    So let’s get started! First create a function promisify() that takes a function …

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    function promisify(fn) {\n  ...\n}
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    5. \n
  5. \n

    … and returns another function (that uses rest parameters).

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    function promisify(fn) {\n  return function (...args) {\n    ...\n  }\n}
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    6. \n
  6. \n

    Let this function return a Promise object.

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    function promisify(fn) {\n  return function (...args) {\n    return new Promise((resolve, reject) => {\n      ...\n    });\n  }\n}
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    7. \n
  7. \n

    And inside the callback of the Promise object, call the original function,\npassing the rest parameters to it (using the spread operator) and a callback function.

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    function promisify(fn) {\n  return function (...args) {\n    return new Promise((resolve, reject) => {\n      fn(...args, (error, result) => {\n        ...\n      });\n    });\n  }\n}
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    8. \n
  8. \n

    Inside the callback function of the Promise object check if there is an error. If so,\nthen call the method reject() passing the object object as argument.

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    function promisify(fn) {\n  return function (...args) {\n    return new Promise((resolve, reject) => {\n      fn(...args, (error, result) => {\n        if(error) {\n          return reject(error);\n        }\n      });\n    });\n  }\n}
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    9. \n
  9. \n

    Otherwise call the method resolve() passing the result object as argument.

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    function promisify(fn) {\n  return function (...args) {\n    return new Promise((resolve, reject) => {\n      fn(...args, (error, result) => {\n        if(error) {\n          return reject(error);\n        } else {\n          return resolve(result);\n        }\n      });\n    });\n  }\n}
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    10. \n
  10. \n

    Optimization: combine the last two steps.

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    function promisify(fn) {\n  return function (...args) {\n    return new Promise((resolve, reject) => {\n      fn(...args, (error, result) => {\n        return error ? reject(error) : resolve(result);\n      });\n    });\n  }\n}
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    11. \n
  11. \n

    Optimization: replace function-style functions by arrow functions.

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    const promisify = (fn) => {\n  return (...args) => {\n    return new Promise((resolve, reject) => {\n      fn(...args, (error, result) => {\n        return error ? reject(error) : resolve(result);\n      });\n    });\n  }\n}
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    12. \n
  12. \n

    Optimization: replace the return keywords and the brackets where possible.

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    const promisify = fn =>                         // take the original function\n  (...args) =>                                  // return another function\n  new Promise((resolve, reject) =>              // that returns a promise\n  fn(...args, (error, result) =>                // which calls the original\n  error ? reject(error) : resolve(result)));    // and handles the results
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    13. \n
  13. \n

    Voilá, a perfect generic function for converting asynchronous functions to promises.

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    const promisify = fn =>\n  (...args) =>\n  new Promise((resolve, reject) =>\n  fn(...args, (error, result) =>\n  error ? reject(error) : resolve(result)));\n\nconst add = (x, y, z, callback) => {\n  setTimeout(() => {\n    callback(null, x + y + z);\n  }, 5000);\n}\n\nconst promisifiedAdd = promisify(add);\npromisifiedAdd(2,3,4)\n  .then(result => console.log(result))\n  .catch(error => console.error(error));
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    14. \n
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Notes

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Alternative recipes

\n","fields":{"slug":"/posts/en/2016-07-13-promisify/","tagSlugs":["/tag/java-script/"]},"frontmatter":{"date":"2016-07-13T08:00:00.000Z","description":null,"tags":["JavaScript"],"title":"Create a generic function to promisify asynchronous functions"}}},"pageContext":{"isCreatedByStatefulCreatePages":false,"slug":"/posts/en/2016-07-13-promisify/"}}}