{"componentChunkName":"component---src-templates-index-template-js","path":"/page/5","webpackCompilationHash":"a3ac22c47fe06e2afd02","result":{"data":{"allMarkdownRemark":{"edges":[{"node":{"html":"

Problem

\n

You want to be sure that the logic inside a function is executed only once.

\n

Ingredients

\n\n

Directions

\n
    \n
  1. \n

    Given: a function that is called twice, but should not do anything the second time it is called.

    \n
    function printSomething() {\n  console.log('something');\n}\nprintSomething();            // (1) Ok\nprintSomething();            // (2) Should not be allowed
    \n
    2. \n
  2. \n

    Add a variable to memorize if the function has been called.

    \n
    function printSomething() {\n  let called = false;\n  console.log('something');\n}
    \n
    3. \n
  3. \n

    If it is false, then return undefined.

    \n
    function printSomething() {\n  let called = false;\n  if(called) {\n    return undefined;\n  }\n  console.log('something');\n}
    \n
    4. \n
  4. \n

    … otherwise set the variable to true …

    \n
    function printSomething() {\n  let called = false;\n  if(called) {\n    return undefined;\n  } else {\n    called = true;\n  }\n}
    \n
    5. \n
  5. \n

    … execute the remaining logic of the function …

    \n
    function printSomething() {\n  let called = false;\n  if(called) {\n    return undefined;\n  } else {\n    called = true;\n    console.log('something');\n  }\n}
    \n
    6. \n
  6. \n

    … and overwrite the function to do nothing.

    \n
    function printSomething() {\n  let called = false;\n  if(called) {\n    return undefined;\n  } else {\n    called = true;\n    console.log('something');\n    printSomething = function() {};\n  }\n}
    \n
    7. \n
  7. \n

    Voilá, now if printSomething() is called a second time, it simply does nothing.

    \n
    printSomething();        // (1) Ok\nprintSomething();        // (2) Does nothing
    \n

    Notes

    \n
    8. \n
  8. A function that overwrite itself is also called a self-defining function.
    9. \n
\n

Alternative recipes

\n","fields":{"slug":"/posts/en/2016-07-06-function-called-once/","categorySlug":null},"frontmatter":{"title":"Create a function that only executes once","date":"2016-07-06T08:00:00.000Z","category":null,"description":null}}},{"node":{"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/","categorySlug":null},"frontmatter":{"title":"Return after calling a callback","date":"2016-07-05T08:00:00.000Z","category":null,"description":null}}},{"node":{"html":"

Problem

\n

You want to be sure that the value passed as the callback parameter is really a function.

\n

Ingredients

\n\n

Directions

\n
    \n
  1. \n

    Given: a function that accepts a callback function.

    \n
    function someFunction(callback) {\n  callback();\n}
    \n
    2. \n
\n\n
    \n
  1. \n

    Check that callback is of type function.

    \n
    function someFunction(callback) {\n  if(typeof callback === 'function') {\n    callback();\n  }\n}
    \n
    2. \n
  2. \n

    Optional: otherwise throw an error.

    \n
    function someFunction(callback) {\n  if(typeof callback === 'function') {\n    callback();\n  } else {\n    // handle it, throw error\n    throw new TypeError('The callback parameter is no function.');\n  }\n}
    \n
    3. \n
  3. \n

    Voilá, a delicious type check of the callback parameter.

    \n
    someFunction(() => {console.log(4711)});  // 4711\nsomeFunction({});                         // TypeError
    \n
    4. \n
\n

Variants

\n\n

Alternative recipes

\n","fields":{"slug":"/posts/en/2016-07-04-check-callback-type/","categorySlug":null},"frontmatter":{"title":"Check type of callback parameter","date":"2016-07-04T08:00:00.000Z","category":null,"description":null}}},{"node":{"html":"

Problem

\n

You want to create a function that creates a promise that delays for a given number of milliseconds.

\n

Ingredients

\n\n

Directions

\n
    \n
  1. \n

    Create a function that accepts the number of milliseconds.

    \n
    function delay(ms) {\n  ...\n}
    \n
    2. \n
\n\n
    \n
  1. \n

    Let the function return a promise object.

    \n
    function delay(ms) {\n  return new Promise(...);\n}
    \n
    2. \n
  2. \n

    Add the resolve-/reject-callback to the promise.

    \n
    function delay(ms) {\n  return new Promise(\n    (resolve, reject) => {\n      ...\n    }\n  );\n}
    \n
    3. \n
  3. \n

    Inside that callback function call setTimeout().

    \n
    function delay(ms) {\n  return new Promise(\n    (resolve, reject) => {\n      setTimeout(resolve, ms);\n    }\n  );\n}
    \n
    4. \n
  4. \n

    Voilá, a perfect delaying function in promise-style.

    \n
    delay(5000).then(() => {\n  console.log('Printed after 5 seconds.')\n});
    \n
    5. \n
\n

Variants

\n\n

Notes

\n","fields":{"slug":"/posts/en/2016-07-03-delaying-promise/","categorySlug":null},"frontmatter":{"title":"Create a delaying function in promise-style","date":"2016-07-03T08:00:00.000Z","category":null,"description":null}}},{"node":{"html":"

Problem

\n

You want to create a generic function, that produces for a given function the partial applied function.

\n

Ingredients

\n\n

Directions

\n
    \n
  1. \n

    Define a function that accepts another function (the one that should be partial applicable).

    \n
    function partial(fn /*, args...*/) {\n  ...\n}
    \n
    2. \n
\n\n
    \n
  1. \n

    Let the function return another function (this will be the partial applicable function).

    \n
    function partial(fn /*, args...*/) {\n  ...\n  return function() {\n    ...\n  }\n}
    \n
    2. \n
  2. \n

    Convert all but the first argument to an array by borrowing the slice() method.

    \n
    function partial(fn /*, args...*/) {\n  var args = Array.prototype.slice.call(arguments, 1);\n  return function() {\n    ...\n  };\n}
    \n
    3. \n
  3. \n

    Convert the arguments of the inner function to an array.

    \n
    function partial(fn /*, args...*/) {\n  var args = Array.prototype.slice.call(arguments, 1);\n  return function() {\n    var argsInner = Array.prototype.slice.call(arguments, 0);\n    ...\n  };\n}
    \n
    4. \n
  4. \n

    Combine the arguments of the outer function with the arguments of the inner function.

    \n
    function partial(fn /*, args...*/) {\n  var args = Array.prototype.slice.call(arguments, 1);\n  return function() {\n    var argsInner = Array.prototype.slice.call(arguments, 0);\n    var argsAll = args.concat(argsInner);\n    ...\n  };\n}
    \n
    5. \n
  5. \n

    Call the passed function using the apply() method, passing the combined arguments.

    \n
    function partial(fn /*, args...*/) {\n  var args = Array.prototype.slice.call(arguments, 1);\n  return function() {\n    var argsInner = Array.prototype.slice.call(arguments, 0);\n    var argsAll = args.concat(argsInner);\n    return fn.apply(this, argsAll);\n  };\n}
    \n
    6. \n
  6. \n

    Optional: combine the previous three steps.

    \n
    function partial(fn /*, args...*/) {\n  var args = Array.prototype.slice.call(arguments, 1);\n  return function() {\n    return fn.apply(this, args.concat(Array.prototype.slice.call(arguments, 0)));\n  };\n}
    \n
    7. \n
  7. \n

    Voilá, a perfect, tasteful generic function that creates partial applicable functions.

    \n
    function partial(fn /*, args...*/) {\n  var args = Array.prototype.slice.call(arguments, 1);\n  return function() {\n    return fn.apply(this, args.concat(Array.prototype.slice.call(arguments, 0)));\n  };\n}\nfunction createPerson(firstName, lastName) {\n  return {\n   firstName: firstName,\n   lastName: lastName\n  }\n}\nvar createPersonWithFirstNameJohn = partial(createPerson, 'John');\nvar johnDoe = createPersonWithFirstNameJohn('Doe');\nconsole.log(johnDoe.firstName);  // \"John\"\nconsole.log(johnDoe.lastName);   // \"Doe\"
    \n
    8. \n
\n

Alternative recipes

\n","fields":{"slug":"/posts/en/2016-07-02-partial-application/","categorySlug":null},"frontmatter":{"title":"Create a generic function for partial application","date":"2016-07-02T07:00:00.000Z","category":null,"description":null}}}]}},"pageContext":{"isCreatedByStatefulCreatePages":false,"currentPage":5,"postsLimit":5,"postsOffset":25,"prevPagePath":"/page/4","nextPagePath":"/page/6","hasPrevPage":true,"hasNextPage":true}}}