Getting a better understanding of callback functions in JavaScript

Question

I understand passing in a function to another function as a callback and having it execute  but I m not understanding the best implementation to do that  I m looking for a very basic example  like this   var myCallBackExample         myFirstFunction   function  param1  param2  callback                Do something with param1 and param2          if   arguments length    3                    Execute callback function                 What is the  best  way to do this                       mySecondFunction   function             myFirstFunction  false  true  function                    When this anonymous function is called  execute it                         In myFirstFunction  if I do return new callback    then it works and executes the anonymous function  but that doesn t seem like the correct approach to me

User · Answer

There are 3 main possibilities to execute a function:

var callback = function(x, y) {
    // "this" may be different depending how you call the function
    alert(this);
};

callback(argument_1, argument_2);
callback.call(some_object, argument_1, argument_2);
callback.apply(some_object, [argument_1, argument_2]);

The method you choose depends whether:

You have the arguments stored in an Array or as distinct variables.
You want to call that function in the context of some object. In this case, using the "this" keyword in that callback would reference the object passed as argument in call() or apply(). If you don't want to pass the object context, use null or undefined. In the latter case the global object would be used for "this".

Docs for Function.call, Function.apply

User · Answer

Callbacks are about signals and  new  is about creating object instances    In this case it would be even more appropriate to execute just  callback     than  return new callback    because you aren t doing anything with a return value anyway    And the arguments length  3 test is really clunky  fwiw  better to check that callback param exists and is a function

User · Answer

Callbacks are about signals and  new  is about creating object instances    In this case it would be even more appropriate to execute just  callback     than  return new callback    because you aren t doing anything with a return value anyway    And the arguments length  3 test is really clunky  fwiw  better to check that callback param exists and is a function

User · Answer

You can use   if  callback  amp  amp  typeof callback       function         callback        The below example is little more comprehensive    x000D   x000D  function mySandwich param1  param2  callback    x000D    alert  Started eating my sandwich  n nIt has      param1          param2   x000D    var sandwich     x000D        toppings   param1  param2  x000D         x000D      madeCorrectly    typeof param1       string   amp  amp  typeof param2       string     true   false  x000D    if  callback  amp  amp  typeof callback       function     x000D      callback apply sandwich   madeCorrectly    x000D      x000D    x000D   x000D  mySandwich  ham    cheese   function correct    x000D    if  correct    x000D      alert  Finished eating my     this toppings 0      and     this toppings 1      sandwich     x000D      else   x000D      alert  Gross   Why would I eat a     this toppings 0      and     this toppings 1      sandwich     x000D      x000D      x000D   x000D   x000D

User · Answer

x000D   x000D  function checkCallback cb    x000D    if  cb    cb          x000D      if  typeof window cb       undefined   alert  Callback function not found     x000D      else window cb  call this  Arg1  Arg2   x000D      x000D    x000D   x000D   x000D

User · Answer

Callbacks are about signals and  new  is about creating object instances    In this case it would be even more appropriate to execute just  callback     than  return new callback    because you aren t doing anything with a return value anyway    And the arguments length  3 test is really clunky  fwiw  better to check that callback param exists and is a function

User · Answer

the proper implementation would be   if  callback   callback      this makes the callback parameter optional

User · Answer

You can just say  callback      Alternately you can use the call method if you want to adjust the value of this within the callback   callback call  newValueForThis     Inside the function this would be whatever newValueForThis is

User · Answer

You can just say  callback      Alternately you can use the call method if you want to adjust the value of this within the callback   callback call  newValueForThis     Inside the function this would be whatever newValueForThis is

User · Answer

Callbacks are about signals and  new  is about creating object instances    In this case it would be even more appropriate to execute just  callback     than  return new callback    because you aren t doing anything with a return value anyway    And the arguments length  3 test is really clunky  fwiw  better to check that callback param exists and is a function

User · Answer

Here is a basic example that explains the callback   function in JavaScript    x000D   x000D  var x   0  x000D   x000D  function testCallBack param1  param2  callback    x000D    alert  param1      param1      param2      param2     X     x   x000D    if  callback  amp  amp  typeof callback       function     x000D      x    1  x000D      alert  Calla Back x      x   x000D      x    1  x000D      callback    x000D      x000D    x000D   x000D  testCallBack  ham    cheese   function     x000D    alert  Function X      x   x000D      x000D   x000D   x000D    JSFiddle

User · Answer

the proper implementation would be   if  callback   callback      this makes the callback parameter optional

User · Answer

There are 3 main possibilities to execute a function:

var callback = function(x, y) {
    // "this" may be different depending how you call the function
    alert(this);
};

callback(argument_1, argument_2);
callback.call(some_object, argument_1, argument_2);
callback.apply(some_object, [argument_1, argument_2]);

The method you choose depends whether:

You have the arguments stored in an Array or as distinct variables.
You want to call that function in the context of some object. In this case, using the "this" keyword in that callback would reference the object passed as argument in call() or apply(). If you don't want to pass the object context, use null or undefined. In the latter case the global object would be used for "this".

Docs for Function.call, Function.apply

User · Answer

x000D   x000D  function checkCallback cb    x000D    if  cb    cb          x000D      if  typeof window cb       undefined   alert  Callback function not found     x000D      else window cb  call this  Arg1  Arg2   x000D      x000D    x000D   x000D   x000D

User · Answer

You should check if the callback exists  and is an executable function   if  callback  amp  amp  typeof callback       function            execute the callback  passing parameters as necessary     callback        A lot of libraries  jQuery  dojo  etc   use a similar pattern for their asynchronous functions  as well as node js for all async functions  nodejs usually passes error and data to the callback   Looking into their source code would help

User · Answer

You should check if the callback exists  and is an executable function   if  callback  amp  amp  typeof callback       function            execute the callback  passing parameters as necessary     callback        A lot of libraries  jQuery  dojo  etc   use a similar pattern for their asynchronous functions  as well as node js for all async functions  nodejs usually passes error and data to the callback   Looking into their source code would help

User · Answer

You can use   if  callback  amp  amp  typeof callback       function         callback        The below example is little more comprehensive    x000D   x000D  function mySandwich param1  param2  callback    x000D    alert  Started eating my sandwich  n nIt has      param1          param2   x000D    var sandwich     x000D        toppings   param1  param2  x000D         x000D      madeCorrectly    typeof param1       string   amp  amp  typeof param2       string     true   false  x000D    if  callback  amp  amp  typeof callback       function     x000D      callback apply sandwich   madeCorrectly    x000D      x000D    x000D   x000D  mySandwich  ham    cheese   function correct    x000D    if  correct    x000D      alert  Finished eating my     this toppings 0      and     this toppings 1      sandwich     x000D      else   x000D      alert  Gross   Why would I eat a     this toppings 0      and     this toppings 1      sandwich     x000D      x000D      x000D   x000D   x000D

User · Answer

There are 3 main possibilities to execute a function:

var callback = function(x, y) {
    // "this" may be different depending how you call the function
    alert(this);
};

callback(argument_1, argument_2);
callback.call(some_object, argument_1, argument_2);
callback.apply(some_object, [argument_1, argument_2]);

The method you choose depends whether:

You have the arguments stored in an Array or as distinct variables.
You want to call that function in the context of some object. In this case, using the "this" keyword in that callback would reference the object passed as argument in call() or apply(). If you don't want to pass the object context, use null or undefined. In the latter case the global object would be used for "this".

Docs for Function.call, Function.apply

User · Answer

There are 3 main possibilities to execute a function:

var callback = function(x, y) {
    // "this" may be different depending how you call the function
    alert(this);
};

callback(argument_1, argument_2);
callback.call(some_object, argument_1, argument_2);
callback.apply(some_object, [argument_1, argument_2]);

The method you choose depends whether:

You have the arguments stored in an Array or as distinct variables.
You want to call that function in the context of some object. In this case, using the "this" keyword in that callback would reference the object passed as argument in call() or apply(). If you don't want to pass the object context, use null or undefined. In the latter case the global object would be used for "this".

Docs for Function.call, Function.apply

User · Answer

You can just say  callback      Alternately you can use the call method if you want to adjust the value of this within the callback   callback call  newValueForThis     Inside the function this would be whatever newValueForThis is

User · Answer

You can just say  callback      Alternately you can use the call method if you want to adjust the value of this within the callback   callback call  newValueForThis     Inside the function this would be whatever newValueForThis is

User · Answer

Here is a basic example that explains the callback   function in JavaScript    x000D   x000D  var x   0  x000D   x000D  function testCallBack param1  param2  callback    x000D    alert  param1      param1      param2      param2     X     x   x000D    if  callback  amp  amp  typeof callback       function     x000D      x    1  x000D      alert  Calla Back x      x   x000D      x    1  x000D      callback    x000D      x000D    x000D   x000D  testCallBack  ham    cheese   function     x000D    alert  Function X      x   x000D      x000D   x000D   x000D    JSFiddle

[javascript] Getting a better understanding of callback functions in JavaScript

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