It's current to see the terms callback, trigger and handler in some API documentations. It is just synonymous or each term correspond to a specific concept?

I used to think that this terms are just synonymous, but I must be wrong ...

Thanks !

2 Answers 2


These are general terms in programming. Often can mean different things but generally speaking...

callback is a reference to a function or block of code that is executed by a third party.

trigger is a behavior in response to stimuli, and an event may trigger the change of state or as the result of that trigger execute the associated callback.

handler is a reference to an object or class that is associated with a behavior. A handler is different from a callback because it's an object that represents a state.

Using jQuery AJAX as an example.

   url: "test.html",
   context: document.body
}).done(function() {
  • The function() is a callback.
  • The object passed to $.ajax(...) is a handler.
  • The event done() is a trigger.

The handler has an event done that when triggered calls the callback to perform $(this).addClass("done");.

  • is a handler executed by a third party too ? By a trigger by example? is a handler just a complex callback ?
    – nouney
    Commented Jul 23, 2013 at 19:31
  • 2
    handler is like a callback except that it can do multiple things, where as a callback is singular. A handler might handle success, timeout or errors as an example.
    – Reactgular
    Commented Jul 23, 2013 at 19:35
  • A handler can also be a function.
    – tdammers
    Commented Jul 23, 2013 at 20:15
  • 1
    @tdammers In your opinion, what's the difference between a handler and a callback then ?
    – nouney
    Commented Jul 24, 2013 at 8:13
  • 1
    @nouney: it's a conceptual difference more than a technical one. 'Callback' refers to the technical mechanism of passing function A to function B, and then having B call A as part of its processing. 'Handler' highlights the intent that some sort of event is handled; this is often done with callbacks, but there are other mechanisms that one can use for this.
    – tdammers
    Commented Jul 25, 2013 at 8:33

Handler, an asynchronous callback subroutine. Handler implements interceptor design pattern. Which is used to handle incoming or outgoing messages and manipulate its values.

A callback is a piece of executable code that is passed as an argument to other code, which is expected to call back (execute) the argument at some convenient time. The invocation may be immediate as in a synchronous callback, or it might happen at later time as in an asynchronous callback.
Callbacks implements in different ways Closure , lambda expressions and so on...

Triggers are rules, it executes on events.

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