Thursday, July 31, 2008

Passing function as callback to another function

A callback is a function that is passed as an argument to another function and is executed after its parent function has completed.

Another important thing to know is how to properly pass the callback. This is where I have often forgotten the proper syntax.

Callback without arguments

For a callback with no arguments you pass it like this:

myFunction(myCallBack);

Note that the second parameter here is simply the function name (but not as a string and without parentheses). Functions in Javascript are 'First class citizens' and so can be passed around like variable references and executed at a later time.

Callback with arguments

"What do you do if you have arguments that you want to pass?", you might ask yourself.

Wrong

The Wrong Way (will not work!)


myFunction(myCallBack(param1, param2));


This will not work because it calls myCallBack(param1, param2) and then passes the return value as the second parameter to myFunction.

Right

The Right Way ( note the use of function(){ preceding myCallBack() )


myFunction(function(){
myCallBack(param1, param2);
});

91 comments:

Zang said...

Chamnap,

Thanks for this example!

I was running into a problem where calling myFunction(myCallBack('someArg')) was actually executing myCallBack('someArg') before executing myFunction.

Modifying the code to prepend function(){... to my myCallBack call allows me to pass the reference around nicely. Thanks!

Erin
Seattle, WA

chamnap said...

You're welcome.

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