javascript functions are callbacks mandatory

If I have a function like this

function foo (arg1, arg2, callback) {
  // blah blah
  callback()
}

can I just do:

foo (arg1, arg2)

Sometimes I find that I can't, what's going on here? 开发者_如何学编程Thanks.

the issue is that callback() will throw an error if you only pass in two arguments since it's effectively doing undefined()

If you are writing the function, you can do:

callback && callback()

meaning that it's only called if it exists (for more of a failsafe, check that it's a function).

If you don't control it, you can pass in an empty function:

foo(arg1, arg2, function(){});

If you don't pass a third parameter to foo, "callback" will have a value of "undefined". Hence, you'll likely hit a script exception when you get to the invocation of the callback() function.