Posts Tagged ‘callback’


Tuesday, September 15th, 2009

Since functions are objects they can be passed as arguments to other functions.

function test(a, b, fn) {
    fn(a, b);

In this simple example we have a function test() that takes a function fn as a third parameter and calls that function passing the first two parameters.

When you cal the test() function, you can pass a pointer to an existing function myFunc() or you can also pass an anonymous function.

test(1, 2, myFunc);
test(1, 2, function(one, two){


For examples of this pattern in the wild – well, every time you attach an event listener, you pass a callback function.


Same with timeouts and intervals. setTimeout() and setInterval() expect a function and call it back after the specified number of milliseconds.

var thePlotThickens = function(){
  console.log('500ms later...');
setTimeout(thePlotThickens, 500);

And here's a common anti-pattern – passing a string where a function is expected. The JavaScript engine has no choice but to evaluate the string (like with eval()) and execute it, which is unnecessary work.

// anti-pattern
setTimeout("thePlotThickens()", 500);