Question

I’m learning lots of javascript these days, and one of the things I’m not quite understanding is passing functions as parameters to other functions. I get the concept of doing such things, but I myself can’t come up with any situations where this would be ideal.

My question is:

When do you want to have your javascript functions take another function as a parameter? Why not just assign a variable to that function’s return value and pass that variable to the function like so:

// Why not do this
var foo = doStuff(params);
callerFunction(foo);

//instead of this
callerFunction(doStuff);

I’m confused as to why I would ever choose to do things as in my second example.

Why would you do this? What are some use cases?

Answer 1

Here’s yet another example. Does some formatting operations on an array:

function pctFormatter(num) {
  return num + '%';
}

function centsFormatter(num) {
  return num + '.00';
}

function formatThisArray(array, formatter) {
  var output = [];
  for(var i = 0; i < array.length; i++) {
    output.push( formatter(array[i]) );
  }
  return output;
}

formatThisArray([1,2,3], pctFormatter);// returns ['1%', '2%', '3%']
formatThisArray([1,2,3], centsFormatter);// returns ['1.00', '2.00', '3.00']