Question

In trying to make my Javascript unobtrusive, I’m using onLoads to add functionality to <input>s and such. With Dojo, this looks something like:

var coolInput = dojo.byId('cool_input');
if(coolInput) {
  dojo.addOnLoad(function() {
    coolInput.onkeyup = function() { ... };
  });
}

Or, approximately equivalently:

dojo.addOnLoad(function() {
  dojo.forEach(dojo.query('#cool_input'), function(elt) {
    elt.onkeyup = function() { ... };
  });
});

Has anyone written an implementation of Ruby’s andand so that I could do the following?

dojo.addOnLoad(function() {
  // the input's onkeyup is set iff the input exists
  dojo.byId('cool_input').andand().onkeyup = function() { ... };
});

or

dojo.byId('cool_input').andand(function(elt) {
  // this function gets called with elt = the input iff it exists
  dojo.addOnLoad(function() {
    elt.onkeyup = function() { ... };
  });
});

Answer 1

The exact syntax you want is not possible in JavaScript. The way JavaScript executes would need to change in a pretty fundamental fashion. For example:

var name = getUserById(id).andand().name;
//                        ^
//                        |-------------------------------
// if getUserById returns null, execution MUST stop here |
// otherwise, you'll get a "null is not an object" exception

However, JavaScript doesn’t work that way. It simply doesn’t.

The following line performs almost exactly what you want.

var name = (var user = getUserById(id)) ? user.name : null;

But readability won’t scale to larger examples. For example:

// this is what you want to see
var initial = getUserById(id).andand().name.andand()[0];
// this is the best that JavaScript can do
var initial = (var name = (var user = getUserById(id)) ? user.name : null) ? name[0] : null;

And there is the side-effect of those unnecessary variables. I use those variables to avoid the double lookup. The variables are mucking up the context, and if that’s a huge deal, you can use anonymous functions:

var name = (function() {return (var user = getUserById(id)) ? user.name : null;})();

Now, the user variable is cleaned-up properly, and everybody’s happy. But wow! what a lot of typing! 🙂