Simpltry

Rails introduce a pattern called alias\_method\_chain in 2006. The general idea is you want to add something to an existing method, but you don’t want to monkey patch the whole method.

I found myself wanting this functionality in JavaScript. My use case was adding a callback to the Ajax.Autocompleter in script.aculo.us. Here is how I solved it.

This is the code that defines aliasMethodChain on Object.

  Object.extend(Object, {
    aliasMethodChain: function(object, target,
          feature, implementation) {
      object[target + "Without" +
          feature.charAt(0).toUpperCase() +
          feature.substring(1)] = object[target];
      object[target] = implementation;
    }
  });

And here I am using the aliasMethodChain.

  Object.aliasMethodChain(Autocompleter.Base.prototype,
        "selectEntry", "selectCallback", function() {
    this.selectEntryWithoutSelectCallback();
    this.options.onSelect ? this.options.onSelect() : null;
  });

Nothin’ to it. Now I can pass onSelect into the options of any autocompleter and it will be called when the user makes a selection.