Javascript, passing a function in an object literal and is it callable?

Question

always in the process of learning Javascript and modifying a cool autocomplete library , i am now in front of this :

i need to check if something passed in an object literal is a variable/field (that is to be considered as a simple value) or is something that can be called.

(as MY autocomplete depend on many input fields, i need to "value" the right things, just before the Ajax.Request) so that this declaration (see the 'extra' parts...)

   myAutoComplete = new Autocomplete('query', {
        serviceUrl:'autoComplete.rails',
    minChars:3,
    maxHeight:400,
    width:300,
    deferRequestBy:100,
    // callback function:
    onSelect: function(value, data){
                alert('You selected: ' + value + ', ' + data);
                       }
            // the lines below are the extra part that i add to the library
            //     an optional parameter, that will handle others arguments to pass
            //     if needed, these must be value-ed just before the Ajax Request... 
    , extraParametersForAjaxRequest : { 
                  myExtraID : function() { return document.getElementById('myExtraID').value; } 
    }

see the "1 // here i'm lost..." below, and instead of 1 => i would like to check, if extraParametersForAjaxRequest[x] is callable or not, and call it if so, keeping only its value if not. So that, i get the right value of my other inputs... while keeping a really generic approach and clean modification of this library...

{
  var ajaxOptions = {
    parameters: { query: this.currentValue , },
    onComplete: this.processResponse.bind(this),
    method: 'get'
  };
  if (this.options.hasOwnProperty('extraParametersForAjaxRequest'))
  {
      for (var x in this.options.extraParametersForAjaxRequest)
      {
          ajaxOptions.parameters[x] = 1 // here i'm lost...
      }
  }


  new Ajax.Request(this.serviceUrl, ajaxOptions );

Answer 1

   if (typeof this.options.extraParametersForAjaxRequest[x]==='function') {

   }

You should also do this:

   if (this.options.extraParametersForAjaxRequest.hasOwnProperty(x) {
       if (typeof this.options.extraParametersForAjaxRequest[x]==='function') {

       }
   }

when iterating through properties of objects, otherwise you can end up looking at prototype members too.

Another suggestion is to make this more readable with an alias for the thing you're working with. So the ultimate would be:

  var opts = this.options.extraParametersForAjaxRequest;
  // don't need to check for existence of property explicitly with hasOwnProperty
  // just try to access it, and check to see if the result is
  // truthy. if extraParametersForAjaxRequest isn't there, no error will
  //  result and "opts" will just be undefined
  if (opts)
  {
      for (var x in opts) {
          if (opts.hasOwnProperty(x) && typeof opts[x]==='function') {

          }
       }
  }

Answer 2

You can do a typeof to see if the parameter is a function, and call it if it is.

var value;
for (var x in this.options.extraParametersForAjaxRequest)
{
    value = this.options.extraParametersForAjaxRequest[x];

    if (typeof(value) == 'function') {
        ajaxOptions.parameters[x] = value();
    }
    else {
        ajaxOptions.parameters[x] = value;  
    }
}