Question

So I’m using an object to pass in my optional variables like so:

var foo = function (options) {
            var options = options || {}; //provide a default, so that
            var bar = options.bar || 42; //this doesn't fail on 'foo();'
            //do stuff
        };

JSLint complains that I’m overwriting options, which is what I want if it’s falsy. Actually, I should probably check if it’s an Object and elsewise throw an error to let the user know what’s up. So – on that note – what would be good form here? Anybody know a good read on this? Additionally, how would I go about writing a bunch of functions with this pattern in a DRY style?

Answer 1

First, you need to not reassign the options argument to a var. Second, be careful with this for certain values:

var foo = function (options) {
            options = options || {};
            var bar = options.bar || 42;
            var baz = options.baz || 'fubar';
          };
foo({ bar: 0, baz: "" });

Inside foo, bar == 42 & baz == 'fubar' because 0 and "" are falsy. It’s probably better to be more verbose so as to be more precise:

var foo = function (options) {
            options = options || {};
            var bar = typeof options.bar == 'undefined' ? 42 : options.bar;
            var baz = typeof options.baz == 'undefined' ? 'fubar' : options.baz;
          };

But, to be DRY, you can create a defaults object and just extend both objects to a settings object:

// generic shallow object extension, mutates obj1
var extend = function (obj1, obj2) {
  for (var prop in obj2) {
    if (obj2.hasOwnProperty(prop)) {
      obj1[prop] = obj2[prop];
    }
  }
  return obj1;
};

var foo = function (options) {
  var defaults = {
        bar: 42,
        baz: 'fubar'
      },
      settings = extend(defaults, options);
};

Since you won’t be using defaults any more in this invocation it doesn’t matter that it gets mutated. Using this method:

foo();                    // -> settings = { bar: 42, baz: 'fubar' }
foo({ bar: 0 });          // -> settings = { bar: 0, baz: 'fubar' }
foo({ bar: 0, baz: "" }); // -> settings = { bar: 0, baz: '' }