defined.js

// returns the first defined argument, which is useful for cleanly
// setting values with multiple potential defaults
function defined () {
  for (var i=0; i<arguments.length; i++)
    if (typeof arguments[i] !== 'undefined') 
      return arguments[i]
}

var baz = false

// example
function example (options) {
  options = defined(options, {})
  var foo = defined(options.foo, 'foo-default-value')
    , bar = defined(options.bar, baz, 'bar-default-value')
  console.log(options, foo, bar)
}
example({ foo: false })

// typical pseudo-equivalent, which merely tests for falsy values
function badIdea (options) {
  options = options || {}
  var foo = options.foo || 'foo-default-value'
    , bar = options.bar || baz || 'bar-default-value'
  console.log(options, foo, bar)
}
badIdea({ foo: false })

// actual equivalent, which is fairly verbose
function equivalent (options) {
  var foo
    , bar
  if (typeof options == 'undefined') {
    options = {}
  }
  if (typeof options.foo != 'undefined') {
    foo = options.foo
  }
  else {
    foo = 'foo-default-value'
  }
  if (typeof options.bar != 'undefined') {
    bar = options.bar
  }
  else if (typeof baz != 'undefined') {
    bar = baz
  }
  else {
    bar = 'bar-default-value'
  }
  console.log(options, foo, bar)
}
equivalent({ foo: false })