Question

I am trying to clean up some old code, and was wondering if there was a clean one line solution to deal with something like the following.

function dates(format) {
    var formats = ['DD/MM/YYYY','MM/DD/YYYY'];
    // is there a one liner for something like this? If format is undefined this will throw an error when evaluating formats.indexOf(format)
    format = (typeof format !== 'undefined' && formats.indexOf(format) >= 0) ? format : 'MM/DD/YYYY';

    // ...

format is an argument to a function, and I would like to do some prelim checks to make sure the input is given, and meets criteria of being in an array.

Answer 1

Your code looks fine to me, except that ==undefined check is superfluous.

function dates(format) {
    var formats = ['DD/MM/YYYY','MM/DD/YYYY'];
    format = formats.indexOf(format) >= 0 ? format : 'MM/DD/YYYY';
}

Note that this will not throw an exception if you call dates() with no arguments at all.

If you’ve a lot of code like this, you might consider using an utility function like

function oneOf(item /*, args */) {
    args = [].slice.call(arguments, 1)
    return args.indexOf(item) >= 0 ? item : args[0];
}

and then:

function dates(format) {
    format = oneOf(format, 'MM/DD/YYYY', 'DD/MM/YYYY')
}

If you decide to use an object as Felix suggested, it’s better to structure it like this:

var formats = {
    'DD/MM/YYYY': 'DD/MM/YYYY',
    'MM/DD/YYYY': 'MM/DD/YYYY',
};

This looks stupid at the first glance, but has two important advantages. First, the in check is as simple as

 format = formats[format] || 'DD/MM/YYYY'

and second, you can extend that to provide aliases for your values:

var formats = {
    'DD/MM/YYYY': 'DD/MM/YYYY',
    'MM/DD/YYYY': 'MM/DD/YYYY',
    'english': 'M/D/YYYY'
    'german':   'DD.MM.YYYY'
};

dates('german')