Add _.withDefault combinator

[joshuacc]

The _.withDefault function would accept a function as its argument, and returns a new function that transparently replaces null/undefined return values with a default value.

Example:

function getArrLength (arr) {
  if (_.isArray(arr)) return arr.length;
}

getArrLengthOrZero = _.withDefault(getArrLength, 0);

getArrLengthOrZero({});
// => 0

May need a better name for the function, though.

[fogus]

Hi @joshuacc,

Take a look a fnull at https://github.com/documentcloud/underscore-contrib/blob/master/underscore.function.combinators.js#L192 and see if that gives you what you need.

[fogus]

Ahhh! Wait, I see that your proposal works on the return. Maybe we could talk about how this operates with or augments fnull instead?

[joshuacc]

@fogus I saw that, but if I'm reading right it operates on the opposite end of things. My understanding is that _.fnull fills in non-existy arguments.

What I'm looking for in _.withDefault is filling in non-existy return values.

[fogus]

Right. I saw that right after hitting send. :(

[joshuacc]

Hehe. Replying at the same time. 😁

One common use case that I could see for _.withDefault is when using _.map to pluck properties that may or may not be defined.

var arr = [
    { prop: "val" },
    { prop: "val" },
    {}
];

_.map(arr, function (val) { return val; });
// => ["val", "val", undefined]

_.map(arr, _.withDefault(function (val) { return val; }, "default"));
// => ["val", "val", "default"]

I think this fits as a nice complement to _.fnull operating on the other end of things.

[jgonggrijp]

I like this idea a lot. The names withDefault and fnull don't reveal the duality between them, though. Maybe one of them can get an alias to emphasize this duality?

[joshuacc]

Maybe fnull could get an alias like withDefaultArgs?