_.constant Function

[OttoAllmendinger]

It would be nice to have a function like

_.constant = function(x) { return function() { return x; }}

I have often needed something like _.constant(true) or _.constant(false) when filtering collections with a configurable iterator.

[jashkenas]

This is a very interesting function -- but I'm afraid that I fail entirely to see the use-case where this would be helpful. Instead of filtering a collection by true, why not just duplicate the collection ... instead of filtering it by false, why not just use an empty array?

Provide some real-world code that makes use of this idiom, and we'll reopen the ticket.

[OttoAllmendinger]

There is an analogue in Clojure core: constantly

I admit the usage is a little subtle, but I'll try to demonstrate.

Consider

_.isUndefined(filterFunc) ? collection : _.filter(collection, filterFunc);

_.select(collection, filterFunction || function() { return true; })

_.select(collection, filterFunction || _.constant(true))

Graphics

// api: object.position = function(time) { return coords }
circle.position = _.constant( [10, 10] )

// api: image.fill( function(x, y) { return color })
image.fill( _.constant( black ) );

Testing/Stubbing

textDisplay.source = { getText: _.constant("text") }

[jashkenas]

So, mainly for passing constants to APIs that expect functions. Nice.

Still, I think it would be clearer for most readers, and not much longer, to simply write the function:

circle.position = function(){ return [10, 10]; };

image.fill(function(){ return black; });

[OttoAllmendinger]

That is correct, though the same can be said for _.identity.

I also think the word constant is more expressive than having to parse the code and see if the return value is indeed constant.

[sophiebits]

This would also be useful if you need a function that returns a particular this:

var self = this;
return $.ajax(...).then(function() { return self });

// vs

return $.ajax(...).then(_.constant(this));

I agree that there isn't a huge difference.