Underscore.js: how to chain custom functions

Question

Using Underscore.js, I can write the following which returns 42:

_([42, 43]).chain()     .first()     .value() 

I have custom function, not part of Underscore.js called double():

function double(value) { return value * 2; }; 

I would like to be able to call this function in an Underscore chain, as if it was part of Underscore. I would like to write the following, which I'd like to return 84:

_([42, 43]).chain()     .first()     .double()     .value() 

This can't work since Underscore doesn't define double(). I could use tap() as in:

_([42, 43]).chain()     .first()     .tap(double)     .value() 

This is valid, but tap applies the function to its argument and returns the argument, not the result of the function. So it looks to me like I would need a sort of tap that returns the result of the function applied to its argument. Is there anything like this in Underscore.js? Am I missing something terribly obvious?

Answer 1

Not finding a tap that returns the value returns by the function is runs, I define one which I can take and add to _:

_.mixin({take: function(obj, interceptor) {     return interceptor(obj); }}); 

Then assuming I have:

function double(value) { return value * 2; }; 

I can write:

_([42, 43]).chain()     .first()             // 42     .take(double)        // Applies double to 42     .value()             // 84 

You can look at take as map on objects, instead of lists. Want to experiment with this? See this example on jsFiddle.