Say I have an array of arrays, and I want to return the first element of each array within the array:
array = [[["028A","028B","028C","028D","028E"],
["028F","0290","0291","0292","0293"],
["0294","0295","0296","0297","0298"],
["0299","029A","029B","029C","029D"],
["029E","029F","02A0","02A1","02A2"]],
[["02A3","02A4"],
["02A5", "02A6"]];
I know I can do something like this:
var firsts = [];
_.each(array, function(item){
_.each(item, function(thisitem){
firsts.push(_.first(thisitem));
});
});
but what if I want to do it with underscore's _.chain()
method? Just learning underscore, and so far seems very useful.
You could do it with flatten
and map
thusly:
var firsts = _.chain(array)
.flatten(true) // This true is important.
.map(function(a) { return a[0] })
.value();
Demo: http://jsfiddle.net/ambiguous/cm3CJ/
You use flatten(true)
to convert your array-of-arrays-of-arrays into an array-of-arrays and then the map
peels off the first element of each inner array.
If you want something shorter than the map
, you could use pluck
to pull out the first element of the inner arrays:
var firsts = _.chain(array)
.flatten(true) // This true is important.
.pluck(0)
.value();
Demo: http://jsfiddle.net/ambiguous/pM9Hq/
_.pluck
is just a map
call anyway:
// Convenience version of a common use case of `map`: fetching a property.
_.pluck = function(obj, key) {
return _.map(obj, function(value){ return value[key]; });
};
This one looks a lot more like the .map(&:first)
that you'd use in Ruby so it might be more familiar to some people and more concise once you're used to pluck
. If you really want something Rubyish, you could use a non-anonymous function with map
:
var first = function(a) { return a[0] };
var firsts = _.chain(array)
.flatten(true) // This true is important.
.map(first)
.value();