Hey I'm new to underscore.js and I'm trying to figure out how to perform an operation on a map. I read through the API and I'm afraid I'm missing something.
Here's what I'd like to do:
doubled = _.someFunction( { bob: 25, sally: 30, tom: 5 }
, function(value){ return value*2; }
);
Returning:
{ bob: 50, sally: 60, tom: 10 }
Any idea how to do this? Should I create a new function with _.mixin()?
You could make a double function like this:
function double(data) {
var doubled = {};
_.each(data, function(value, key) {
doubled[key] = 2 * value;
});
return doubled;
};
double({ bob: 25, sally: 30, tom: 5 });
_.double = double that would do it.No, Underscore indeed does not provide a map function for objects. (You can use _.map on objects, but it will return an array)
So, you will have to do it manually:
_.someFunction = function(o, f) {
var res = {};
for (var p in o)
res[p] = f(o[p]);
return res;
};
Of course you might use some iteration functions from underscore. Without a helper function, these snippets might be more or less expressive:
var doubled = {};
_.each({ bob: 25, sally: 30, tom: 5 }, function(p, value){
doubled[p] = value*2;
});
var doubled = _.reduce({ bob: 25, sally: 30, tom: 5 }, function(m, value, p){
m[p] = value*2;
return m;
}, {});
var obj = { bob: 25, sally: 30, tom: 5 };
var doubled = _.object(
_.keys(obj),
_.map(_.values(obj), function(value){ return value*2; })
);
For those interested, here's what I ended up doing: (See the reduce function API)
_.mapValues = function( object, iterator, context ){
if (context) iterator = _.bind(iterator, context);
return _.reduce(
object
, function( memo, value, key ){
memo[key] = iterator( value, key );
return memo;
}
, {} )
};
