Iterate over object attributes and modify them

Question

Underscore.js provides _.each and _.map on collections, which is nice, but I need to iterate over all attributes of my object. I need to modify the values and preserve the keys. E.g. I've got something like: {a:1, b:2, c:3} and I need to perform an operation that changes the value but keeps the keys. Let's say, I'll calculate squares, I should get {a:1, b:4, c:9}. The question is: how to do that using underscore (not interested in vanilla javascript)? I'd love a method like:

var a = {a:1, b:2, c:3} _.magic(a, function(item){ return item*item; }); 

Additionally, it would be great if this was possible to chain it, since I'm doing a map, dump result to perform each and then use a map again (because I need to).

Answer 1

_.mapObject (added in version 1.8) is exactly what you're looking for.

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For previous versions, can mutate the original object using the third argument to the _.each callback.

_.each({a:1, b:2, c:3}, function(value, key, obj) { obj[key] = value * value; }); 

Using _.reduce with an initial memo can create a brand new object as well.

_.reduce({a:1, b:2, c:3}, function(memo, value, key) {     memo[key] = value * value;     return memo; }, {}); 

I'd prefer the _.reduce approach. A simple mixin will make it behave exactly like _.map.

_.mixin({     magic: function(obj, iterator, context) {         return _.reduce(obj, function(memo, value, key) {             memo[key] = iterator.call(context, value, key, obj);             return memo;         }, {});     } });