Javascript reduce() on Object

Question

There is nice Array method reduce() to get one value from the Array. Example:

[0,1,2,3,4].reduce(function(previousValue, currentValue, index, array){
  return previousValue + currentValue;
});

What is the best way to achieve the same with objects? I'd like to do this:

{ 
    a: {value:1}, 
    b: {value:2}, 
    c: {value:3} 
}.reduce(function(previous, current, index, array){
  return previous.value + current.value;
});

However, Object does not seem to have any reduce() method implemented.

Answer 1

One option would be to reduce the keys():

var o = { 
    a: {value:1}, 
    b: {value:2}, 
    c: {value:3} 
};

Object.keys(o).reduce(function (previous, key) {
    return previous + o[key].value;
}, 0);

With this, you'll want to specify an initial value or the 1st round will be 'a' + 2.

If you want the result as an Object ({ value: ... }), you'll have to initialize and return the object each time:

Object.keys(o).reduce(function (previous, key) {
    previous.value += o[key].value;
    return previous;
}, { value: 0 });