Reduce array to object using arrow function

问题

I'm playing around with the limits of array and arrow functions and I'm trying to convert this reduce function into an arrow function:

var monthsById = months.reduce(function(result, month) {
                            result[month.Id] = month;
                            return result;
                        }, {});

But I'm having trouble to return the map, since result[month.Id] = month; will return the month and not the map like in this approach:

var monthsById = months.reduce((byId, month) => byId[month.Id] = month, {});

So I'm looking for a single statement, that sets the value AND returns the object. (new Map() is not an option, since I need it in the regular {} format).

var months = [ { Id: 1 }, { Id: 2 }, { Id: 3 } ];

var monthsById = months.reduce((byId, month) => byId[month.Id] = month, {});

console.log(monthsById);

回答1:

You can return byId in each iteration and wrap function body in parentheses ()

var months = [ { Id: 1 }, { Id: 2 }, { Id: 3 } ];

var monthsById = months.reduce((byId, month) => (byId[month.Id] = month, byId), {});
console.log(monthsById);

回答2:

You could use Object.assign where you set a new property with computed property names and return the whole object

var months = [ { Id: 1 }, { Id: 2 }, { Id: 3 } ];

var monthsById = months.reduce((byId, month) => Object.assign(byId, { [month.Id]: month }), {});

console.log(monthsById);

来源：https://stackoverflow.com/questions/49209046/reduce-array-to-object-using-arrow-function