Reduce array to object using arrow function

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);

2 Answers

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);

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

An example with spreading.

var months = [{ Id: 1 }, { Id: 2 }, { Id: 3 }], monthsById = months.reduce((byId, month) => ({ ...byId, [month.Id]: month }), {});
console.log(monthsById);