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(resul

Solution 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);

Solution 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 }],
    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);