Reduce array to object using arrow function

Question

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);

Nenad Vracar · Accepted Answer · 2018-03-10 12:29:31Z

9

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);

answered Mar 10, 2018 at 12:29

Nenad Vracar

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3 Comments

Basti Over a year ago

Wow! Exactly what I was looking for, I knew there are no limits :D may you add a reference what => (a, b) does? I don't fully understand it yet.

Nenad Vracar Over a year ago

It will return last expression inside () in this case its accumulator parameter in reduce or byId

Slai Over a year ago

@Basti it's comma operator developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

Nina Scholz · Accepted Answer · 2020-08-02 07:05:48Z

6

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);

edited Aug 2, 2020 at 7:05

answered Mar 10, 2018 at 12:28

Nina Scholz

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2 Comments

Basti Over a year ago

great! I thought of something like this, but didn't know how to assign the dynamic key :)

Mosh Feu Over a year ago

Thanks! You already received my +1 before ;) (And even posted about it).

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