Using array.reduce to count occurances in an array of objects

Question

I have this array of voters:

let voters = [
    {name:'Bob' , age: 30, voted: true},
    {name:'Jake' , age: 32, voted: true},
    {name:'Kate' , age: 25, voted: false},
    {name:'Sam' , age: 20, voted: false},
    {name:'Phil' , age: 21, voted: true},
    {name:'Ed' , age:55, voted:true},
    {name:'Tami' , age: 54, voted:true},
    {name: 'Mary', age: 31, voted: false},
    {name: 'Becky', age: 43, voted: false},
    {name: 'Joey', age: 41, voted: true},
    {name: 'Jeff', age: 30, voted: true},
    {name: 'Zack', age: 19, voted: false}
];

That I need to count how many voted (true).

I was able to find it using a for loop, but I'm learning the reduce method, and would like to apply it here but can't seem to figure out how.

// works perfectly
function totalVotes(voters){
      let total = 0;
      for (i=0; i < voters.length; i++){
        if (voters[i].voted === true) {
          total++
        }
      }
      return total
    }

function totalVotes2(voters){
      voters.reduce(function (contador, item){
        if (item.voted === true){
          console.log(item)
          console.log(contador)
          //cant figure out how to make it count
          }
          // and how to return it
      }, 0)
    }

Unmitigated · Accepted Answer · 2020-08-20 00:30:59Z

2

You can set the accumulator's initial value to 0 and add 1 to it if the current element's voted property is true and add 0 otherwise.

let voters = [
    {name:'Bob' , age: 30, voted: true},
    {name:'Jake' , age: 32, voted: true},
    {name:'Kate' , age: 25, voted: false},
    {name:'Sam' , age: 20, voted: false},
    {name:'Phil' , age: 21, voted: true},
    {name:'Ed' , age:55, voted:true},
    {name:'Tami' , age: 54, voted:true},
    {name: 'Mary', age: 31, voted: false},
    {name: 'Becky', age: 43, voted: false},
    {name: 'Joey', age: 41, voted: true},
    {name: 'Jeff', age: 30, voted: true},
    {name: 'Zack', age: 19, voted: false}
];
let res = voters.reduce((acc, {voted})=>acc + (voted ? 1 : 0), 0);
console.log(res);

You could also make use of the fact that true can be coerced to 1 and false can be coerced to 0 in JavaScript, obviating the need for the ternary operator.

let voters = [
    {name:'Bob' , age: 30, voted: true},
    {name:'Jake' , age: 32, voted: true},
    {name:'Kate' , age: 25, voted: false},
    {name:'Sam' , age: 20, voted: false},
    {name:'Phil' , age: 21, voted: true},
    {name:'Ed' , age:55, voted:true},
    {name:'Tami' , age: 54, voted:true},
    {name: 'Mary', age: 31, voted: false},
    {name: 'Becky', age: 43, voted: false},
    {name: 'Joey', age: 41, voted: true},
    {name: 'Jeff', age: 30, voted: true},
    {name: 'Zack', age: 19, voted: false}
];
let res = voters.reduce((acc, {voted})=>acc + voted, 0);
console.log(res);

answered Aug 20, 2020 at 0:30

Unmitigated

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6 Comments

Leonardo Nobre Ghiggino Over a year ago

Dear @hev1, thanks! This works but now I don't understand how does {voted} captures the value of that KVP. Could you help me figure that out?

Leonardo Nobre Ghiggino Over a year ago

I just used {age}, with the accumulator set to zero, and it added up all ages, as expected. But I still don't understand how could {age} pick the value of age:number

Unmitigated Over a year ago

It's a destructuring assignment.

Leonardo Nobre Ghiggino Over a year ago

ooooooooooooooooooooooohhhhhhhhhhhhhhhhhhhhh! : ) Thanks!

Unmitigated Over a year ago

@LeonardoNobreGhiggino No problem. Does that answer your question?

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