CODEWITHSUNDEEP
javascript
José Carlos
José Carlos

Reputation: 2922

Javascript: reduce function of an array of json objects don't return the addition of values

I've got this array of json objects with these data:

      let data = [
    {"id_player_team" : 1, "name" : "Jose", "t2p_conv": 3, "t3p_conv": 5},
    {"id_player_team" : 2, "name" : "Jesus", "t2p_conv": 2, "t3p_conv": 1},
    {"id_player_team" : 3, "name" : "Maria", "t2p_conv": 3, "t3p_conv": 0},
    {"id_player_team" : 4, "name" : "Irene", "t2p_conv": 4, "t3p_conv": 2},
    {"id_player_team" : 5, "name" : "Carmen", "t2p_conv": 1, "t3p_conv": 2},
  ];

I want to get the result of the addition of key "t2p_conv". To do this, I use reduce function of javascript, like this:

let sumt2p = data.reduce((acc, item) => {
     return acc + item.t2p_conv;
});
console.log("Result: " + sumt2p);

When I try to show the value of sumt2p I've got this result:

Result: [object Object]2341

How is possible that? Am I doing something wrong?

Upvotes: 1

Views: 128

Answers (1)

T.J. Crowder
T.J. Crowder

Reputation: 1074198

When you're picking out a property like that, you need to supply the second argument to reduce to provide a seed value for acc parameter:

    let sumt2p = data.reduce((acc, item) => {
         return acc + item.t2p_conv;
    }, 0);
// −−^^^

If you don't supply the seed, the first call to your callback uses the first two entries in the array. Since you're using + on an object (the first object in acc), it gets converted to a string. :-)

This is one of the many reasons that reduce is usually more complicated than necessary. Here, for instance, a simple loop, perhaps with destructuring, does the job:

let sumt2p = 0;
for (const {t2p_conv} of data) {
     sumt2p += t2p_conv;
}

Upvotes: 7

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