Sum the same object of array

Question

var data = [ {id:1, qty:100}, {id:2, qty:200}, {id:1, qty:100}, {id:2, qty:200} ];

How to sum this array become to [ {id:1, qty:200}, {id:2, qty:

Answer 1

You can do it by using the functional way, with the .reduce() and .map() method.

In my example, i've made a function to make the process more generic.

  var data = [ {id:1, qty:100}, {id:2, qty:200}, {id:1, qty:100}, {id:2, qty:200} ];

  function reducer(data, groupBy, reduceValue){
    //Reduce your data and initialize it with an empty object
    return [].concat.apply(data.reduce(function(hash, current){
      //If hash get current key, retrieve it and add sum property
      hash[current[groupBy]] = (hash[current[groupBy]] || 0) + current[reduceValue];
      //Return our current hash
      return hash;
    }, {})).map(function(elm){
      //Retrieve object keys
      return Object.keys(elm).map(function(key){
        //Build our object
        var obj = {};
        obj[groupBy] = +key;
        obj[reduceValue] = elm[key];
        return obj;
      });
    })[0];
  }

  console.log(reducer(data, 'id', 'qty'));
  //[ {id:1, qty:200}, {id:2, qty:400} ];

This function takes 3 arguments

• the data which will be reduced
• the groupBy parameter
• the key of the object to sum data