Group objects by multiple properties in array then sum up their values

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Use Array#reduce with a helper object to group similar objects. For each object, check if the combined shape and color exists in the helper. If it doesn’t, add to the helper using Object#assign to create a copy of the object, and push to the array. If it does, add it’s values to used and instances.

var arr = [{"shape":"square","color":"red","used":1,"instances":1},{"shape":"square","color":"red","used":2,"instances":1},{"shape":"circle","color":"blue","used":0,"instances":0},{"shape":"square","color":"blue","used":4,"instances":4},{"shape":"circle","color":"red","used":1,"instances":1},{"shape":"circle","color":"red","used":1,"instances":0},{"shape":"square","color":"blue","used":4,"instances":5},{"shape":"square","color":"red","used":2,"instances":1}];

var helper = {};
var result = arr.reduce(function(r, o) {
  var key = o.shape + '-' + o.color;
  
  if(!helper[key]) {
    helper[key] = Object.assign({}, o); // create a copy of o
    r.push(helper[key]);
  } else {
    helper[key].used += o.used;
    helper[key].instances += o.instances;
  }

  return r;
}, []);

console.log(result);

If you can use ES6, you use a Map to collect the values, and then convert it back to an array by spreading the Map#values:

const arr = [{"shape":"square","color":"red","used":1,"instances":1},{"shape":"square","color":"red","used":2,"instances":1},{"shape":"circle","color":"blue","used":0,"instances":0},{"shape":"square","color":"blue","used":4,"instances":4},{"shape":"circle","color":"red","used":1,"instances":1},{"shape":"circle","color":"red","used":1,"instances":0},{"shape":"square","color":"blue","used":4,"instances":5},{"shape":"square","color":"red","used":2,"instances":1}];

const result = [...arr.reduce((r, o) => {
  const key = o.shape + '-' + o.color;
  
  const item = r.get(key) || Object.assign({}, o, {
    used: 0,
    instances: 0
  });
  
  item.used += o.used;
  item.instances += o.instances;

  return r.set(key, item);
}, new Map).values()];

console.log(result);

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