Grouping elements in array by multiple properties is the closest match to my question as it indeed groups objects by multiple keys in an array. Problem is this solution doesn’t sum up the properties value then remove the duplicates, it instead nests all the duplicates in a two-dimensional arrays.
Expected behavior
I have an array of objects which must be grouped by shape
and color
.
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} ];
Objects in this array are considered duplicates only if both their shape
and color
are the same. If they are, I want to respectively sum up their used
and instances
values then delete the duplicates.
So in this example result array may only contain four combinations : square red
, square blue
, circle red
, circle blue
Problem
I tried a simpler approach here:
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: 'red', used: 4, instances: 4}, {shape: 'square', color: 'red', used: 2, instances: 2} ]; result = []; arr.forEach(function (a) { if ( !this[a.color] && !this[a.shape] ) { this[a.color] = { color: a.color, shape: a.shape, used: 0, instances: 0 }; result.push(this[a.color]); } this[a.color].used += a.used; this[a.color].instances += a.instances; }, Object.create(null)); console.log(result);
but it outputs
[{shape: "square", color: "red", used: 11, instances: 9}, {shape: "circle", color: "blue", used: 4, instances: 4}]
instead of expected result:
[{shape: "square", color: "red", used: 5, instances: 3}, {shape: "circle", color: "red", used: 2, instances: 1}, {shape: "square", color: "blue", used: 11, instances: 9}, {shape: "circle", color: "blue", used: 0, instances: 0}]
How can I get my function to properly group the objects by shape and color ? i.e. sum up their values and remove the duplicates ?
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Answer
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);