Filter an object object in javascript (filter or reduce?)

Question

What the best practice to filter or reduce an object? I would have done this via for-loop and created a new array but understand that you can do it somehow via filter property in JavaScript?

var myObject = [
  {dimensions: [451, 255], margins: [0, 2, 0, 29]}, 
  {dimensions: [222, 390], margins: [0, 5, 0, 37]},
  {dimensions: [333, 390], margins: [0, 8, 0, 37]}
];

I would like to filter out separately the dimensions first, property and margins second and fourth, property in a array to have:

var dimension = [ 451, 222, 333 ];
var margins = [ 2, 29, 5, 37, 8, 37 ];

Also if I filter an object and update these variables, is there a way to map them back? Or should I have it as below to map afterwards:

var dimension = [ a: 451, b: 222, c: 333];
var margins = [ a: [2, 29], b : [5, 37], c: [8, 37] ];

Answer 1

You can use reduce :

var myObject = [{
  dimensions: [451, 255],
  margins: [0, 2, 0, 29]
}, {
  dimensions: [222, 390],
  margins: [0, 5, 0, 37]
}, {
  dimensions: [333, 390],
  margins: [0, 8, 0, 37]
}];


var results = myObject.reduce(function(acc, item) {
  acc.dimensions.push(item.dimensions[0]);
  acc.margins.push(item.margins[1], item.margins[3]);
  return acc;
}, { dimensions: [], margins: [] });

console.log(results); // {dimensions: Array[3], margins: Array[6]}

Working fiddle: https://jsfiddle.net/9ynez03c/

To map back to myObject :

var dimensions = results.dimensions;
var margins = results.margins;

var updateMyObject = function() {
  myObject.forEach(function(item, i) {
    item.dimensions[0] = dimensions[i];
    item.margins[1] = margins[i * 2];
    item.margins[3] = margins[i * 2 + 1];
  })
};
updateMyObject();

This is very clunky. I'm not sure how you're using this data, but something tells me that you should be using an object in the first place and then accessing/updating properties as needed instead of creating additional arrays.

New fiddle with update: https://jsfiddle.net/adrice727/g01s80qf/

Answer 2

Filtering works fine, if you only want a subset of an array which meets some conditions (like all numbers greater than 5 from an array of arbitrary numbers). In your case I would definitely go for a loop.

 var myObject = [ {dimensions: [451, 255], margins: [0, 2, 0, 29]}, {dimensions: [222, 390], margins: [0, 5, 0, 37]}, {dimensions: [333, 390], margins: [0, 8, 0, 37]} ] var dimensions = []; var margins = []; myObject.forEach(function(item) { dimensions.push(item.dimensions[0]); margins.push(item.margins[1]); margins.push(item.margins[3]); // or by using concat(): // dimensions = dimensions.push(item.dimensions[0]); // margins = margins.concat([item.margins[1], item.margins[3]]); }); console.log("dimensions:", dimensions); console.log("margins:", margins);