Sort array containing objects based on another array using _Underscore

Question

I already read the previous questions answered, but it didn't fit with my need.

I have an array of objects such as

var Widgets = [
             [{Id: 'abcdef', post_id: 12345}],
             [{Id: 'ghijkl', post_id: 45678}],
             [{Id: 'mnoptq', post_id: 90123}]
];

I have a second array :

var sortArray = ['ghijkl', 'mnoptq', 'abcdef'];

I need To reorder Widgets with the initial order of elements which appears on sortArray

i've succeed to do it this way

sortArray.forEach(function(Id) {
                    var found = false;                  
                    Widgets = Widgets.filter(function(Widget) {
                        if(!found && Widget.Id == Id) {
                            NewWidgets.push(Widget);
                            found = true;
                            return false;
                        } else {
                            return true;
                        }
                    });
                });

But I wish to improve this code by using _SortBy but I didn't succeed so far...

Anyhelp ?

Edit

Final result should be

var Widgets = [
             [{Id: 'ghijkl', post_id: 45678}],
             [{Id: 'mnoptq', post_id: 90123}],
             [{Id: 'abcdef', post_id: 12345}]
];

This is unclear, the array has no order, it's all the same ? — adeneo
– adeneo, Commented Feb 23, 2015 at 12:19

georg · Accepted Answer · 2015-02-23 12:34:23Z

5

Like this?

sorted = _.sortBy(Widgets, function(x) {
    return _.indexOf(sortArray, x[0].Id)
})

This is not very efficient, a faster way is to convert sortArray to an object key=>index and use hash lookups in sortBy:

sortObj = _.invert(_.object(_.pairs(sortArray)));

sorted = _.sortBy(Widgets, function(x) {
    return sortObj[x[0].Id]
})

edited Feb 23, 2015 at 12:34

answered Feb 23, 2015 at 12:27

georg

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2 Comments

Toucouleur Over a year ago

Works like a charm even if I made this change to fit with my needs : sortArray = _.invert(_.object(_.pairs(sortArray))); Widgets = _.sortBy(Widgets, function(Widget) { return sortArray[Widget.objectId]; });

JayRu Over a year ago

I had to add parseInt to make this work properly. I'm pretty sure it's because my sort array was > 10 items and the sortObj was a collection of strings. So: return parseInt(sortObj[x[0].Id]), 10)

astorije · Accepted Answer · 2016-07-13 05:26:40Z

On top of @georg's solution, if performance is really important to you, and you have to keep something like this¹, then preparing the key/index object using the following will be much faster:

// Example: ['a', 'b', 'c'] becomes { 'a': 0, 'b': 1, 'c': 2 }
var sortObj = sortArray.reduce(function(acc, value, index) {
  acc[value] = index;
  return acc;
}, {});

// Same as posted by @georg, with no need for `parseInt`
sorted = _.sortBy(Widgets, function(Widget) {
  return sortObj[Widget[0].Id];
});

It's a bit longer but also I'd argue it's also easier to read than the invert/object/pairs.

Also, note that there is no need for parseInt with this solution, because it builds an object of integer values, while the other solution returns strings.

¹ Because let's face it, there are other ways to improve this if you actually can give it a bigger refactor.

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