Concatenate an unknown number of arrays in Javascript/NodeJS

Question

I have a function in one of my controllers where I populate an array of references to a document, which, when populated, have embedded arrays themselves.

Here's an example:

The mongoose populate function gives me an array of objects. Within each of those objects is an array:

[{ name: Test, array: [ 1, 2, 3, 4, 5 ] }, { name: TestAgain, array: [1, 2, 3, 4, 5] }, { name: Test^3, array: [1, 2, 3, 4, 5]}, {...

The desired output would be:

[1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, ...]

I need to concatenate all of the "arrays" within the populated references. How can I do this without knowing how many arrays there are?

For reference, here is (generally) what my function looks like:

exports.myFunctionName = function ( req, res, next )

Document.findOne({ 'document_id' : decodeURIComponent( document_id )}).populate('array_containing_references').exec(function( err, document) 
{
   //Here I need to concatenate all of the embedded arrays, then sort and return the result as JSON (not worried about the sorting).
});

That's not clear at all. simplify this by 2 factors at least (since it looks like you're after something very simple). Show input, desired output, and what you've tried so far. — Amit
– Amit, Commented Jul 15, 2015 at 22:11

Amit · Accepted Answer · 2015-07-15 22:43:51Z

1

Assuming your input is in the document variable, try this:

var output = document.reduce(function (res, cur) {
  Array.prototype.push.apply(res, cur.array);
  return res;
}, []);

Or this:

var output = [];
document.forEach(function(cur) {
  Array.prototype.push.apply(output, cur.array);
});

edited Jul 15, 2015 at 22:43

answered Jul 15, 2015 at 22:25

Amit

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1 Comment

Amit Over a year ago

@RobG - yeah.. that was an afterthought... fixing

Tony · Accepted Answer · 2015-07-15 22:46:56Z

You want to take each document and do something with a property from it. Sounds like a great use case for Array.prototype.map!

map will get each document's array value and return you an array of those values. But, you don't want a nested array so we simply use Array.prototype.concat to flatten it. You could also use something like lodash/underscore.js flatten method.

var a = [
  { name: 'test1', array: [1, 2, 3, 4, 5]}, 
  { name: 'test2', array: [1, 2, 3, 4, 5]}, 
  { name: 'test3', array: [1, 2, 3, 4, 5]}
];

var results = Array.prototype.concat.apply([], a.map(function(doc) { return doc.array; }));

document.body.innerHTML = results;

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