Javascript create array in for loop

Question

I'm pretty sure this is a basic question but my PHP background is making me unable to solve this. I was also unable to find a solution that came even close in SO.

I have an array of objects containing a value I want to loop and sum. Everything works well but I would like to directly sum those values to another array in the output.

So this is the working function:

 function postsCalc(arrayEntry) {
 var postsOutput = [];
 var postLikes = 0;
 for (var i = 0, len = arrayEntry.length; i < len; i++) {
 postLikes += arrayEntry[i].likes.summary.total_count;
 } 
 postsOutput.likes = postLikes;
 return postsOutput; 
 }

Output:

likes : 55555;

Which works well, but can't I push it directly to the array key and avoid having to do the postsOutput.likes = postLikes?

Like this:

 function postsCalc(arrayEntry) {
 var postsOutput = [];
 for (var i = 0, len = arrayEntry.length; i < len; i++) {
 postsOutput.likes += arrayEntry[i].likes.summary.total_count;
 // or even a multidimensional:
 postsOutput.totals.likes += arrayEntry[i].likes.summary.total_count;
 } 
 return postsOutput; 
 }

and that would output the same as above but avoiding the extra step, is this possible in Javascript?

Thanks in advance

• Dekel: In php you have associative arrays, but in javascript arrays are non-associative (only contain values, not keys). If you want keys - you are talking about objects (and not Arrays).

Answer 1

You can use the reduce function on a list to do this:

function postsCalc(arrayEntry) {
 var postLikes = arrayEntry.reduce(function(a, b) {return a + b.likes.summary.total_count}, 0);
 return {likes: postLikes}; 
}

It works like this: Array.reduce(callback, initialValue) where callback = function(a, b) where a is the accumulator (e.g. tracks the sum, etc) and b is a representation of the item you're iterating over on the list. initialValue is the starting point for the function (e.g. on the first iteration).

• nunorbatista: This worked like a charm, it's a very neat solution. I'm going to read more about .reduce because it seems very interesting, thanks!
• abigperson: Happy to help. The Array prototype methods are all very helpful once you get the hang of them.

Answer 2

Idiomatic javascript would look like (for your first function):

function postsCalc(arrayEntry) {
 var postsOutput = {
 postLikes: 0
 };
 for (var i = 0, len = arrayEntry.length; i < len; i++) {
 postsOutput.postLikes += arrayEntry[i].likes.summary.total_count;
 } 
 return postsOutput; 
}

or, now that we can assume .forEach exists:

function postsCalc(arrayEntry) {
 var postsOutput = {
 postLikes: 0
 };
 arrayEntry.forEach(function(entry) {
 postsOutput.postLikes += entry.likes.summary.total_count;
 } 
 return postsOutput; 
}

• nunorbatista: In your example I still have to create an empty object to assign the values later and that's what I was trying to avoid. The accepted solution is simpler but that you very much for your help!