Flatten a JavaScript array -- why isn't this working?

Question

I'm trying to flatten an array with randomly nested arrays inside. I'm not sure why the function I wrote ends up in an infinite loop:

let array = [1, 2, [3]]
var final_array = [] 

function flattener(array){
  for(i = 0; i < array.length; i++){
    if(array[i] instanceof Array){
      flattener(array[i])
    }else{
      final_array.push(array[i])
    }
  }
}

flattener(array)

What I think SHOULD happen is:

When I'm in the for loop checking for [3], it goes into the if statement, flattener gets called again, it resolves, and then I exit the if statement.

Instead, the if statement keeps calling to check [3] infinitely, and I'm not sure why this happens.

Setting a breakpoint and running through line by line would've probably shown you what is going wrong. — nnnnnn
– nnnnnn, Commented Mar 12, 2017 at 4:30

CrackpotCrocodile · Accepted Answer · 2017-03-12 04:28:36Z

8

The problem is you didn't declare the i variable, so it's leaking into the global space and being reset when it recurses.

Change:

for(i = 0; i < array.length; i++){

To:

for(var i = 0; i < array.length; i++){

answered Mar 12, 2017 at 4:28

CrackpotCrocodile

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1 Comment

pineapplecakes Over a year ago

oh my god. THANK YOU SO MUCH I LOVE YOU.

jherax · Accepted Answer · 2017-03-12 04:32:20Z

2

This is another approach using Array.reduce and Array.concat.

/**
 * Flattens an array of arrays into one-dimensional array
 * @param  {Array} arr: the array to be flattened
 * @return {Array}
 */
function flatten(arr) {
  return arr.reduce(function (flattened, cvalue) {
    return flattened.concat(Array.isArray(cvalue) ? flatten(cvalue) : cvalue);
  }, []); // initial value of flattened array []
}

Testing it...

let array = [1, 2, [3]]
var falttened = flatten(array)

Take a look at his gist: Array flatten

answered Mar 12, 2017 at 4:32

jherax

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Comments

Jaromanda X · Accepted Answer · 2017-03-12 04:35:11Z

2

Array#concat plus Array#map is yet another way you can achieve this

var flattener = arr => [].concat.apply([], arr.map(item=>Array.isArray(item) ? flattener(item) : item));

or the ES5 version

var flattener = function flattener(arr) {
  return [].concat.apply([], arr.map(function (item) {
    return Array.isArray(item) ? flattener(item) : item;
  }));
};

answered Mar 12, 2017 at 4:35

Jaromanda X

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Comments

Saurabh Agrawal · Accepted Answer · 2017-03-12 04:52:01Z

1

Change

 for(i = 0; i < array.length; i++)

to

 for(var i = 0; i < array.length; i++)

answered Mar 12, 2017 at 4:52

Saurabh Agrawal

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