Javascript Recursive Error

Question

My goal is to write a function that will flat an array. So for example [1,[2,3]] should turn to [1,2,3]. I tried it with a recursive method as seen below, but it produces an endless loop.

function steamrollArray(arr) {

 var resultArray = [];

 function flatArray(array) {    
   for (i = 0; i < array.length; i++){
     if (Array.isArray(array[i])) {
         flatArray(array[i]);
     } else resultArray.push(array[i]);
   }
 }

 flatArray(arr);         
 return resultArray;
}

steamrollArray([1, [2,3]]);

What is my mistake?

Thanks in advance

Also, steamrollArray([1, [2, 3]]) produces [1, 2, 3] as expected. — Assan
– Assan, Commented Jan 4, 2017 at 16:23
Declare your variables, i is global, hence every call to flatArray makes it start from zero. — Teemu
– Teemu, Commented Jan 4, 2017 at 16:23
Additionally, if you use lodash, you can take a look at _.flattenDeep — Assan
– Assan, Commented Jan 4, 2017 at 16:24

Nina Scholz · Accepted Answer · 2017-01-04 16:54:57Z

5

You need to move resultArray inside of flatArray and concat the recursive call of flatArray to the result as well.

It is a good approach to see a recursion from the end, that means, you need an array as result, then you need to declare an array at start and return the array at last, or inbetween of the function.

While processing the data, you need to append the array with either a single item or with an array as result of a recursive call of a sub array. Both parts are in the if clause.

function steamrollArray(arr) {
    function flatArray(array) {
        var resultArray = [], // move inside
            i;                // declare as well
        for (i = 0; i < array.length; i++) {
            if (Array.isArray(array[i])) {
                resultArray = resultArray.concat(flatArray(array[i]));
                // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            } else {
                resultArray.push(array[i]);
            }
        }
        return resultArray; // return result here
    }

    return flatArray(arr); // return the result of the call
}

console.log(steamrollArray([1, [2, 3]]));
console.log(steamrollArray([[1, 9, [7, 8]], [2,3]]));

edited Jan 4, 2017 at 16:54

answered Jan 4, 2017 at 16:23

Nina Scholz

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1 Comment

Bene Over a year ago

Ok, it works fine. Can you tell my why I have to make these changes?

Ala Eddine JEBALI · Accepted Answer · 2017-01-04 16:29:24Z

1

You can simply do that:

var yourArray = [1, [2,3,4,5], [6,7], 8, [9]];
var result = yourArray.reduce(function(a, b) {
	if( a.constructor === Array){
		return a.concat(b);
	}
	return a;
}, []);
console.log(result);

answered Jan 4, 2017 at 16:29

Ala Eddine JEBALI

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1 Comment

Bene Over a year ago

this doesn't work for 'deeper' arrays e.g. [1, [[[2,3],4],5]]

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