How do I recursively use the reduce() method in javascript?

Question

The question is to use reduce() to operate on an array of arrays and return a homogeneous array with no sub-arrays. Eg - [1,2,[3,[4,5]]] will return [1,2,3,4,5].

Here is code that works, considering the sub-array itself is not another array of arrays-

var a = [3,[4,[5,[6,7]]]];
var b = 8;
var c = [9,10];
var x = []

var arr = [1,2,a,b,c];
arr = [1,2,3,4,c];

console.log(arr.reduce(function oneArray(a,b)
    {
        return a.concat(b); 
    },x))

Notice that I change my arr array as the code does not work for the first arr array i.e [1,2,a,b,c]. For arr [1,2,3,4,c] the object returned is [1,2,3,4,9,10]

Here is my attempt to make it work for the first arr-

function oneArray(a,b)
    {
        if (typeof b == "object")
            b = b.reduce(oneArray(x,y),[]);
        return a.concat(b); 
    }

    console.log(arr.reduce(oneArray(a,b),x));

This gives me the error

Uncaught TypeError: [object Array] is not a function(anonymous function) @ Flattening.html:27

It interprets my function oneArray as an array not a function

I understand the problem comes from the fact that reduce can (I think) work only with a function defined as a parameter. Does anyone have a solution so that I get [1,2,3,4,5,6,78,9,10] as my result working around the limitations of reduce() unless I missed something?

Solution

Reduce with a test for an array and a recursive call of the reduce function.

Edit: Kudos to Jonathan Lonowski for the hint.

function flat(r, a) {
    if (Array.isArray(a)) {
        return a.reduce(flat, r);
    }
    r.push(a);
    return r;
}

var array = [3, [4, [5, [6, 7]]]];
document.write('<pre>' + JSON.stringify(array.reduce(flat, []), 0, 4) + '</pre>');

Comments

[Stephan Bijzitter]

I have only read the title, but in the title you're asking the wrong question. You have situation A and you want to get to situation B. Explain what you want; not how you think you want it. If you knew how, you would not have to ask a question.

[Jonathan Lonowski]

To provide a function as an argument, pass it by name alone – arr.reduce(onArray, x). Including parenthesis after the name invokes the function first and provides its return value to arr.reduce().

[Dmitri Pavlutin]

Why duplication? Here is required recursion, in the other question are just 2 level arrays.

[Jonathan Lonowski]

Note: The line for reducing embedded arrays could be shortened to return a.reduce(flat, r);. It'll .push() to the same array throughout the call stack vs. creating a new array to concat.

[Nina Scholz]

@JonathanLonowski, thank you, pl see edit.