A couple of functions to flatten arrays, with tests. See http://jsbin.com/zoqorudoka/edit?js,console

flattenArrays.js

/**
 * Test Inputs
 */
a = [[1,2,[3.23,4,"dfger",[5]]],4];
a2 = [[[[[0]], [1]], [[[2,5,6], [3]]], [[4], [5]]]];

/**
 * Recursively flatten an array
 */
function flatten (array) {
    var result = [];
    for (var i = 0; i < array.length; i++) {
        var value = array[i];
        Array.isArray(value) ? result = result.concat(flatten(value)) : result.push(value);
    }
    return result;
}

/**
 * Flatten an array using a string trick
 */
function flt(a){return (""+a).split(',')}
function fltNums(a){return (""+a).split(',').map(Number)}

/**
 * Demonstrate each function with each input
 */
console.log("Flatten mixed types recursively", flatten(a));
console.log("Flatten mixed depths recursively", flatten(a2));
console.log("Flatten mixed types using a string trick", flt(a));
console.log("Flatten mixed depths using a string trick", flt(a2));
console.log("Flatten mixed depths using a string trick, converting each value to a number", fltNums(a2));



/***********
 * Testing
 ***********/

/**
 * Simple array comparison
 */
function arraysAreEqual (array, array2) {
    return JSON.stringify(array) === JSON.stringify(array2);
}

/**
 * Assert a thing to be true
 */
function assert (outcome, description) {
    if (!outcome) throw "error attempting to " + description;
}


/**
 * Test Battery
 */
function runTests () {
  assert(arraysAreEqual(flatten(a), [1, 2, 3.23, 4, "dfger", 5, 4]), "Flatten mixed types recursively");
  assert(arraysAreEqual(flatten(a2), [0, 1, 2, 5, 6, 3, 4, 5]), "Flatten mixed depths recursively");
  assert(arraysAreEqual(flt(a), ["1", "2", "3.23", "4", "dfger", "5", "4"]), "Flatten mixed types using a string trick");
  assert(arraysAreEqual(flt(a2), ["0", "1", "2", "5", "6", "3", "4", "5"]), "Flatten mixed depths using a string trick");
  assert(arraysAreEqual(fltNums(a2), [0, 1, 2, 5, 6, 3, 4, 5]), "Flatten mixed depths using a string trick, converting each value to a number");
}
  
/**
 * Test Runner
 */
runTests();