array flatten

flatten.js

function flatten(arr) {
    var result=[],
        concat = Array.prototype.concat,
        push = Array.prototype.push,
        element,
        array = arr.slice(); //shadow copy the array
    for(var i = 0, l = array.length; i < l; i++) {
        element = arr[i];
        if(isArray(element)) {
            //for proformance reason, if all element flattened already, do not recurcivly call
            if(isAllElementFlatten(element)) {
                result = concat.call(result,element);
            }
            else {
                push.apply(result,flatten(element));
            }
        }
        else {
            result.push(element);
        }
    }
    return result;

    /** helper methods  */
    function isAllElementFlatten(arr) {
        return isAll(arr, function(ele) {
            return !(isArray(ele));
        });
    }

    function isAll(arr, predict) {
        return arr.map(function(ele){
            return predict(ele) ?  1 : 0;
        }).reduce(function(pre, curr){
            return pre + curr
        },0) === arr.length
    }

    function isArray(arr) {
        if(Array.isArray) return Array.isArray(arr);
        return Object.prototype.toString.call(arr) === '[object Array]';
    }
}


//poorman's unittest
function test(){
    var testcases = {
        'test1' : [1,3,[2,3,[3,4]],2,3,[1,2,3]],
        'test2' : ['a',['b',['c']]],
        'test3' : [1,[2,[3,[4,[5]]]]]
    },
    baselines = {
        'test1' : [1,3,2,3,3,4,2,3,1,2,3],
        'test2' : ['a','b','c'],
        'test3' : [1,2,3,4,5]
    },
        baseline,
        actual;

    for(var testcase in testcases) {
        baseline = baselines[testcase];
        actual = flatten(testcases[testcase]);
        if(baseline.toString() === actual.toString()) {
            console.log('Testcase: '+testcase+' passed');
        }
        else {
            console.log('Testcase: '+testcase+' failed');
        }
    }
}
test();