function testForEach(array) {
    array.reduce((res, cur) => {
        if (Array.isArray(cur)) {
            cur.forEach(i => res.push(i));
        } else {
            res.push(i);
        }
        return res;
    }, []);
}

function testConcat(array) {
    array.reduce((res, cur) => {
        if (Array.isArray(cur)) {
            res.concat(cur);
        } else {
            res.push(i);
        }
        return res;
    }, []);
}

function testSpread(array) {
    array.reduce((list, i) => (Array.isArray(i) ? [...list, ...i] : [...list, i]), []);
}

function measureTime(fun) {
    console.time(fun.name);
    fun(Array.prototype.slice.call(arguments, 1));
    console.timeEnd(fun.name);
}

function generateInput() {
    let result = [];

    function generateMember() {
        let rand = Math.random();
        if (rand < 0.05) {
            return new Array(5);
        } else if (rand < 0.7) {
            return rand;
        } else {
            return 'AAAAAAAAAAA';
        }
    }
    for (let i = 0; i < 1000000; i++) {
        result.push(generateMember());
    }
    return result;
}

let input = generateInput();


measureTime(testForEach, input);
measureTime(testSpread, input);
measureTime(testConcat, input);