function insertSort(arr=[]){
    let length = arr.length,
        current,
        preIndex;

    for(let i=1;i<length;i++){
        current = arr[i];
        preIndex = i - 1;

        // current 每次是不变的，相当于是把值暂存起来，
        // 每次比较都是把前面一个元素的值赋给后面一个
        while( preIndex >= 0 && current < arr[preIndex] ){
            arr[preIndex+1] = arr[preIndex];
            preIndex--;
        }

        arr[preIndex+1] = current;
    }

    return arr;
}

console.log( insertSort([4,3,1,72,5,2]) ) //[ 1, 2, 3, 4, 5, 72 ]