var arr = [
    [0,1,2,3],
    [4,5,6,7],
    [8,9,0,1],
    [2,3,4,5]
];

1.兩層迴圈

var result = [];
for(var i=0;i<arr.length;i++){
    for(var j=0;j<arr[i].length;j++){
        result.push(arr[i][j]);
    }
}

2.用concat代替內層迴圈

var result = [];
for(var i=0;i<arr.length;i++){
    result = result.concat(arr[i]);
}
 

3.用apply代替外層遍歷

var result = [].concat.apply([],arr);