JS合并数组的几种方法及优劣比较
本文属于JavaScript的基础技能. 我们将学习结合/合并两个JS数组的各种常用方法,并比较各种方法的优缺点.
我们先来看看具体的场景:
var q = [ 5, 5, 1, 9, 9, 6, 4, 5, 8];
var b = [ "tie", "mao", "csdn", "ren", "fu", "fei" ];
很明显,数组 q 和 b 简单拼接的结果是:
[
5, 5, 1, 9, 9, 6, 4, 5, 8,
"tie", "mao", "csdn", "ren", "fu", "fei"
]
concat(..)方法
最常见的用法如下:
var c = q.concat( b );
q; // [5,5,1,9,9,6,4,5,8]
b; // ["tie","mao","csdn","ren","fu","fei"];
c; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]
如您所见, c 是一个全新的数组, 表示 q 和 b 这两个数组的组合, 但是 q 和 b 现在没用了是吧?
如果 q 数组有10000个元素, b 数组也有有10000个元素? 那么数组c现在就有20000个元素, 这种方式占用了2倍的内存.
“这没问题!”,你可能会觉得. 只要将 q 和 b 置空就行, 然后就会被垃圾回收,对吗?问题解决了!
q = b = null; // `q` and `b` 现在可以被垃圾回收了
额? 如果数组都很小,那自然没问题. 但对大型的数组,或需要多次重复处理时, 内存就被限制了, 它还需要进行优化.
循环插入
OK, 让我们把一个数组的内容加入到另一个中试试,使用 Array#push() 方法:
// 将数组 `b` 插入 `q`
for (var i=0; i < b.length; i++) {
q.push( b[i] );
}
q; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]
b = null;
现在, q中存放了两个原始数组的内容(q + b).
看样子对内存优化做的不错.
但如果 q 数组很小而 b 又很大呢? 出于内存和速度的考虑,这时想把较小的 q 插入到 b 前面. 没问题,只要用 unshift() 方法代替 push() 即可, 对应的也要从大到小进行循环遍历:
// `q` into `b`:
for (var i=q.length-1; i >= 0; i--) {
b.unshift( q[i] );
}
b; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]
q = null;
实用技巧
悲催的是,for循环很土并且难以维护. 我们能做得更好吗?
我们先试试 Array#reduce :
// `b` onto `q`:
q = b.reduce( function(coll,item){
coll.push( item );
return coll;
}, q );
q; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]
// or `q` into `b`:
b = q.reduceRight( function(coll,item){
coll.unshift( item );
return coll;
}, b );
b; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]
Array#reduce() 和 Array#reduceRight() 很高大上,但有点笨重,而且一般人也记不住. JS规范6 中的 => 箭头函数(arrow-functions) 能让代码量大大减少, 但需要对每个数组元素执行函数调用, 也是很渣的手段.
那么下面的代码怎么样呢?
// `b` onto `q`:
q.push.apply( q, b );
q; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]
// or `q` into `b`:
b.unshift.apply( b, q );
b; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]
