Javascript 中几种数组赋值的区别及=号的作用

Wayne posted @ Fri, 18 Nov 2016 15:17:26 +0000 in Experience with tags Javascript , 2359 readers

Javascript中，目前要把一个数组的内容等于另一个数组，我会的手段大致有三种：

  1. a = b;

  2. a = b.slice(0);

  3. b.forEach(function(element){ a.push(element);});

这三种有什么不同呢？

 

这三种写法的不同究其原因都出在=号的作用上。数组间的等号可以认为是一个别名。也就是 a = b 的效果等同于 b.alias => a， 给 b 取了个叫a的别名。数组还是同一个数组，多了个名字而已。于是不论用名字 a 操作还是用名字 b 操作，改变的都是同一个数组，换个名字都能看到效果。比如：

var b = [1];
var a = b;
a.push(2);
b.push(3);
console.log(a);  //[1,2,3]
console.log(b);  //[1,2,3]

而 b.slice(0) != b 本身， 只是等于 b 的内容。 a = b.slice(0); 相当于先 new 了一个新的匿名 array，内容为 b 的内容，然后取了个名字叫 a。因为是new出来的，所以用 a = b.slice(0) 后，操作 a  和操作 b 是操作两个不同的数组。

 

那么，什么时候用第三种呢？要用第三种，很明显，首先 a 必须是一个已经存在的数组，不然会报 undefined。第三种的使用场景有比如这样的：

var a = [];
var b = [1];
var c;
afunction(a)｛ c = a;｝;  //一个函数，使用a作为传入参数,只调用一次

//此时想把 b 的值赋给c。

因为 c 只在函数体内才能被赋值，而这个值只能通过a来进行。 如果用 a = b， 那么效果等同于 (Original Array a) . removeName(a), 然后 b.alias(a);  而 c 依然是那个 Original Array a，只是丢失了原本a这个名字，只剩下c这个名字了。类似的，如果用 a = b.slice(0), 也无法改变c，只是让c这个数组丢失了另一个叫a的名字而已。这时候，用第三种，把 b 的内容逐个加到已有的 a 里面，才能同时操作到 c。

 

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