在JavaScript中声明多个变量

在JavaScript中,可以声明多个这样的变量:

var variable1 = "Hello World!";
var variable2 = "Testing...";
var variable3 = 42;

......或者像这样:

var variable1 = "Hello World!",
    variable2 = "Testing...",
    variable3 = 42;

一种方法比另一种方法更好/更快吗？

第一种方式更容易维护.每个声明都是一行中的单个语句,因此您可以轻松添加,删除和重新排序声明.

使用第二种方法,删除第一个或最后一个声明是很烦人的,因为它们包含var关键字和分号.每次添加新声明时,都必须将旧行中的分号更改为逗号.

除可维护性外,第一种方法消除了事故全局变量创建的可能性:

(function () {
var variable1 = "Hello World!" // semicolon is missed out accidently
var variable2 = "Testing..."; // still a local variable
var variable3 = 42;
}());

虽然第二种方式不太宽容:

(function () {
var variable1 = "Hello World!" // comma is missed out accidently
    variable2 = "Testing...", // becomes a global variable
    variable3 = 42; // a global variable as well
}());

每个范围的组织使用一个var语句是很常见的.所有"范围"的方式都遵循类似的模式,使代码更具可读性.此外,无论如何,发动机将它们"升起"到顶部.所以保持你的声明在一起模仿实际发生的事情会更紧密.

这样做时更具可读性:

var hey = 23;
var hi = 3;
var howdy 4;

但是这样占用更少的空间和代码行:

var hey=23,hi=3,howdy=4;

它可以是节省空间的理想选择,但让JavaScript压缩器为您处理它.

也许是这样的

var variable1 = "hello world"
, variable2 = 2
, variable3 = "how are you doing"
, variable4 = 42;

除了更改第一个或最后一个变量外,它易于维护和读取.

这只是个人偏好的问题.这两种方式没有区别,除了用第二种形式保存的几个字节,如果你去掉空白区域.

ECMAScript6引入了解构赋值,它非常好用:

[a, b] = [1, 2] a将等于1和b将等于2.

var variable1 = "Hello World!";
var variable2 = "Testing...";
var variable3 = 42;

比以下更具可读性:

var variable1 = "Hello World!",
    variable2 = "Testing...",
    variable3 = 42;

但他们做同样的事情.

使用ES6 Destructuring赋值:它将数组中的值或对象的属性解包为不同的变量.

let [variable1 , variable2, variable3] = 
["Hello World!", "Testing...", 42];

console.log(variable1); // Hello World!
console.log(variable2); // Testing...
console.log(variable3); // 42

for (var i = 0, n = a.length; i < n; i++) {
  var e = a[i];
  console.log(e);
}

a=[3,5,7,11];
(function l () { for (var i = 0, n = a.length; i < n; i++) {
  var e = a[i];
  console.log(e);
}}) ();
console.log(typeof n == "undefined" ?
  "as expected, n was local" : "oops, n was global");

   var /* Vars */
            me = this, that = scope,
            temp, tempUri, tempUrl,
            videoId = getQueryString()["id"],
            host = location.protocol + '//' + location.host,
            baseUrl = "localhost",
            str = "Visit W3Schools",
            n = str.search(/w3schools/i),
            x = 5,
            y = 6,
            z = x + y
   /* End Vars */;