在JavaScript中，链接分配好吗？ [英] In JavaScript, is chained assignment okay?

问题描述

对JS或其语法不是新手，但有时候，语言的语义有时让我感到难过。在今天的工作中，一位同事提到了这一点：

  var a = b = []; 
  

与

<$ p $不同p> var a = []，b = [];


或

  var a = []; var b = []; 
  

因为第一个版本实际上将对空数组的引用分配给a和b。我不能完全接受这一点，但我不确定。你们都在想什么？

解决方案

是的，它们不一样。 var a = b = [] 相当于

  var a; 
 b = []; 
 a = b; 
  

不仅 a 和 b 获得相同的值（对同一个空数组的引用）， b 根本没有声明。在ECMAScript 5及更高版本的严格模式中，这将抛出 ReferenceError ;否则，除非范围内已有变量 b ，否则 b 将自动创建为全局对象的属性，类似于全局变量，无论代码在哪里，甚至在函数内部。哪个不好。

你可以很容易地看到这个：

 （function（）{
 var a = b = []; 
}）（）; 
 
 window.console.log（b）; //显示[] 
  

Am not new to JS or its syntax, but sometimes, the semantics of the language has me stumped at times. At work today, a colleague mentioned this:

var a = b = [];

is not the same as

var a = [], b = [];

or

var a = []; var b = [];

since the first version actually assigns the reference to an empty array to a and b. I couldn't quite accept this as true, but I'm not sure. What do you all think?

解决方案

Yes, they're not the same. var a = b = [] is equivalent to

var a;
b = [];
a = b;

Not only do both a and b get assigned the same value (a reference to the same empty array), b is not declared at all. In strict mode in ECMAScript 5 and later, this will throw a ReferenceError; otherwise, unless there is already a variable b in scope, b is silently created as a property of the global object and acts similarly to a global variable, wherever the code is, even inside a function. Which is not good.

You can see this quite easily:

(function() {
    var a = b = [];
})();

window.console.log(b); // Shows []

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